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Brandon Chinn pushed to branch wip/interpolated-strings at Glasgow Haskell Compiler / GHC

Commits:

7a108e43

by Simon Jakobi at 2026-09-12T18:35:59-04:00

FastString: Drop mkFastStringWith's constructor callback

All three callers passed the same callback, a partial application of
mkNewFastStringShortByteString to the string being interned. That
partial application is allocated as a closure before the table lookup,
on the common hit path too, although the callback is needed only after
a miss.

Drop the parameter and call mkNewFastStringShortByteString directly
after a miss. Since nothing is passed "with" anymore, rename the
function to internSB.

Suggested by Simon PJ in #27528:
https://gitlab.haskell.org/ghc/ghc/-/work_items/27528#note_687031

Assisted-by: Claude Fable 5

82c73b22

by Alan Zimmerman at 2026-09-12T18:36:38-04:00

EPA: More targeted HsDo exact print annotation

HsDo is multi-purpose, as encoded in its HsDoFlavour field. Some of
these are in a layout context (DoExpr, MDoExpr), others are not
(ListComp, MonadComp).

We are moving towards using AnnList only in layout contexts, so we
switch the HsDo TTG annotation from holding an AnnList for this, to
holding

  Either (EpToken "[", EpToken "]") AnnList

This also allows us to trim down AnnListBrackets to only have braces
or None, thereby opening the door for unification with the existing
layout context data type EpLayout.

749d2448

by Luite Stegeman at 2026-09-15T14:07:01-04:00

rts: handle large CONSTR closures in compacting GC

The function update_fwd_large in the compacting GC could run into
an unexpected object with the following error:

  internal error: update_fwd_large: unknown/strange object  1

Closure type 1 is CONSTR. This patch adds cases for CONSTR and
CONSTR_NOCAF.

This is the same problem as #27434 (large AP closures, fixed by
cca0d58963), which missed this case.

Fixes #27649

177d6155

by Luite Stegeman at 2026-09-15T14:07:02-04:00

testsuite: fix T27434 test with compacting GC

Ensure that the interpreter is actually run with the compacting GC

5796aa63

by Luite Stegeman at 2026-09-15T14:07:49-04:00

rts: initialise the stack frame header for mask_frame and apply_mask_frame

We must leave the stack in consistent state before jumping to
mask_frame or apply_mask_frame because they may result. Failing
to do so could lead to a crash if there were waiting exceptions.

Fixes #27651

2486cc92

by Simon Peyton Jones at 2026-09-15T14:08:36-04:00

Re-introduce the "weird SpecDict" case

In this commit

    commit f80375dd4945350a1d784e436975b48b9c736eaf
    Author: Simon Peyton Jones <simon.peytonjones@gmail.com>
    Date:   Sun Jun 29 15:26:58 2025 +0100

      Refactor of Specialise.hs

I removed a test for a dictionary with unbound type variables.  This
turned out to be wrong; see #27629.

So this MR re-introduces it.

8e09a46f

by Duncan Coutts at 2026-09-15T20:11:35-04:00

Refactor (and rename) removeFromQueues, to simplify I/O managers

Rename it to unblockAndAppendToRunQueue which better reflects what it is
intended to do.

The post-condition for unblockAndAppendToRunQueue is that the TSO is on
the run queue or it is in the process of migrating to another cap.
Previously it achieved that by always directly adding the TSO to the run
queue itself.

But this actually made things more complicated for the I/O managers,
because it meant they needed a separate code path for notifying for
cancellation compared to notifying for completion. The general
notification code would always add the TSO to the run queue itself.

So the improvement is to allow different cases in
unblockAndAppendToRunQueue to achieve the same outcome in different
ways: either directly adding to the run queue or calling helper
functions that do so themselves.

This then allows the new I/O managers to share code between the sync and
async cancellation, and to reuse their notifyIOCompletion helpers for
cancellation. This avoids a source of bugs where the completion path may
be updated but the cancellation path may be forgotten, or similarly in
future for sync/async operations.

Update all the existing in-RTS I/O managers, and the posix timeout code.

d668782c

by Duncan Coutts at 2026-09-15T20:11:35-04:00

Document that awaitCompletedTimeoutsOrIO expects an empty run queue

This was true before but implicit and not relied on much. It's better to
be explicit, and allow things to depend on it.

11d938bb

by Duncan Coutts at 2026-09-15T20:11:35-04:00

Store the I/O opcode and fd in the StgAsyncIOOp

This will be useful in several I/O managers and it is handy for logging
and debugging.

It also doesn't increase the size of the StgAsyncIOOp structure. There
was enough spare padding space already.

Update the poll I/O manager to set the new fields.

Add a helper function to convert the enum IOReadOrWrite into the enum
IOOpCode. Also change IOReadOrWrite to be an enum without a typedef, for
consistency with other enumerations in IOManager.h

bfa87882

by Duncan Coutts at 2026-09-15T20:11:36-04:00

Add a new I/O manager based on select()

Yes, this is the second such I/O manager, but it is a modern
re-implementation based on the new in-RTS I/O manager infrastructure. So
it is cleaner and faster than the old select I/O manager.

Why do we need another I/O manager based on select? Why isn't the poll()
one good enough as a baseline portable unix I/O manager? Because macOS.
Apple Inc. is why we cannot have nice things.

The man page for poll on macOS documents the fact that it does not work.
At least, it does not work for all files. Specifically, it does not work
for device files. Whereas macOS select() does work for device files.
Aaaaarg!

We _do_ want to deprecate and remove the old select I/O manager, but due to
macOS we cannot do that until we have a replacement. This is that
replacement. Until of course a nice new k-queue I/O manager arrives,
which could become the new default for macOS and FreeBSD.

Interestingly, this select I/O manager is actually faster than the poll
one, on Linix, in some circumstances: specifically when many Haskell
threads are waiting on the same fd. The poll I/O manager does O(n) work
for n threads waiting on I/O, whereas the select one does O(fds) work
for the number of fds that threads are waiting on. Usually this is 1:1,
so it's not noticable, but one can concoct extreme benchmarks to show
the difference.

8e9de15a

by Duncan Coutts at 2026-09-15T20:11:36-04:00

Minor updates in the poll I/O manager to keep in sync with select

This keeps it in sync with select one. The changes are based on code review
while implementing the new select I/O manager. The two I/O managers are so
similar in structure that it makes sense to try to minimise the diff between
them. This should aid understanding, and fixes to both in future.

222668bb

by Duncan Coutts at 2026-09-15T20:11:36-04:00

Document the new select I/O manager in the user guide

in the RTS section about I/O managers.

And add a changelog entry.

0f48735b

by Cheng Shao at 2026-09-15T20:12:23-04:00

Revert "wasm: workaround WebKit bug in dyld"

This reverts commit c9b8465c2c338176fcab9d197e9d31f4aee11f68. The
underlying webkit bug has been fixed in
https://commits.webkit.org/314219@main. Closes #27564.

809294f1

by Luite Stegeman at 2026-09-15T20:13:08-04:00

rts: make stg_threadLabelzh return a valid pointer for unlabeled threads.

This fixes a segfault in the GC caused by stg_threadLabelzh returning
a 0 pointer in a GC pointer field.

stg_threadLabelzh returns a tuple of type (# Int#, ByteArray# #). If a
thread has no label, the second field is unused. We must still return
a valid heap object pointer.

Instead of returning 0, we now return stg_DEAD_SLOT_closure.

fixes #27618

e2d57026

by Luite Stegeman at 2026-09-15T20:13:54-04:00

JS: fix selector thunk handling

fix a bug where entering a selector thunk could lead to a crash
if the selected field was still a thunk.

fixes #27788

1b4a622f

by sheaf at 2026-09-17T02:39:13-04:00

Improvements to type/data family handling

This commit makes several improvements to the treatment of type and data
families in GHC:

  - We consistently use "type family" instead of "type synonym family"
    to avoid a confusion with type synonyms. For example:

      - OpenSynFamilyTyCon ==> OpenTypeFamilyTyCon
      - ClosedSynFamilyTyCon => ClosedTypeFamilyTyCon

  - Various functions that were specific to data families have been
    renamed to make this clear. For example:

      - tyConFamInst_maybe ==> tyConDataFamInst_maybe
      - isFamInstTyCon ==> isDataFamInstTyCon

  - 'FamTyConFlav' has been refactored so that all closed type families
    share a single 'ClosedTypeFamilyTyCon' constructor.
    The corresponding 'IfaceFamTyConFlav' type was updated similarly.

