GitLab

sheaf pushed to branch wip/andreask/ticked_joins at Glasgow Haskell Compiler / GHC

Commits:

41bdb16f
by Andreas Klebinger at 2025-10-06T18:04:34-04:00
```
Add a perf test for #26425
```

1da0c700

by Andreas Klebinger at 2025-10-06T18:05:14-04:00

Testsuite: Silence warnings about Wx-partial in concprog001

7471eb6a

by sheaf at 2025-10-07T21:39:43-04:00

Improve how we detect user type errors in types

This commit cleans up all the code responsible for detecting whether a
type contains "TypeError msg" applications nested inside it. All the
logic is now in 'userTypeError_maybe', which is always deep. Whether
it looks inside type family applications is determined by the passed-in
boolean flag:

  - When deciding whether a constraint is definitely insoluble, don't
    look inside type family applications, as they may still reduce -- in
    which case the TypeError could disappear.
  - When reporting unsolved constraints, look inside type family
    applications: they had the chance to reduce but didn't, and the
    custom type error might contain valuable information.

All the details are explained in Note [Custom type errors in constraints]
in GHC.Tc.Types.Constraint.

Another benefit of this change is that it allows us to get rid of the
deeply dodgy 'getUserTypeErrorMsg' function.

This commit also improves the detection of custom type errors, for
example in equality constraints:

  TypeError blah ~# rhs

It used to be the case that we didn't detect the TypeError on the LHS,
because we never considered that equality constraints could be insoluble
due to the presence of custom type errors. Addressing this oversight
improves detection of redundant pattern match warnings, fixing #26400.

29955267

by Rodrigo Mesquita at 2025-10-07T21:40:25-04:00

cleanup: Drop obsolete settings from config.mk.in

These values used to be spliced into the bindist's `config.mk` s.t. when
`make` was run, the values were read and written into the bindist installation `settings` file.

However, we now carry these values to the bindist directly in the
default.target toolchain file, and `make` writes almost nothing to
`settings` now (see #26227)

The entries deleted in this MR were already unused.

Fixes #26478

f7adfed2

by ARATA Mizuki at 2025-10-08T08:37:24-04:00

T22033 is only relevant if the word size is 64-bit

Fixes #25497

ff1650c9

by Ben Gamari at 2025-10-08T08:38:07-04:00

rts/posix: Enforce iteration limit on heap reservation logic

Previously we could loop indefinitely when attempting to get an address
space reservation for our heap. Limit the logic to 8 iterations to
ensure we instead issue a reasonable error message.

Addresses #26151.

01844557

by Ben Gamari at 2025-10-08T08:38:07-04:00

rts/posix: Hold on to low reservations when reserving heap

Previously when the OS gave us an address space reservation in low
memory we would immediately release it and try again. However, on some
platforms this meant that we would get the same allocation again in the
next iteration (since mmap's `hint` argument is just that, a hint).

Instead we now hold on to low reservations until we have found a
suitable heap reservation.

Fixes #26151.

b2c8d052

by Sven Tennie at 2025-10-08T08:38:47-04:00

Build terminfo only in upper stages in cross-builds (#26288)

Currently, there's no way to provide library paths for [n]curses for
both - build and target - in cross-builds. As stage0 is only used to
build upper stages, it should be fine to build terminfo only for them.

This re-enables building cross-compilers with terminfo.

c58f9a61

by Julian Ospald at 2025-10-08T08:39:36-04:00

ghc-toolchain: Drop `ld.gold` from merge object command

It's deprecated.

Also see #25716

2b8baada

by sheaf at 2025-10-08T18:23:37-04:00

Improvements to 'mayLookIdentical'

This commit makes significant improvements to the machinery that decides
when we should pretty-print the "invisible bits" of a type, such as:

  - kind applications, e.g. '@k' in 'Proxy @k ty'
  - RuntimeReps, e.g. 'TYPE r'
  - multiplicities and linear arrows 'a %1 -> b'

To do this, this commit refactors 'mayLookIdentical' to return **which**
of the invisible bits don't match up, e.g. in

  (a %1 -> b) ~ (a %Many -> b)

we find that the invisible bit that doesn't match up is a multiplicity,
so we should set 'sdocLinearTypes = True' when pretty-printing, and with
e.g.

  Proxy @k1 ~ Proxy @k2

we find that the invisible bit that doesn't match up is an invisible
TyCon argument, so we set 'sdocPrintExplicitKinds = True'.

We leverage these changes to remove the ad-hoc treatment of linearity
of data constructors with 'dataConDisplayType' and 'dataConNonLinearType'.
This is now handled by the machinery of 'pprWithInvisibleBits'.

Fixes #26335 #26340

129ce32d

by sheaf at 2025-10-08T18:23:37-04:00

Store SDoc context in SourceError

This commits modifies the SourceError datatype which is used for
throwing and then reporting exceptions by storing all the info we need
to be able to print the SDoc, including whether we should print with
explicit kinds, explicit runtime-reps, etc.

This is done using the new datatype:

  data SourceErrorContext
    = SEC
        !DiagOpts
        !(DiagnosticOpts GhcMessage)

Now, when we come to report an error (by handling the exception), we
have access to the full context we need.

Fixes #26387

f9790ca8

by Ben Gamari at 2025-10-08T18:24:19-04:00

gitlab-ci: Make RELEASE_JOB an input

Rather than an undocumented variable.

14281a22
by Ben Gamari at 2025-10-11T14:06:47-04:00
```
rts/nonmoving: Fix comment spelling
```

bedd38b0

by Ben Gamari at 2025-10-11T14:06:47-04:00

rts/nonmoving: Use atomic operations to update bd->flags

215d6841

by Ben Gamari at 2025-10-11T14:06:47-04:00

nonmoving: Use get_itbl instead of explicit loads

This is cleaner and also fixes unnecessary (and unsound) use of
`volatile`.

2c94aa3a

by Ben Gamari at 2025-10-11T14:06:47-04:00

rts/Scav: Handle WHITEHOLEs in scavenge_one

`scavenge_one`, used to scavenge mutable list entries, may encounter
`WHITEHOLE`s when the non-moving GC is in use via two paths:

  1. when an MVAR is being marked concurrently
  2. when the object belongs to a chain of selectors being short-cutted.

Fixes #26204.

6bd8155c

by Matthew Pickering at 2025-10-11T14:07:29-04:00

Add support for generating bytecode objects

This commit adds the `-fwrite-byte-code` option which makes GHC emit a
`.gbc` file which contains a serialised representation of bytecode.

The bytecode can be loaded by the compiler to avoid having to
reinterpret a module when using the bytecode interpreter (for example,
in GHCi).

There are also the new options:

* -gbcdir=<DIR>: Specify the directory to place the gbc files
* -gbcsuf=<suffix>: Specify the suffix for gbc files

The option `-fbyte-code-and-object-code` now implies
`-fwrite-byte-code`.

These performance tests fail due to https://github.com/haskell/directory/issues/204

-------------------------
Metric Increase:
    MultiComponentModules
    MultiLayerModules
    MultiComponentModulesRecomp
    MultiLayerModulesRecomp
    MultiLayerModulesTH_Make
    MultiLayerModulesTH_OneShot
    T13701
-------------------------

The bytecode serialisation part was implemented by Cheng Shao

Co-authored-by: Cheng Shao <terrorjack@type.dance>

dc8f9599

by Matthew Pickering at 2025-10-11T14:07:30-04:00

Revert "Add a perf test for #26425"

This test has a large memory spike currently, which makes the test
sensitive, since if you allocate a little more or less, the precise
location where GC happens shifts and you observe a different part of the
spike.

Andreas told me to revert the patch for now, and he will add it back
when he fixes the memory spike.