    This is then used to tidy up the implementation of
    GHC.Core.FamInstEnv.reduceTyFamApp_maybe.

c5aa9098

by sheaf at 2026-09-17T02:39:13-04:00

Small refactor of lookupFamInstEnv

This commit reworks the code for looking up family instances in
GHC.Core.FamInstEnv, reducing duplication a little bit.

No change in behaviour.

a3ad4588

by sheaf at 2026-09-17T02:39:14-04:00

Use a type family for boxing unboxed types

This commit overhauls the way in which unboxed types are boxed, as per
the updated Note [Boxing constructors] in GHC.Builtin.WiredIn.Types.Box.

The main change is to allow boxing to be parametrised by representation,
e.g. making 'Box ty' a valid type. We achieve this by implementing 'Box'
using a closed type family:

  newtype Box (a :: TYPE r) = Box (BoxTF r)

  type BoxTF :: RuntimeRep -> Type
  type family BoxTF r where
    BoxTF IntRep    = BoxInt
    BoxTF Int8Rep   = BoxInt8
    BoxTF WordRep   = BoxWord
    BoxTF LiftedRep = Any @Type
    BoxTF (TupleRep [r1, r2]) = (BoxTF r1, BoxTF r2)
    ...
    BoxTF (SumRep rs) = BoxSum rs

This new design solves the following problems:

  - It allows boxing a type whose 'RuntimeRep' has not yet been
    determined (e.g. because we only learn the 'RuntimeRep' after
    constraint solving). That is, 'Box @r a' is a perfectly valid type
    even if 'r' is an unfilled metavariable. This is a requirement
    to fix #25065 (in which we need to box a type whose 'RuntimeRep'
    is only filled in during constraint solving).

  - It handles boxing of unboxed tuples and unboxed sums, via recursive
    boxing. See Note [Boxing TupleRep and SumRep] in
    GHC.Builtin.WiredIn.Types.Box.

'Box', 'BoxTF' and all of the boxing data constructors are defined in
a new 'ghc-internal' module, 'GHC.Internal.Box'. This module contains
nothing but wired-in declarations, and GHC never loads its interface on
account of a wired-in thing (GHC.Builtin.Modules.isWiredInOnlyModule);
see Note [Loading instances for wired-in things] in GHC.Iface.Load.

The individual boxing data constructors are deliberately left as an
internal implementation detail, and not exported. The intended API is
via the two new magic 'Id's:

  box :: forall {r} (a :: TYPE r). a -> Box a
  unbox :: forall {r} (a :: TYPE r). Box a -> a

exported from 'GHC.PrimOps' (in 'ghc-experimental'). The old boxing
data types (IntBox, WordBox, FloatBox, DoubleBox, DictBox) are no longer
exported from base (CLC proposal #421).

We use this new design to consistently desugar:

  - parallel comprehensions (GHC.HsToCore.ListComp.{deListComp,dsMcStmt})
  - then/group/using statements (GHC.HsToCore.ListComp.{dsTransStmt,dsMcStmt})
  - recursive do blocks (mdo/rec) (GHC.HsToCore.Expr.dsDo)
  - arrows (GHC.HsToCore.Arrows)

by consistently using 'box'/'unbox' to deal with unboxed types being put
into big tuples. We also adapt 'GHC.Tc.Gen.Match.{tcMcStmt,tcDoStmt}'
to create fresh /representation-polymorphic/ type variables for the
binders that we are going to put into a big tuple (previously, GHC only
allowed lifted types in that position).

These desugaring of box and unbox is explained in
Note [Desugaring box & unbox] in GHC.Core.Make.Box. Roughly speaking,
the desugaring looks like:

  box   @r @ty e  ==>  K e
  unbox @r @ty e  ==>  case e of K x -> x

where 'K' is the boxing data constructor at RuntimeRep 'r'. It's
slightly more complicated than that in practice: we first need to
insert a cast (see Note [The canonical type of a RuntimeRep]) using the
new 'CanonicalProv' 'UnivCoProvenance', and unboxed tuples/sums need
recursive logic explained in Note [box/unbox for unboxed tuples and sums],
including a specific 'BoxSum' datatypes for unboxed sum (which also
needed a wired-in type-level list indexing type family (!!)).

The desugaring happens in two ways:

  - Occurrences of 'box'/'unbox' emitted by the typechecker are
    eliminated in the 'ds_app_var' case of GHC.HsToCore.Expr.

  - Other code that needs to box/unbox emits the appropriate code inline.
    This includes the big tuple machinery of GHC.Core.Make used by the
    desugarer (GHC.HsToCore.{Utils,Binds,Arrows,ListComp}) as well as
    the float-out pass (GHC.Core.Opt.SetLevels).

Core Lint checks that no occurrence of 'box'/'unbox' survives desugaring.

On the way, we overhaul the litany of big tuple functions by introducing

  data BigTupleLayout = BoxedElements | BareElements

to distinguish the two kinds of "big tuples" that GHC routinely deals
with; for example 'mkBigCoreVarTupSolo' becomes 'mkBigCoreVarTup BareElements'.
All of this code is moved to the new module GHC.Core.Make.BigTuple.

Fixes #20864 #22336 #22473 #25065 #27331

-------------------------
Metric Decrease:
    T9872a
-------------------------

4cf7628a

by Evgeny Malyshev at 2026-09-17T02:39:56-04:00

Fix the CompactionFailed documentation link to compact

The old Haddock reference links into base instead of ghc-compact.
Use an explicit URL because ghc-compact depends on base, not vice versa.

Fixes #27558

Assisted-By: OpenAI Codex

098bba70

by Facundo Domínguez at 2026-09-17T14:57:41-04:00

Remove the call to hoistFlag from processCmdLineP

This call was undesirable because it was causing a reconstruction
of the list of flag processors.

Even relying on list fusion would not be sufficient because we want
to replace the list with a data structure with faster lookups to
address #25763.

The removal is done in two moves:

* We move CmdLineP to CmdLine.hs
* Then we move the effect of hoisting to processArgs via a new function
  hoistEwM

e01799c7

by Facundo Domínguez at 2026-09-17T14:57:42-04:00

Do not create duplicate flag processors for the -W flag

This makes simpler implementing the command line parser with a trie map
holding flag processors. If we were to allow duplicate flag processors
for any flag, the trie map would need to be a multimap.

3875b257

by Facundo Domínguez at 2026-09-17T14:57:42-04:00

Replace the list of flag processors with a trie map in `processArgs`

Previously `GHC.Driver.CmdLine.findArg` was called for each
command-line argument, doing a linear search on the list of flag
processors each time. This was somewhat costly as the list of flag
processor has near 2000 elements.

A lookup with a trie map is faster in general, but the speed up is
only noticeable when calling `GHC.Driver.CmdLine.processArgs` sufficient
times, so the amount of lookups piles up. This could happen in large
projects with many units each having many dependencies. If only a
module needs to be recompiled, the user still needs to wait for all of
the flag parsing to complete.

The benchmark at testsuite/tests/perf/compiler/cmdline-parser shows a
10x speed up when using the trie.

The trie map is implemented in `GHC.Data.StringTrie`.

The design is explained in Note [Processing command line argument fast]
in GHC.Driver.Session.

The commit also:
* adds two property-based tests for trie map operations,
* adds a benchmark of the command line parser, and
* renames the predicate `arg_ok` to `valid_flag_suffix` to decide what
  flag processor should be used when there are multiple matching
  prefixes on a flag.

Fixes #25763.

5236634a

by Andreas Klebinger at 2026-09-17T14:58:24-04:00

Compile Word64{Map,Set} with -O2 always.

This is the hottest code and even if we build GHC with -O0 otherwise -O2
pays off for these modules.

Fixes #27814.

545fd4fb

by ARATA Mizuki at 2026-09-22T15:03:24-04:00

Change the C types of hs_{bitrev,bswap}{16,32} to match the Haskell types

bitReverse{16,32}# and byteSwap{16,32}# have the type
Word# -> Word#, but their C correspondents took and
returned StgWord{16,32}.