This reverts commit 41bdb16fd083110a06507248f648c507a2feb4af.

e10dcd65

by Sven Tennie at 2025-10-12T10:24:56+00:00

T22859: Increase threadDelay for small machines

The previously used thread delay led to failures on my RISC-V test
setups.

d59ef6b6

by Hai / @BestYeen at 2025-10-14T21:51:14-04:00

Change Alex and Happy m4 scripts to display which version was found in the system, adapt small formatting details in Happy script to be more like the Alex script again.

c98abb6a

by Hai / @BestYeen at 2025-10-14T21:52:08-04:00

Update occurrences of return to pure and add a sample for redefining :m to mean :main

70ee825a

by Cheng Shao at 2025-10-14T21:52:50-04:00

testsuite: fix T3586 for non-SSE3 platforms

`T3586.hs` contains `-fvia-C -optc-msse3` which I think is a
best-effort basis to harvest the C compiler's auto vectorization
optimizations via the C backend back when the test was added. The
`-fvia-C` part is now a deprecated no-op because GHC can't fall back
to the C backend on a non-unregisterised build, and `-optc-msse3`
might actually cause the test to fail on non x86/x64 platforms, e.g.
recent builds of wasi-sdk would report `wasm32-wasi-clang: error:
unsupported option '-msse3' for target 'wasm32-unknown-wasi'`.

So this patch cleans up this historical cruft. `-fvia-C` is removed,
and `-optc-msse3` is only passed when cpuid contains `pni` (which
indicates support of SSE3).

4be32153

by Teo Camarasu at 2025-10-15T08:06:09-04:00

Add submodules for template-haskell-lift and template-haskell-quasiquoter

These two new boot libraries expose stable subsets of the
template-haskell interface.

This is an implemenation of the GHC proposal https://github.com/ghc-proposals/ghc-proposals/pull/696

Work towards #25262

0c00c9c3

by Ben Gamari at 2025-10-15T08:06:51-04:00

rts: Eliminate uses of implicit constant arrays

Folding of `const`-sized variable-length arrays to a constant-length
array is a gnu extension which clang complains about.

Closes #26502.

bf902a1d

by Fendor at 2025-10-15T16:00:59-04:00

Refactor distinct constructor tables map construction

Adds `GHC.Types.Unique.FM.alterUFM_L`, `GHC.Types.Unique.DFM.alterUDFM_L`
`GHC.Data.Word64Map.alterLookup` to support fusion of distinct
constructor data insertion and lookup during the construction of the `DataCon`
map in `GHC.Stg.Debug.numberDataCon`.

Co-authored-by: Fendor <fendor@posteo.de>
Co-authored-by: Finley McIlwaine <finleymcilwaine@gmail.com>

b3585ba1

by Fendor at 2025-10-15T16:00:59-04:00

Allow per constructor refinement of distinct-constructor-tables

Introduce `-fno-distinct-constructor-tables`. A distinct constructor table
configuration is built from the combination of flags given, in order. For
example, to only generate distinct constructor tables for a few specific
constructors and no others, just pass
`-fdistinct-constructor-tables-only=C1,...,CN`.
This flag can be supplied multiple times to extend the set of
constructors to generate a distinct info table for.

You can disable generation of distinct constructor tables for all
configurations by passing `-fno-distinct-constructor-tables`.

The various configurations of these flags is included in the `DynFlags`
fingerprints, which should result in the expected recompilation logic.

Adds a test that checks for distinct tables for various given or omitted
constructors.

Updates CountDepsAst and CountDepsParser tests to account for new dependencies.