The mismatch on bitReverse16# led to the problem reported
in #27299. Although no similar issue is reported for
bitReverse32# and byteSwap{16,32}#, it seemed wise to
change them proactively.

This commit fixes the mismatch by changing the C type.

Fixes #27299

bf17f289

by fendor at 2026-09-22T15:04:05-04:00

Don't lose instances from shadowed imported modules with the same name

When we look for instances below the current module, we need to check
the unit id as well when we check for whether the module is the current
module, otherwise we ignore modules we import that have the same name as
the import module.

Adds regression test.

53eead15

by Wolfgang Jeltsch at 2026-09-24T20:12:03-04:00

Correct the documentation of `explicitUnits`

Resolves #27787.

35bf6f4b

by Simon Jakobi at 2026-09-24T20:12:49-04:00

Remove stale specHeader bullet from Note [Don't make fillers for constraint types]

The Note said that Specialise.specHeader also calls mkAbsentFiller.
abfc224a27c removed that call: the specialiser no longer drops dead
arguments at all, see Note [Do not drop dead args from specialisations]
in GHC.Core.Opt.Specialise.

Assisted-by: Claude Fable 5.1

1abef1ef
by Wolfgang Jeltsch at 2026-09-25T17:02:07-04:00
```
Improve the `mhu-closure` makefile
```
a79a68ea
by Wolfgang Jeltsch at 2026-09-25T17:02:07-04:00
```
Make `ghci-multi` hide all unneeded packages
```

d6e72450

by Wolfgang Jeltsch at 2026-09-25T17:02:07-04:00

Improve the definition of the home unit closure property

c9a2766d

by Wolfgang Jeltsch at 2026-09-25T17:02:07-04:00

Re-implement the home unit closure check

Compared to the previous implementation, the new one has the following
advantages:

  * It is correct.
      - It distinguishes between units that have the same unit ID but
        different ABI hashes.
      - When `-hide-all-packages` is not used, it considers as home unit
        dependencies also units that are made implicitly available
        because they are in the package database.
  * It seems to be faster in usual settings.
      - In particular, it does not have a preparation phase in which it
        merges the dependency information from all home units, so that
        its running time does not grow linearly with the size of the
        package database in normal Cabal scenarios.
  * It is (hopefully) clearer.
  * It is better documented.

fa96acf8

by Wolfgang Jeltsch at 2026-09-25T17:02:07-04:00

Remove erroneous GHC flag in `hadrian-multi`

The flag was `-Werror=-Wno-error=incomplete-record-selectors`. Its
purpose was to permit incomplete record selectors, but because of the
illegal combination of `-Werror=` and `-Wno-error` it was ignored,
without any negative consequences.

9f48a5b9

by Alan Zimmerman at 2026-09-25T17:02:50-04:00

EPA: Add specific layout field to AnnList

Up to now AnnList has effectively served to represent lists of things
having layout, regardless of whether they do or not.

In Haskell, layout is optional in many places, as there is also an
alternative syntax using braces and semi-colons, which is free-form.

This commit introduces AnnListLayout, a data structure capturing this
distinction.

It is deliberately set up similarly to EpLayout, so that these two can
be merged in time.

The transition is done mechanically, where AnnListBraces is not used
yet, the original Nothing case maps to AnnListNoLayout, and the Just
case to AnnListLayout.

Later commits will fine tune these against the actual parser layout.

234bab08

by Vladislav Zavialov at 2026-09-29T17:22:54-04:00

Add regression test for #7803

Commit 2effe18ab5, first shipped with GHC 8.2.1, fixed #7803 "Superclass
methods are left unspecialized".

This change turns the example from the ticket description into a regression
test, checking that foo1 is fully specialised at Double, with no calls to the
overloaded exp5Tail.

Closes #7803

Assisted-by: Claude Opus 5

c406135c

by Andreas Klebinger at 2026-10-01T06:57:28-04:00

RTS: Update flag parsing/handling:

Using 0 as argument to any of these flags now resets the flags setting to default:

    - ``-n0`` disables nursery chunking.
    - ``-K0`` (or a bare ``-K``) removes the stack size limit.
    - ``-M0`` removes the heap size limit.
    - ``-H0`` resets the suggested heap size, including the nursery scaling set by ``-H``.
    - ``-AL0`` makes the large object allocation limit follow ``-A`` again.

GC Generations are now limited to 64.

-n larger than -A now emits a warning and disables chunking.

Fixes #27830 #27829

984ee82b

by Andreas Klebinger at 2026-10-01T06:58:10-04:00

Fix MultiLayerModules timeout multiplier.

We accidentally used the compile multiplier for ghci commands which
use the run timeout setting to set the timeout.

Fixes #26263 and #27796.

50a9cf26
by Andreas Klebinger at 2026-10-01T06:58:10-04:00
```
testsuite: Include runtime in JUnit report.
```

e358619f

by Alan Zimmerman at 2026-10-01T06:59:01-04:00

EPA: Use EpaLocation in EpVirtualBraces

Haddock processing uses EpLayout.EpVirtualBraces to record the current
layout column, for allocating haddock comments correctly.

For exact printing, we need the line as well as the col, so we know
how many lines to skip when the indented code block starts.

This commit updates EpLayout.EpVirtualBraces so that it carries an
EpaLocation instead of just the Int column, to allow it to be used for
both purposes.

It is a simple mechanical change, with no other impact, but it allows
us to use EpLayout instead of AnnListLayout in AnnList, in the next
commit in this series.

1d2a26ee

by Andrew Lelechenko at 2026-10-01T20:22:04-04:00

Remove bitSize from MINIMAL pragma of class Bits

Cf. https://github.com/haskell/core-libraries-committee/issues/441

The function is deprecated for 12 years and has a default implementation,
yet it was never removed from the MINIMAL pragma, putting library authors
in a very awkward position. Let's finally remove bitSize from MINIMAL.

3cc21e4c
by Hannes Siebenhandl at 2026-10-01T20:22:59-04:00
```
Add regression test for #27641
```

c35096f5

by Hannes Siebenhandl at 2026-10-01T20:23:00-04:00

Don't `:add` modules the loader will reject

Only home modules can be actually added to the loader context, thus we
only accept the module if it belongs to a home unit.

a563cb27

by Simon Jakobi at 2026-10-02T17:37:36-04:00

ci: accept modified changelog.d entries in lint-changelog

The changelog guide says to extend an existing entry rather than add a
new one when the feature is already documented. lint_changelog only
counted added files, so such MRs failed before the validator ran.

Fixes #27779.

Assisted-by: Claude Fable 5.1

03082cd4

by Simon Jakobi at 2026-10-02T17:38:17-04:00

changelog.d: Fix warning flag name in the semaphore-v2 entry

The entry refers to -Wsemaphore-version-mismatch, which does not exist.
The warning GHC emits when it cannot use a -jsem semaphore, including
on a protocol version mismatch, is -Wsemaphore-open-failure.

Context: !15729, #25087

Assisted-by: Claude Fable 5.1

a93705ee

by Simon Jakobi at 2026-10-02T17:38:17-04:00

changelog.d: Fix the CLC proposal link in the T27455 entry

The entry used the Sphinx :ref: role with a URL as its target. :ref:
resolves only internal labels, so the generated release notes would
show the proposal name without a link. Use the plain hyperlink form
like the other entries.

Context: !16274, #27455

Assisted-by: Claude Fable 5.1

54381079

by Hannes Siebenhandl at 2026-10-02T17:39:01-04:00

Extend the `TrustOverlay` for wired-in units

The TrustOverlay only contained the `UnitId` of all on-disk units.
However, wired-in units are referred to by their wired-in unit id, which
is not the same as the on-disk unit id.

Thus, trust checks for wired-in units failed because the lookup was
using the wired-in unit, which wasn't present in the `TrustOverlay`.

We fix #27834, by copying the trust info for wired-in unit ids.

f8cfb1a5

by Hécate Kleidukos at 2026-10-04T19:36:39+02:00

[CLC 341] Strict variants of mutable types

This commit is the implementation of CLC Proposal 341
https://github.com/haskell/core-libraries-committee/issues/341

575c4bc6

by Rodrigo Mesquita at 2026-10-05T12:32:02-04:00

ttg: HsModuleName in the AST, ModuleName in GHC

This commit makes the AST fully independent from GHC by parsing a
"module name" as an `HsModuleName (= HText)` rather than constructing an
`ModuleName (= FastString)` straight away.

In the GHC side, everything remains as a `ModuleName`. We turn AST
`HsModuleName`s into fast strings with the cheap `mkFastStringShortText`
when we process the AST for manipulation within the compiler.

There are no performance regressions introduced by this approach.

This is the final piece of #21592

Alternative plans were discussed in
gitlab.haskell.org/ghc/ghc/-/merge_requests/16546#note_694647,
but the Simon suggested one won in simplicity and performance when
compared against my attempt at implementing my original plan of having
a family `ModuleNameP`.

b3b89d32

by mangoiv at 2026-10-05T22:03:32-04:00

hadrian: Persist the used flavour name in the build root (#20650)

This executes the idea proposed in #20650, teaching Hadrian to persist
the flavour used to build a build root. This both makes use of multiple
build roots more ergonomic (as the user no longer needs to repeat
themselves) and avoids a tiresome foot-gun (forgetting to specify a
flavour, often wholly invalidating a build root).

Closes #20650.

Co-authored-by: Sylvain Henry <sylvain@haskus.fr>

f977837b

by mangoiv at 2026-10-05T22:04:21-04:00

configure: fix sed script and subst tooldir script

Fixes #27672

Adds sanity checking for SUBST_TOOLDIR

Metric Decrease:
    LinkableUsage01
    MultiLayerModulesTH_OneShot
    T10547
    T12234
    T12425
    T13035
    T13820
    T18140
    T18730
    T18923
    T9198
    TcPlugin_RewritePerf
    mhu-perf

f1cb08be

by Hannes Siebenhandl at 2026-10-05T22:05:03-04:00

Prepend in-memory unit database for backpack

The packageDbFlags are stored in reverse order. Thus, prepending will
ensure that the in-memory unit database is the last item on the unit
database stack.

Further, the in-memory unit database needs to be able to shadow units
from earlier unit databases in the unit database stack.

Moreover, a `-clear-package-db` would remove the in-memory database if
it weren't the last item on the stack.

Closes #27835

8d49ed42

by Kazu Yamamoto at 2026-10-06T19:37:25-04:00

Do not leave standard file descriptors free at startup (#24450)

A process may be started with stdout closed -- `prog >&-` from a shell does
exactly that.  Nothing then reserves descriptor 1, and open(), pipe() and
socket() hand out the lowest free descriptor, so the next descriptor the
runtime creates for itself takes it: the ticker's pipe, or a capability's IO
manager control pipe.

'GHC.Internal.IO.StdHandles.stdout' is a CAF, built the first time the program
writes to stdout, which is long after that has happened.  It builds a 'Handle'
over descriptor 1, which now belongs to the runtime, and nothing reports an
error because the descriptor is perfectly good.  The program then writes its
output into the runtime's own plumbing.

What that looks like from the outside is a hang.  The descriptor the Handle has
taken over is the read end of a pipe, so poll() for POLLOUT on it never says
yes and epoll_ctl() accepts a registration for it; the thread waiting to write
blocks for ever.  Closing the Handle at exit closes the runtime's descriptor,
which is worse: the ticker then fails on every tick.

'hs_init_ghc' now opens /dev/null onto whichever of descriptors 0, 1 and 2 are
closed, before the runtime creates any descriptor of its own.  Each is opened
for the access that descriptor would normally have, read-only for 0 and
write-only for 1 and 2, so that writing to 'stdin' or reading from 'stdout'
still fails rather than quietly succeeding against /dev/null.  See
Note [Standard file descriptors must be open].

Moving only the descriptors the runtime opens would not do: the first file the
program itself opened would land on the free number instead.  The fix has to
leave no free number.  A program run this way therefore finds stdout connected
to /dev/null rather than failing with EBADF on the first write, which is what
such a program would get from most other language runtimes.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

65bcf9e9

by Simon Jakobi at 2026-10-06T19:38:09-04:00

changelog: Repair the sentence describing #27477

The description read "programs using could crash", the object of
"using" having been lost. The synopsis already says which programs are
affected, so drop the dangling word.

Assisted-by: Claude Fable 5.1

15ece296

by mangoiv at 2026-10-08T15:14:56-04:00

ci: move hadrian/ghci into the lint category so it can be picked up by more runners

Fixes #27817

c0823135

by mangoiv at 2026-10-08T15:14:56-04:00

ci: make lint-adjacent jobs into platform independent

- move lint jobs into lint-anyarch tag
- unify lint jobs under the same platform independent container

f76464cf

by Zubin Duggal at 2026-10-08T15:15:39-04:00

DmdAnal: Make AbsDmd neutral in maxDmd

fixes #27845

ffa56ae4

by sheaf at 2026-10-09T17:15:59+02:00

Preserve sign of zero when parsing -0.0#

This commit leverages the recently added GHC.Types.Literal.Floating
module in order to preserve the sign of negative zero from
source-written floating-point literals.

Fixes #27545

d538cac9

by Facundo Domínguez at 2026-10-09T20:26:38-04:00

Propagate annotations to simple interfaces

This allows annotations to be visible to plugins when run by
haddock. The LiquidHaskell plugin needs the annotations, in particular.

Fixes #26761

32c4f4ab

by meooow25 at 2026-10-09T20:27:28-04:00

Bump containers submodule to 0.8.1

And fix affected tests.

* T4086a: containers depends on template-haskell-lift now instead of
  template-haskell.
* T22611: Updated to be less dependent on containers internals.

c786065e

by sheaf at 2026-10-09T20:28:20-04:00

Use an explicit pattern match in toIfaceIdDetails

The old wildcard pattern let this code bit rot: it did not properly account
for recently added 'IdDetails' such as 'RepPolyId', which resulted in
spurious 'pprTrace' messages that could even pollute HLS sessions.

Fixes #23785

e7cacf59

by sheaf at 2026-10-09T20:29:09-04:00

Take into account ifKind in freeNamesIfDecl

Commit 3c2f4bb4 added the 'ifKind' field to various constructors
of 'IfaceDecl', which preserves the original kind (before type synonym
expansion) for better error messages.

However, that commit forgot to update 'freeNamesIfDecl' correspondingly.
This could result in a "urk! lookup local fingerpring" crash when e.g.
a type's kind uses a type synonym defined later, as we failed to add a
dependency on the type synonym during dependency analysis.

Fixes #27916

41e80f41

by Simon Peyton Jones at 2026-10-11T14:38:51-04:00

Simplify the error-suppression mechanism in GHC.Tc.Errors

In `GHC.Tc.Errors.reportWanteds` we suppress some errors in favour of
others.  But the mechanism had grown crufty, and #27331 exposed a bug.

This MR makes it simpler and more uniform, by doing everything via
the `ei_suppress` field of the ErrorItem.

Claude then found another closely-related bug (see the review linked
on !16564).  That bug is immortalised in new test T18851d.

I'm still not happy with `ignoreConstraint` but we can worry about
that another day; I have not touched it.

Main changes

* Instead of /predicting/ when an error will be reported, we set
  `cec_error_reported` when an error is /actually/ reported.  (I renamed
  it from `cec_suppress` to reflect this change.)

  Now errors are suppressed only when a preceding error has actually been
  reported.  Simpler and more robust.

  See `Note [cec_error_reported: suppressing less-serious errors]`

  The big changes are in `reportWanteds` which looks very different.

* New `Note [ei_suppress: suppressing confusing errors]` describes the
  (quite separate) `ei_suppress` mechanism for suppressing confusing
  errors, mainly arising from non-empty rewriter sets

* In `tryTypeClassDefaulting`, instead of switching off type-class defaulting
  if there is an insoluble /anywhere/, we just switch it off locally when
  we find an insoluble sub-tree. See Note [When to do type-class defaulting]
  and (W5) in Note [ApproximateWC]

* Greatly simplify `insolubleWantedCt`.