Fixes #23703

Co-authored-by: Fendor <fendor@posteo.de>
Co-authored-by: Finley McIlwaine <finleymcilwaine@gmail.com>

e17dc695

by fendor at 2025-10-15T16:01:41-04:00

Fix typos in haddock documentation for stack annotation API

f85058d3

by Zubin Duggal at 2025-10-17T13:50:52+05:30

compiler: Attempt to systematize Unique tags by introducing an ADT for each different tag

Fixes #26264

Metric Decrease:
    T9233

c85c845d

by sheaf at 2025-10-17T22:35:32-04:00

Don't prematurely final-zonk PatSyn declarations

This commit makes GHC hold off on the final zonk for pattern synonym
declarations, in 'GHC.Tc.TyCl.PatSyn.tc_patsyn_finish'.
This accommodates the fact that pattern synonym declarations without a
type signature can contain unfilled metavariables, e.g. if the RHS of
the pattern synonym involves view-patterns whose type mentions promoted
(level 0) metavariables. Just like we do for ordinary function bindings,
we should allow these metavariables to be settled later, instead of
eagerly performing a final zonk-to-type.

Now, the final zonking-to-type for pattern synonyms is performed in
GHC.Tc.Module.zonkTcGblEnv.

Fixes #26465

ba3e5bdd

by Rodrigo Mesquita at 2025-10-18T16:57:18-04:00

Move code-gen aux symbols from ghc-internal to rts

These symbols were all previously defined in ghc-internal and made the
dependency structure awkward, where the rts may refer to some of these
symbols and had to work around that circular dependency the way
described in #26166.

Moreover, the code generator will produce code that uses these symbols!
Therefore, they should be available in the rts:

PRINCIPLE: If the code generator may produce code which uses this
symbol, then it should be defined in the rts rather than, say,
ghc-internal.

That said, the main motivation is towards fixing #26166.

Towards #26166. Pre-requisite of !14892

f31de2a9

by Ben Gamari at 2025-10-18T16:57:18-04:00

rts: Avoid static symbol references to ghc-internal

This resolves #26166, a bug due to new constraints placed by Apple's
linker on undefined references.

One source of such references in the RTS is the many symbols referenced
in ghc-internal. To mitigate #26166, we make these references dynamic,
as described in Note [RTS/ghc-internal interface].

Fixes #26166

Co-authored-by: Rodrigo Mesquita <rodrigo.m.mesquita@gmail.com>
Co-authored-by: Cheng Shao <terrorjack@type.dance>

43fdfddc

by Ben Gamari at 2025-10-18T16:57:18-04:00

compiler: Rename isMathFun -> isLibcFun

This set includes more than just math functions.

4ed5138f

by Ben Gamari at 2025-10-18T16:57:18-04:00

compiler: Add libc allocator functions to libc_funs

Prototypes for these are now visible from `Prim.h`, resulting in
multiple-declaration warnings in the unregisterised job.

9a0a076b

by Ben Gamari at 2025-10-18T16:57:18-04:00

rts: Minimize header dependencies of Prim.h

Otherwise we will end up with redundant and incompatible declarations
resulting in warnings during the unregisterised build.

26b8a414

by Diego Antonio Rosario Palomino at 2025-10-18T16:58:10-04:00

Cmm Parser: Fix incorrect example in comment

The Parser.y file contains a comment with an incorrect example of textual
Cmm (used in .cmm files). This commit updates the comment to ensure it
reflects valid textual Cmm syntax.

Fixes #26313

d4a9d6d6

by ARATA Mizuki at 2025-10-19T18:43:47+09:00

Handle implications between x86 feature flags

This includes:

* Multiple -msse* options can be specified
* -mavx implies -msse4.2
* -mavx2 implies -mavx
* -mfma implies -mavx
* -mavx512f implies -mavx2 and -mfma
* -mavx512{cd,er,pf} imply -mavx512f

Closes #24989

Co-authored-by: sheaf <sam.derbyshire@gmail.com>

c9b8465c

by Cheng Shao at 2025-10-20T10:16:00-04:00

wasm: workaround WebKit bug in dyld

This patch works around a WebKit bug and allows dyld to run on WebKit
based platforms as well. See added note for detailed explanation.