* I made type-class-defaulting be a bit more eager, only switched off when
  a peer or ancestor (but not child) constraint is insoluble.  See
  Note [When to do type-class defaulting].

Small refactorings

* Rename `askNoErrs` to `askErrsFound` and flip its polarity

* The code for "check if ei_suppress is true of all items" code is
  moved to `mkErrorItems` rather than in the large `reportWanteds`.

* Code for `has_gadt_match` moves to `mkGivenErrorReporter`, rather than
  polluting the large `reportWanteds`.

* Many tests with `-fdefer-type-errors` get more warnings, especially
  about ambiguous (Num a) constraints. This is by-design: see
  Note [Always warn with -fdefer-type-errors].  Previously GHC was not
  honouring this Note but now it is, hence the extra errors.

f217fc31

by Cheng Shao at 2026-10-11T14:39:33-04:00

compiler: ensure cross ghc --info output target unit ids

Previously, for cross ghcs, the base/ghc/ghc-internal unit ids in
--info output do not match the target unit ids reported by ghc-pkg,
breaking cross ghc release builds with hash-unit-ids when being used
with cabal 3.18. This patch fixes it by making --info output target
unit ids. Fixes #27919.

Co-authored-by: Codex <codex@openai.com>

d07a5176

by Simon Peyton Jones at 2026-10-13T18:10:39-04:00

Remove the Eq and Ord instances for ForAllTyFlag

In Note [Comparing visibility] in GHC.Core.TyCo.Compare we read that
"we always compare ForAllTyFlags with `eqForAllVis` or `cmpForAllVis`"
But alas there was an Eq and Ord instance for ForAllTyFlag, which made it
impossible to check this claim.   So I made these changes:

* Move the definition of ForAllTyFlag and its related functions from
  L.H.S.Specificity (where it did not belong, since L.H.S didn't use it)
  into GHC.Types.Var.

* Remove the Eq and Ord instances for ForAllTyFlag

* Deal with the fallout, by using `eqForAllVis` or related functions.

* In interface files we use IfaceType as a key for a de-deduplication map
  See Note [Deduplication during iface binary serialisation]
  in GHC.Iface.Binary.  For this we need /exact/ comparison, so we use a
  newtype wrapper for IfaceForAllTyFlag, and give use `cmpForAllTyFlagExactly`
  to implement its Ord instance.  Tiresome but easy.

* Rename `toIfaceForAllBndrs` to `toIfaceForAllTyBndrs`

ea5c054d

by Simon Peyton Jones at 2026-10-13T18:10:39-04:00

Refactor type and coercion substitution

This MR refactors type and coercion substitution so that they use
TyCoMapper, rather than hand-rolled code.  Fundamentally it's a simple
idea, but I had to do quite a bit of refactoring to make it fly.

I got Claude to help me this this refactoring, but I paid careful
attention to every line of code.

Main changes:

* New module GHC.Core.TyCo.Make which defines TyCoMapper, and a bunch of other
  simple "smart constructors" for types and coercions.  See
  GHC.Core.TyCo.Rep Note [The Type-related module hierarchy], esp (TRM1)

* The new TyCoMapper has new fields `tcm_tcapp_ty` and `tcm_tcapp_co` to handle
  type-constructor applications.  These replace `tcm_tycon`.

* Rewrite GHC.Core.TyCo.Subst to use TyCoMapper

* Rewrite GHC.Core.Type.expandTypeSynonyms to use TyCoMapper.

* Implement `seqType` and `seqCo` with TyCoFolder. To to this I had to add
  a `tcf_lit` field to TyCoFolder.

* A new module GHC.Utils.StrictIdentity to define a strict identity monad,
  which itself is used for
      substTyCoMapper :: TyCoMapper Subst StrictIdentity

* I removed an apparently-unused instance for Binary Levity

Refactoring `seq` functions

* In GHC.Core.Seq, redefine `seqOneShot`, `seqRuleInfo`, and friends to return
  `SeqResult`, a new type defined in GHC.Types.Baseic, rather than `()`.  We
  can define a Monoid instance for `SeqResult` that does the right thing.

* This has knock-on effects across the compiler.

34165e23

by Simon Peyton Jones at 2026-10-13T18:10:39-04:00

Refactor TyCoMapper to take a single monad; add StrictReaderT

TyCoMapper used to be parameterised over both an environment and a
monad, with fields like
    tcm_tyvar :: env -> TyVar -> m Type
Now it is parameterised over just the monad:
    tcm_tyvar :: TyVar -> m Type
If the mapping needs an environment, `m` is a reader monad over that
environment.  Many uses (e.g. zonkTcTypeMapper, swizzleMapper) need no
environment at all, and used to pass a dummy ().  The binder field
becomes
    tcm_tycobinder :: forall r. TyCoVar -> ForAllTyFlag
                   -> (TyCoVar -> m r) -> m r
and runs the continuation in the extended environment.  mapTyCoX is gone;
only mapTyCo remains.

To make this efficient, the reader monad must use the one-shot trick and
be strict in the environment.  So:

* New module GHC.Utils.Monad.StrictReader defines
      newtype StrictReaderT r m a
  with hand-written instances.  The builder of the pattern synonym
  `StrictReaderT` is a smart constructor that adds both `oneShot` and a
  bang on the environment, so every StrictReaderT computation is strict
  in the environment.  See Note [Instances for StrictReaderT].

* ZonkT becomes  type ZonkT = StrictReaderT ZonkEnv
  ZonkM becomes  type ZonkM = StrictReaderT ZonkGblEnv IO
  replacing their own hand-written instances.

* Substitution uses
      type SubstM = StrictReaderT Subst StrictIdentity
      substTyCoMapper :: TyCoMapper SubstM
  and similarly expandTypeSynonymMapper.

* zonk_tycomapper :: TyCoMapper (ZonkT TcM) is now much simpler.

* swizzleMapper uses StrictIdentity rather than Identity.

* Module renaming, so that the strict monads are all peers in GHC.Utils.Monad:
      GHC.Utils.StrictIdentity      ==>  GHC.Utils.Monad.StrictIdentity
      GHC.Utils.Monad.State.Strict  ==>  GHC.Utils.Monad.StrictState
  alongside the new GHC.Utils.Monad.StrictReader.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

c0299e15

by Simon Jakobi at 2026-10-13T18:11:23-04:00

testsuite: Line-buffer the driver's stdout

Python block-buffers stdout when it is a pipe. This is the case for
example in CI. As a result there is often no indication of progress at
all until the run ends.

To fix this, we configure it explicitly to use line buffering instead.

stderr is reconfigured too, for consistency: Python line-buffers it by
default since 3.9.

The type-ignores are for mypy, which types these handles as TextIO — an
interface without reconfigure.

Fixes #12934.

Assisted-by: Claude Fable 5

9b5beaa2

by Florian Ragwitz at 2026-10-13T18:12:05-04:00

hpc: Improve coverage semantics for instance methods and instances

Users care about which of their explicitly written methods are or aren't
covered, not whether all of the inherited methods are covered as well. Those
should be covered where they are implemented.

We therefore skip generating top-level boxes for inherited methods which will
appear as uncovered with no clear indication as to why, and no way to fix other
than exercising code the user has not written.

Previously, top-level boxes for those inherited methods would use the instance
head as their source span. Now that those boxes are gone, we can use that
source span for another purpose:

We introduce a new top-level box for each class instance with its instance head
as the source span, and use that to indicate whether or not an instance has
been covered at all.

We do this by making any method of the instance, inherited/generated or
otherwise, tick the box for the instance head. Excluded from this are
methodless classes, which could never be covered using the current mechanism.
This could be improved upon further.

These changes apply to ticking for program coverage only, and don't affect
source notes. Additionally, we update the scope of record selector ticking to
be limited to program coverage as well, as the oversight to also extend it to
source notes has previously caused bugs and because record selector ticking is
coverage-specific.

Fixes #17155, #15932.

836ec50a

by Simon Peyton Jones at 2026-10-13T18:12:54-04:00

Refactor fundeps again

This MR fixes #27828; there was a bad scoping error in `trim_qtvs`.

The main changes are

* Refactor `trim_qtvs` completely to eliminate the bug.

* Remove a redundant `apartnessCheck` from `mkTopOpenFamEqFDs`.

* Lots of improvements to the Notes in FunDeps.hs

I also did a couple of pure refactors:

* I realised that we were carefully caching `cab_incomps` in each
  `CoAxBranch`, to make it more efficient to reduce a type family application,
  but we were not doing the same thing when computing relevance for functional
  dependencies; see Note [Relevance] in GHC.Tc.Solver.FunDeps.  So I added
  `cab_overlaps`, which is now used in `noEarlierMatch`.

* Use a proper data types, `AmIUnifying` and `OuterKindsFlag`, for two
  previously-Boolean arguments to `tc_unify_tys`.  This is a pure refactor.

889ef371

by Simon Peyton Jones at 2026-10-13T18:13:36-04:00

Refactor and simplify solver's unification code

Ticket #27844 showed that GHC's constraint solver could miss an
occurs check, and thereby build an infinite type.

This patch significantly simplifes the code in
GHC.Tc.Solver.Equality.canEqCanLHSFinish_try_unification
GHC.Tc.Solver.Equality.canEqCanLHSFinish_no_unification

It is now significantly simpler, and requires much less subtle
reasoning to understand.

Main changes

* If `canEqCanLHSFinish_try_unification` fails to unify, it now /always/ calls
`canEqCanLHSFinish_no_unification` rather than trying to finish the job itself.

* `GHC.Tc.Solver.Monad.checkTypeEq` is moved from Monad into Equality, and renamed
to `GHC.Tc.Solver.Equality.checkTypeEqNoUnification`.

* `canEqCanLHSFinish_no_unification` calls `checkTypeEqNoUnification` to test
the invariants for an inert equality. Before this MR there was a special case for
`do_not_prevent_rewriting`, but that has gone; instead it's just a consequence of
the flags passed to `checkTyEqRhs`, documented in the `PuOK` case of
`canEqCanLHSFinish_no_unification`.

* There is only one bit of "swap back" code, in `finish_no_unify` in
`canEqCanLHSFinish_try_unification. Previously this code was duplicated.

* `kickOutAfterFillingCoercionHole` should account for equalities with a
TyFamLHS. See (KOC5) in Note [Kick out after filling a coercion hole]

* No need to gather coercion holes in the `PuOK` case of `canEqCanLHSFinish_no_unification`.
See documentation with that code.

f9193e53

by Simon Peyton Jones at 2026-10-13T18:13:37-04:00

One line fix to fallout from

...#27731, !16564, simplifying Tc.Errors machinery

There was an over-eager assert!

cdd56b7a

by Florian Ragwitz at 2026-10-13T18:14:19-04:00

Fix several sphinx errors and warnings from rendering the release notes

The extension references which are removed here are for extension which were
already previously undocumented, so just removing them seems fine for now.

4f00da25
by Florian Ragwitz at 2026-10-13T18:14:19-04:00
```
Link to QualifiedStrings documentation
```

97adaf89

by Simon Peyton Jones at 2026-10-14T00:07:27+01:00

Line up simpleUnifyCheck and check_ty_eq_rhs

Even after #26543 was allegedly fixed, the original repo case in the
Description continued to fail.  The reason was that the QuickLook
unifier, `qlUnify`, used `simpleUnifyCheck` for checking unification
invariants; and `simpleUnifyCheck` conservatively rejected any RHS
with a coercion hole in it.  The upshot was that QuickLook was not
as clever as it should be, wrongly failing to do an impredicative
instantiation.

Interestingly `check_ty_eq_rhs`, which does the same job during
constraint solving, was more liberal: it just looked at the free vars
of the coercion, and allowed coercion holes.

This MR lines them up, adding some careful notes.  See
    Note [simpleUnifyCheck]
    Note [Occurs check and coercions]
    Note [Unification preconditions] esp (COERCIONS)

Things are better than before, but I am still uncomfortable about the
possibilty that a hole might be filled with a coercion that mentions
the LHS type variable; for now I have left this discomfort documented
in Note [Occurs check and coercions].

Main changes

* `qlUnify` now only unifies /instantiation variables/ not regular unification
  variables.  This makes QuickLook more predictable.  See (UQL6) in
  Note [QuickLook unification].

* `qlUnify` no longer needs to call `simpleUnifyCheck`, so we can get rid of
  `UC_QuickLook` and its code in `simpleUnifyCheck`.

* I found I could get rid of `mkOccFolders` in favour of `anyFreeVarsOfType`.
  Hooray.

* Improve documentation of CycleBreakerTv, and rename it to GivenCycleBreakerTv
  because they are only uesd for Givens!

* I accepted a 9% worsening in compile time allocation on T5030
  because the newly-consistent story triggered #27934.

Other changes

* I refactored `atvFolder` so that it pairs the "interesting-var" function
  with the in-scope set, to avoid building a closure with a free variable.
  Now we get nice tight loops for `anyFreeVarsOfType` and friends.
  See Note [Any-free-var folder].

Metric Increase:
    T5030

5f7b3047

by Simon Peyton Jones at 2026-10-14T00:07:27+01:00

Fix order-dependence in QuickLook

Ticket #27917 pointed out that QuickLook was order-dependent
-- very bad!

The solution is in `qlUnify`: spot when we are unifying a type with a
definite monotype, and then monomorphise any free instantiation
variables of that type.

See (UQL8) and (UQL9) in Note [QuickLook unification].

c391c70c

by Simon Peyton Jones at 2026-10-14T00:07:28+01:00

Add some comments about RuntimeUnk

This commit just adds comments explaining why we don't need to worry about
RuntimeUnkTv in the constraint solver.

It also tightens up docmentation of earlier (overlapping) commits in
this MR.

7df3ba2f

by Andrea Vezzosi at 2026-10-14T06:15:48-04:00

add runGhcWithSignalHandlers and remove handling from runGhc[T]

it only ever makes sense to call `withSignalHandlers` from `main`, otherwise
signals can be too easily swallowed without notification to the rest of your
process.

This commit takes withSignalHandlers out of `runGhc/runGhcT` and provides
`runGhcWithSignalHandlers` for the old behaviour.

3a72b8a2

by Hannes Siebenhandl at 2026-10-14T13:48:32-04:00

Fix #27836, normalise unit database path in ExternalUnitDatabases

Add a regression test, proofing that all variations:

- local-unit-db/my-test.conf
- ./local-unit-db/my-test.conf
- ./local-unit-db/my-test.conf/

are parsed only once.

8ddbc6f9
by Florian Ragwitz at 2026-10-14T13:49:28-04:00
```
Remove obsolete HPC types

Fixes #27946.
```

f34c79ee

by Alan Zimmerman at 2026-10-15T04:54:47-04:00

EPA: Start using vocurly as basis for AnnListLayout

For the existing cases that populate AnnList.

This uses AnnListBraces when braces are explicitly supplied, and
AnnListLayout when vocurly is used

The change also shows up AnnList usages that can never capture layout,
these are cleared up later.

033ba59c

by sheaf at 2026-10-15T04:55:44-04:00

Make exprFreeVars deep

exprFreeVars used to be a shallow free-variable finder (it didn't look
into kinds), but most callers wanted it to be deep. In particular,
'exprFreeVars' is often used to seed an 'InScopeSet', and that must
contain the /deep/ free variables.

This commit makes it deep, which allows us to get rid of calls to
'closeOverKinds' or other workarounds for shallowness in:

  - 'GHC.Core.Opt.Simplify.Utils.abstractFloats'
  - 'GHC.HsToCore.Binds.dsSpec_help'
  - 'GHC.Core.Opt.Arity.etaBodyForJoinPoint'
  - 'GHC.Core.FVs.freeVars'
  - 'GHC.Core.Lint.lintCoreRule'

We also remove 'deepExprsFreeVarsDSet' and 'closeOverKindsDSet'.

Other places needed a little bit more care, in particular to properly
handle coercion variables free in kinds:

  - 'GHC.Core.Opt.Specialise.specHeader' quantifies over the binder of
    an unspecialised argument directly, instead of computing the free Ids
    of the argument.
  - 'GHC.HsToCore.dsWarnRuleShadowing' filters out 'CoVar's.
  - 'GHC.Core.Opt.SpecConstr.callToPat' had some ad-hoc logic to compute
    argument pattern types and a workaround with 'expandTypeSynonyms'.
    That logic is removed in favour of a new 'argsToPats' function which
    directly computes the argument pattern types by instantiating the
    type of the head of the call pattern.