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

91b6be10

by Julian Ospald at 2025-10-20T18:21:03-04:00

Improve error handling in 'getPackageArchives'

When the library dirs in the package conf files are not set up correctly,
the JS linker will happily ignore such packages and not link against them,
although they're part of the link plan.

Fixes #26383

6c5269da

by Sven Tennie at 2025-10-20T18:21:44-04:00

Align coding style

Improve readability by using the same style for all constructor calls in
this function.

3d305889

by Sven Tennie at 2025-10-20T18:21:44-04:00

Reduce complexity by removing joins with mempty

ldArgs, cArgs and cppArgs are all `mempty`. Thus concatenating them adds
nothing but some complexity while reading the code.

38d65187

by Matthew Pickering at 2025-10-21T13:12:20+01:00

Fix stack decoding when using profiled runtime

There are three fixes in this commit.

* We need to replicate the `InfoTable` and `InfoTableProf`
approach for the other stack constants (see the new Stack.ConstantsProf
file).

* Then we need to appropiately import the profiled or non-profiled
  versions.

* Finally, there was an incorrect addition in `stackFrameSize`. We need
  to cast after performing addition on words.

Fixes #26507

17231bfb
by fendor at 2025-10-21T13:12:20+01:00
```
Add regression test for #26507
```

4f5bf93b

by Simon Peyton Jones at 2025-10-25T04:05:34-04:00

Postscript to fix for #26255

This MR has comments only

6ef22fa0

by IC Rainbow at 2025-10-26T18:23:01-04:00

Add SIMD primops for bitwise logical operations

This adds 128-bit wide and/or/xor instructions for X86 NCG,
with both SSE and AVX encodings.

```
andFloatX4# :: FloatX4# -> FloatX4# -> FloatX4#     -- andps / vandps
andDoubleX2# :: DoubleX2# -> DoubleX2# -> DoubleX2# -- andpd / vandpd
andInt8X16# :: Int8X16# -> Int8X16# -> Int8X16#     -- pand / vpand
```

The new primops are available on ARM when using LLVM backend.

Tests added:
- simd015 (floats and doubles)
- simd016 (integers)
- simd017 (words)

Fixes #26417

fbdc623a

by sheaf at 2025-10-26T18:23:52-04:00

Add hints for unsolved HasField constraints

This commit adds hints and explanations for unsolved 'HasField'
constraints.

GHC will now provide additional explanations for an unsolved constraint
of the form 'HasField fld_name rec_ty fld_ty'; the details are laid out in
Note [Error messages for unsolved HasField constraints], but briefly:

  1. Provide similar name suggestions (e.g. mis-spelled field name)
     and import suggestions (record field not in scope).
     These result in actionable 'GhcHints', which is helpful to provide
     code actions in HLS.

  2. Explain why GHC did not solve the constraint, e.g.:

    - 'fld_name' is not a string literal (e.g. a type variable)
    - 'rec_ty' is a TyCon without any fields, e.g. 'Int' or 'Bool'.
    - 'fld_ty' contains existentials variables or foralls.
    - The record field is a pattern synonym field (GHC does not generate
      HasField instances for those).
    - 'HasField' is a custom 'TyCon', not actually the built-in
      'HasField' typeclass from 'GHC.Records'.

On the way, we slightly refactor the mechanisms for import suggestions
in GHC.Rename.Unbound. This is to account for the fact that, for
'HasField', we don't care whether the field is imported qualified or
unqualified. 'importSuggestions' was refactored, we now have
'sameQualImportSuggestions' and 'anyQualImportSuggestions'.

Fixes #18776 #22382 #26480

99d5707f

by sheaf at 2025-10-26T18:23:52-04:00

Rename PatSyn MatchContext to PatSynCtx to avoid punning

5dc2e9ea
by Julian Ospald at 2025-10-27T18:17:23-04:00
```
Skip uniques test if sources are not available
```

544b9ec9

by Vladislav Zavialov at 2025-10-27T18:18:06-04:00

Re-export GHC.Hs.Basic from GHC.Hs

Clean up some import sections in GHC by re-exporting GHC.Hs.Basic
from GHC.Hs.