Fixes #27936

f93d523c

by Luite Stegeman at 2026-10-15T04:56:34-04:00

testsuite: Use mkWeakMVar in memo001/memo002

mkWeakPtr in the memo001/memo001 tests created weak pointers
with the MVar box as the key, which is fragile in the presence
of optimization. The fix is using mkWeakMVar, keying on the
primitive MVar# instead.

Fixes #27924 and #27396

8c947573

by Simon Peyton Jones at 2026-10-16T13:11:53+01:00

Improve exprOkForSpeculation

Fixes #27910, which exposed an infelicity in exprOkForSpeculation

The new story is carefully described in new Note [exprOkForSpeculation],
esp (a-c) and (EOKS1).

7dedcf13

by Alan Zimmerman at 2026-10-16T19:03:17-04:00

EPA: Add TTG fields to FamilyInfo

This commit is a simple add, with unused extension fields.

It lays the groundwork for using them for exact print annotations

26b9a8a0

by Arkissa at 2026-10-17T16:04:10+00:00

ghci: Prefer exact source spans in :type-at

Prefer expression types for identical source spans.

Fixes #27412.

70dc4e61

by sheaf at 2026-10-18T18:15:15-04:00

Carefully implement coercion zapping

This commit implements coercion zapping in the rewriter, controlled by
the -fzap-coercions flag (on by default). This significantly reduces
the size of coercions produced when rewriting type family applications,
resulting in much faster compilation. See Note [Coercion zapping]
in GHC.Tc.Solver.Rewrite for an explanation and further details.

The rewriter now keeps track of the `CoVar`s and `CoercionHole`s it uses
when rewriting, which allows computing the dependencies of a zapped
coercion without traversing the coercion the rewriter produced, as
explained in Note [Computing coercion dependencies] in GHC.Tc.Solver.Rewrite.

Accompanying changes:

  - Rework how dependencies of 'UnivCo's are tracked, purely for
    performance reasons.

    We now use the 'UnivCoDeps' datatype instead of '[Coercion]', and
    we now insist that 'uco_deps' also contains the CoVars that appear
    in the LHS and RHS types of the UnivCo (as a caching mechanism).

  - Several performance improvements to the rewriter (e.g. increased
    strictness).

  - Improvements to the family application cache, including:
    - Better documentation.
    - Only cache a whole chain of reduction steps.
    - Don't put coercion holes in the cache, fixing #27518.

  - Allow Core Lint to be called from within the typechecker (and from
    within the coercion optimiser).

  - Move out "annotation linting" from Core Lint into the general Core
    pipeline, to avoid the dependency GHC.Core.Coercion.Opt->GHC.Core.Lint
    causing a regression in CountDepsParser due to transitive
    dependencies of GHC.Core.Opt.Monad and GHC.Unit.Module.ModGuts.

Fixes #8095 #27336 #27518

Summary of performance numbers

  Compile-time byte allocated in GHC's testsuite (0 regressions)

          CoOpt_Singletons(normal) ghc/alloc    634,040,332    595,448,728  -6.1% GOOD
           FamAppCachePerf(normal) ghc/alloc    495,629,812    191,612,920 -61.3% GOOD
               LargeRecord(normal) ghc/alloc  1,226,806,434    966,260,976 -21.2% GOOD
             SimplCastPerf(normal) ghc/alloc    160,505,678    148,112,688  -7.7% GOOD
                    T12227(normal) ghc/alloc    244,668,982    233,037,216  -4.8% GOOD
                    T12545(normal) ghc/alloc    697,804,842    424,705,152 -39.1% GOOD
                    T13386(normal) ghc/alloc    422,871,076     35,213,656 -91.7% GOOD
                    T15703(normal) ghc/alloc    288,402,364    273,692,560  -5.1% GOOD
                    T26426(normal) ghc/alloc     86,002,052     78,152,104  -9.1% GOOD
                    T27336(normal) ghc/alloc    498,932,268     35,886,016 -92.8% GOOD
                     T5030(normal) ghc/alloc    117,057,706     88,785,648 -24.2% GOOD
                  T5321Fun(normal) ghc/alloc    252,552,628    243,820,176  -3.5% GOOD
                     T8095(normal) ghc/alloc    768,198,564    127,779,520 -83.4% GOOD
                    T9872a(normal) ghc/alloc  1,655,675,020  1,406,017,008 -15.1% GOOD
                    T9872b(normal) ghc/alloc  1,864,180,628  1,644,706,696 -11.8% GOOD
              T9872b_defer(normal) ghc/alloc  2,834,585,468  1,657,729,760 -41.5% GOOD
                    T9872c(normal) ghc/alloc  1,572,573,116  1,354,055,424 -13.9% GOOD
                    T9872d(normal) ghc/alloc    351,516,004    331,071,424  -5.8% GOOD
      TcPlugin_RewritePerf(normal) ghc/alloc  2,063,065,680  1,847,464,272 -10.5% GOOD

  Chesskell (github.com/Proxymoron461/chesskell/issues)
    Typechecking that the first N plies of kholmovVsBronsteinShort
    are a valid chess game:

      Plies | GHC 9.14 time/alloc | This commit time/alloc |
      -----:|---------------------|------------------------|
      2     | 41s/20GB            | 0.2s/19MB              |
      4     | 95s/49GB            | 0.3s/25MB              |
      6     | timed out           | 0.6s/522MB             |
      8     | timed out           | 7s/673MB               |
      10    | timed out           | timed out              |

  Compilation of singletons-base: compile time -28%, allocations -25%

-------------------------
Metric Decrease:
    CoOpt_Singletons
    FamAppCachePerf
    LargeRecord
    SimplCastPerf
    T12227
    T12545
    T13386
    T15703
    T24984
    T26426
    T27336
    T5030
    T5321Fun
    T8095
    T9872a
    T9872b
    T9872b_defer
    T9872c
    T9872d
    TcPlugin_RewritePerf
-------------------------

Co-Authored-By: Sylvain Henry <sylvain@haskus.fr>

3d326d7d

by Sylvain Henry at 2026-10-18T18:16:09-04:00

JS: fix stack corruption in h$ap_gen_fast's pushAllRegs (#27624)

The loop added in 9e46990976 ("JS: support more than 128 registers
(#26558)") pushed register N to stack[sp - N - 2] while the switch it
falls through into, and every reader of the h$ap_gen frame, uses
stack[sp - (N - 2)]. Registers >= R32 were therefore written 4 slots
too low: the slots the readers consult were left stale and the frame
below was silently corrupted.

Fix the loop to use the same slot formula as the switch, and add a
regression test: an unknown call to a thunk with 35 argument register
slots, which goes through the generic apply.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

6bbca569

by Andreas Klebinger at 2026-10-21T10:01:10+00:00

Fix a few tag inference bugs.

* Treat bottoming functions and nullary join points as functions.
* When generating bytecode don't rely on imported tag sigs.

Fixes #27831, #27841 and #27842.

71808e29

by Sylvain Henry at 2026-10-22T04:50:24-04:00

Add GHC.Internal.Unboxed.Do: do-notation for unboxed values (#27944)

QualifiedDo operators can be representation-polymorphic: a do-block is
expanded into ordinary applications before typechecking, and the only
check on a bound variable is the deferred fixed-runtime-rep check. A
representation-polymorphic (>>=) cannot be defined directly, though,
because its body would bind a variable of unknown representation.

The new module provides such operators for State#-passing code as
methods of a class indexed by RuntimeRep, with one instance per
representation. With optimisation the dictionaries disappear and a
block compiles to the usual nested case expressions:

  Ubx.do { x <- readWordArray# mba 0#     -- x :: Word#
         ; writeWordArray# mba 1# x        -- unit actions are statements
         ; Ubx.pure x }

Also add users guide documentation and tests.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

33247780

by Sylvain Henry at 2026-10-22T04:50:24-04:00

Bignum: use Ubx.do for State#-passing code (#27944)

Rewrite the State#-passing code of the GHC.Internal.Bignum modules
(Primitives, WordArray, BigNat, Natural, Integer, Backend.Native,
Backend.GMP) with the qualified do-notation of GHC.Internal.Unboxed.Do.
Nested case chains such as

    case newWordArray# sz s of { (# s, mwa #) ->
    case act mwa s          of { s ->
    unsafeFreezeByteArray# mwa s }}

become

    Ubx.do
       mwa <- newWordArray# sz
       act mwa
       unsafeFreezeByteArray# mwa

The optimised Core is unchanged (compared per top-level binding).

BigNat now enables MonoLocalBinds: local loops written with Ubx.do
must stay monomorphic in the state type, otherwise GHC does not turn
them into join points.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

593f5f5b

by Sylvain Henry at 2026-10-22T04:51:16-04:00

JS: remove archive padding after wasm objects (#25665)

On Darwin hosts, emar (llvm-ar) writes archives in the Darwin format when
the first member is not a known object file, which is the case for the
archives of the JS backend. This format pads each member to 8 bytes with
'\n' and includes the padding in the member size. Wasm objects do not
accept trailing bytes: wasm-ld failed with "section too large".

The JS linker now removes this padding when it extracts a wasm object from
an archive. In addition, Hadrian passes --format=gnu to emar for the JS
target. See Note [Wasm objects in Darwin archives].

Also document the format of Cc (wasm) objects in Note [Cc object format].

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

13dff5d0

by L0neGamer at 2026-10-22T21:37:31-04:00

add options for resizing bytearrays (#27244)

add five mutable bytearray resizing operations
for various kinds of pinnedness and aligning.

  - prefer unpinned when allocating new array (``resizeMutableByteArrayPreferUnpinned#``; the ``resizeMutableByteArray#`` default)
  - prefer pinned when allocating new array (``resizeMutableByteArrayPreferPinned#``)
  - prefer aligned and pinned when allocating new array (``resizeMutableByteArrayPreferAlignedPinned#``)
  - prefer same as input bytearray when allocating new array (``resizeMutableByteArrayKeepPinnedness#``)
  - ensure the output array is aligned and pinned after operation (``resizeMutableByteArrayMakeAlignedPinned#``)

0c17201c

by Sylvain Henry at 2026-10-22T21:38:23-04:00

JS: implement performMinorGC and performBlockingMajorGC (#28012)

GHC.Internal.System.Mem imports three C entry points: performMajorGC,
performGC (for performMinorGC) and performBlockingMajorGC. The JS RTS
only defined h$performMajorGC, so calling performMinorGC or
performBlockingMajorGC threw "ReferenceError: h$performGC is not
defined" at runtime.

The JS RTS has a single non-generational collector, so both missing
entry points are now aliases of h$performMajorGC.

Add a test that calls every GC entry point of System.Mem.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

7a631ff2

by L0neGamer at 2026-10-23T09:11:39-04:00

Mon(a|oi)d of No (Return|Mappend) progress (!15795)

Make non-canonical definitions of return and mappend
non-suppressible errors, and make canonical definitions
of those methods warnings.

CLC proposal: https://github.com/haskell/core-libraries-committee/issues/328

14656fd4

by Simon Jakobi at 2026-10-23T09:12:27-04:00

Cmm: keep the CBE substitution idempotent

When blocks are merged over multiple rounds during common block
elimination, a block that won a merge in an earlier round can still
lose a merge in a later round. The substitution map (Subst) used to
record this as a chain: the earlier loser mapped to a block that was
itself eliminated. In PLT terminology, the substitution became
*non-idempotent*. Applying such a substitution with a single lookup
left edges to unreachable blocks in the Cmm graph, eventually leading
to a setInfoTableStackMap panic (#27368).

With this patch, each round's fresh substitutions are applied to the old
ones as they are added (extendSubst), so every eliminated block maps
directly to its final replacement and the substitution stays idempotent.

Fixes #27368.

Assisted-by: Claude Fable 5.1

92848e61

by Evgeny Malyshev at 2026-10-23T20:26:04-04:00

hadrian: Include transitive dependencies in Haddock builds

Haddock needs indirect dependency interfaces to resolve references to
definitions outside a package's direct dependencies. Expose Cabal's
configured dependency closure through ContextData and use it alongside
the direct list, preserving direct-interface precedence.

Bump Shake's cache version for the serialized oracle change. Add a
Hadrian selftest requiring template-haskell's indirect ghc-prim interface.

Fixes #25850.
Fixes #27786.

Assisted-By: OpenAI Codex

Metric Increase:
    haddock.Cabal

5e8ae81b

by Zubin Duggal at 2026-10-23T20:26:47-04:00

WinIO: Don't confuse Win32 errors and NTSTATUS codes

WinIO misinterpreted NTSTATUS codes as Win32 errors, and had some ad hoc logic to deal with this.
However, this logic was incomplete and resulted in the mishandling of results of certain I/O operations
(see testcase).

Now the `ErrCode` argument in `CompletionCallback` is always the Win32 error code,
and we convert the NTSTATUS using RtlNtStatusToDosError.

Also when an overlapped/async operation such as ReadFile fails immediately,
with a Win32 error such as ERROR_BROKEN_PIPE, we don't get a further completion
notification, so we never recorded the failure. This worked by coincidence
earlier, because `STATUS_PENDING` happened to have the same numerical code as
`ERROR_NO_MORE_ITEMS`, but it doesn't now that we translate `STATUS_PENDING`
into the correct Win32 error code (`ERROR_IO_PENDING`). Fix this by using
the Win32 error value we obtained when an operation returns `STATUS_PENDING`.

Fixes #27971

c42ed66a

by Martijn Bastiaan at 2026-10-24T16:00:42+02:00

Add single-threaded test for #27526 + #27123

Issue #27526 is a duplicate of #27123, which has already been fixed in
HEAD. The proposed patch in the former carries a "nicer" regression test
in that it doesn't rely on multi-threading to trigger.

Closes #27526

7f73f515

by Simon Peyton Jones at 2026-10-24T17:29:19-04:00

Refactor QuickLook a bit

This MR simply refactors QuickLook a bit.  In particular, it
simplifies `tcApp`, and moves shared code into `finishApp`.

It also adds `Note [QuickLook: arguments before result]`
responding to #26599.

There should be no user-visible changes, except perhaps in error messages,
which improve marginally.

For some reason T24984 compiles 8% faster. I'll take the win!

Metric Decrease:
    T24984

a4a3a964

by Alan Zimmerman at 2026-10-24T17:30:03-04:00

EPA: Use AnnList for XClosedTypeFamily

We now have TTG annotations for FamilyInfo.

Make use of it to store the AnnList annotations for a closed type
family.

953e4c7f

by sheaf at 2026-10-26T18:19:10-04:00

Rework LogQueue to be a proper queue

Now 'LogQueue' uses a 'TQueue' with a separate 'TVar' to indicate
whether the log queue is finished logging, instead of the previous weird
data representation using a reverse-order '[Maybe Msg]' with 'Nothing'
denoting the end of the queue.

ec29c5fa

by Rodrigo Mesquita at 2026-10-26T18:19:10-04:00

Refactor GHC.Driver.MakeAction

Pull out of 'runParPipelines' the logic for creating a 'MakeEnv' ready
to be used by multiple threads, as that will be useful for downsweep as
well (which doesn't fit the 'runPipelines' flow), rather than being just
for upsweep.

The code is moved and re-structured to match the export list, simplify
the sequentiality checks previously both in 'runPipelines' and
'runAllPipelines', which were weirdly similar and confusing; into the
two part step where we need these checks: (1) to construct the MakeEnv,
(2) to run the MakeActions in parallel. These two steps are separate and
used to be too mixed up.

Some additional little simplifications or clean ups here and there.

3fb22073

by Rodrigo Mesquita at 2026-10-26T18:19:11-04:00

Refactor GHC.Driver.Downsweep in preparation for parallel downsweep

Pure refactor to improve the code to facilitate implementing parallel
downsweep in the next commit.

This commit puts a MakeEnv into the DownsweepEnv, gives the fields
proper names and uses RecordWildcards to simplify, rather than passing
around all diagnostic wrappers, driver-message-things and using 10s of
positional fields.

No behavior changes here!

52f385a7