643ce801

by Julian Ospald at 2025-10-28T18:18:55-04:00

rts: remove unneccesary cabal flags

We perform those checks via proper autoconf macros
instead that do the right thing and then add those
libs to the rts buildinfo.

d69ea8fe

by Vladislav Zavialov at 2025-10-28T18:19:37-04:00

Test case for #17705

Starting with GHC 9.12 (the first release to include 5745dbd3),
all examples in this ticket are handled as expected.

4038a28b
by Andreas Klebinger at 2025-10-30T12:38:52-04:00
```
Add a perf test for #26425
```

f997618e

by Andreas Klebinger at 2025-10-30T12:38:52-04:00

OccAnal: Be stricter for better compiler perf.

In particular we are now stricter:

* When combining usageDetails.
* When computing binder info.

In combineUsageDetails when combining the underlying adds we compute a
new `LocalOcc` for each entry by combining the two existing ones.

Rather than wait for those entries to be forced down the road we now
force them immediately. Speeding up T26425 by about 10% with little
effect on the common case.

We also force binders we put into the Core AST everywhere now.
Failure to do so risks leaking the occ env used to set the binders
OccInfo.

For T26425 compiler residency went down by a factor of ~10x.

Compile time also improved by a factor of ~1.6.

-------------------------
Metric Decrease:
    T18698a
    T26425
    T9233
-------------------------

5618645b

by Vladislav Zavialov at 2025-10-30T12:39:33-04:00

Fix namespace specifiers in subordinate exports (#12488)

This patch fixes an oversight in the `lookupChildrenExport` function that
caused explicit namespace specifiers of subordinate export items to be
ignored:

  module M (T (type A)) where   -- should be rejected
  data T = A

Based on the `IEWrappedName` data type, there are 5 cases to consider:
1. Unadorned name:   P(X)
2. Named default:    P(default X)
3. Pattern synonym:  P(pattern X)
4. Type name:        P(type X)
5. Data name:        P(data X)

Case 1 is already handled correctly; cases 2 and 3 are parse errors; and
it is cases 4 and 5 that we are concerned with in this patch.

Following the precedent established in `LookupExactName`, we introduce
a boolean flag in `LookupChildren` to control whether to look up in all
namespaces or in a specific one. If an export item is accompanied by an
explicit namespace specifier `type` or `data`, we restrict the lookup in
`lookupGRE` to a specific namespace.

The newly introduced diagnostic `TcRnExportedSubordinateNotFound`
provides error messages and suggestions more tailored to this context
than the previously used `reportUnboundName`.

f75ab223

by Peter Trommler at 2025-10-31T18:43:13-04:00

ghc-toolchain: detect PowerPC 64 bit ABI

Check preprocessor macro defined for ABI v2 and assume v1 otherwise.
Fixes #26521

d086c474
by Peter Trommler at 2025-10-31T18:43:13-04:00
```
ghc-toolchain: refactor, move lastLine to Utils
```

995dfe0d

by Vladislav Zavialov at 2025-10-31T18:43:54-04:00

Tests for -Wduplicate-exports, -Wdodgy-exports

Add test cases for the previously untested diagnostics:

  [GHC-51876] TcRnDupeModuleExport
  [GHC-64649] TcRnNullExportedModule

This also revealed a typo (incorrect capitalization of "module") in the
warning text for TcRnDupeModuleExport, which is now fixed.

f6961b02

by Cheng Shao at 2025-11-01T00:08:01+01:00

wasm: reformat dyld source code

This commit reformats dyld source code with prettier, to avoid
introducing unnecessary diffs in subsequent patches when they're
formatted before committing.

0c9032a0

by Cheng Shao at 2025-11-01T00:08:01+01:00

wasm: simplify _initialize logic in dyld

This commit simplifies how we _initialize a wasm shared library in
dyld and removes special treatment for libc.so, see added comment for
detailed explanation.

ec1b40bd

by Cheng Shao at 2025-11-01T00:08:01+01:00

wasm: support running dyld fully client side in the browser

This commit refactors the wasm dyld script so that it can be used to
load and run wasm shared libraries fully client-side in the browser
without needing a wasm32-wasi-ghci backend:

- A new `DyLDBrowserHost` class is exported, which runs in the browser
  and uses the in-memory vfs without any RPC calls. This meant to be
  used to create a `rpc` object for the fully client side use cases.
- The exported `main` function now can be used to load user-specified
  shared libraries, and the user can use the returned `DyLD` instance
  to run their own exported Haskell functions.
- The in-browser wasi implementation is switched to
  https://github.com/haskell-wasm/browser_wasi_shim for bugfixes and
  major performance improvements not landed upstream yet.
- When being run by deno, it now correctly switches to non-nodejs code
  paths, so it's more convenient to test dyld logic with deno.

See added comments for details, as well as the added `playground001`
test case for an example of using it to build an in-browser Haskell
playground.

8f3e481f

by Cheng Shao at 2025-11-01T00:08:01+01:00

testsuite: add playground001 to test haskell playground

This commit adds the playground001 test case to test the haskell
playground in browser, see comments for details.

af40606a

by Cheng Shao at 2025-11-01T00:08:04+01:00

Revert "testsuite: add T26431 test case"

This reverts commit 695036686f8c6d78611edf3ed627608d94def6b7. T26431
is now retired, wasm ghc internal-interpreter logic is tested by
playground001.

86c82745

by Vladislav Zavialov at 2025-11-01T07:24:29-04:00

Supplant TcRnExportHiddenComponents with TcRnDodgyExports (#26534)

Remove a bogus special case in lookup_ie_kids_all,
making TcRnExportHiddenComponents obsolete.

fcf6331e

by Richard Eisenberg at 2025-11-03T08:33:05+00:00

Refactor fundep solving

This commit is a large-scale refactor of the increasingly-messy code that
handles functional dependencies.  It has virtually no effect on what compiles
but improves error messages a bit.  And it does the groundwork for #23162.

The big picture is described in
    Note [Overview of functional dependencies in type inference]
in GHC.Tc.Solver.FunDeps

* New module GHC.Tc.Solver.FunDeps contains all the fundep-handling
  code for the constraint solver.

* Fundep-equalities are solved in a nested scope; they may generate
  unifications but otherwise have no other effect.
  See GHC.Tc.Solver.FunDeps.solveFunDeps

  The nested needs to start from the Givens in the inert set, but
  not the Wanteds; hence a new function `resetInertCans`, used in
  `nestFunDepsTcS`.

* That in turn means that fundep equalities never show up in error
  messages, so the complicated FunDepOrigin tracking can all disappear.

* We need to be careful about tracking unifications, so we kick out
  constraints from the inert set after doing unifications.  Unification
  tracking has been majorly reformed: see Note [WhatUnifications] in
  GHC.Tc.Utils.Unify.

  A good consequence is that the hard-to-grok `resetUnificationFlag`
  has been replaced with a simpler use of
  `reportCoarseGrainUnifications`

Smaller things:

* Rename `FunDepEqn` to `FunDepEqns` since it contains multiple
  type equalities.

Some compile time improvement

Metrics: compile_time/bytes allocated
                          Baseline
                  Test       value      New value Change
---------------------- --------------------------------------
         T5030(normal) 173,839,232    148,115,248 -14.8% GOOD
hard_hole_fits(normal) 286,768,048    284,015,416  -1.0%

             geo. mean                             -0.2%
             minimum                              -14.8%
             maximum                               +0.3%

Metric Decrease:
    T5030

231adc30