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1b3d25dd
by Simon Peyton Jones at 2025-07-21T10:51:20+01:00
Improve treatment of SPECIALISE pragmas -- again!
This MR does another major refactor of the way that SPECIALISE
pragmas work, to fix #26115, #26116, #26117.
* We now /always/ solve forall-constraints in an all-or-nothing way.
See Note [Solving a Wanted forall-constraint] in GHC.Tc.Solver.Solve
This means we might have unsolved quantified constraints, which need
to be reported. See `inert_insts` in `getUnsolvedInerts`.
* I refactored the short-cut solver for type classes to work by
recursively calling the solver rather than by having a little baby
solver that kept being not clever enough.
See Note [Shortcut solving] in GHC.Tc.Solver.Dict
* I totally rewrote the desugaring of SPECIALISE pragmas, again.
The new story is in Note [Desugaring new-form SPECIALISE pragmas]
in GHC.HsToCore.Binds
Both old-form and new-form SPECIALISE pragmas now route through the same
function `dsSpec_help`. The tricky function `decomposeRuleLhs` is now used only
for user-written RULES, not for SPECIALISE pragmas.
* I improved `solveOneFromTheOther` to account for rewriter sets. Previously
it would solve a non-rewritten dict from a rewritten one. For equalities
we were already dealing with this, in
Some incidental refactoring
* A small refactor: `ebv_tcvs` in `EvBindsBar` now has a list of coercions, rather
than a set of tyvars. We just delay taking the free vars.
* GHC.Core.FVs.exprFVs now returns /all/ free vars.
Use `exprLocalFVs` for Local vars.
Reason: I wanted another variant for /evidence/ variables.
* Ues `EvId` in preference to `EvVar`. (Evidence variables are always Ids.)
Rename `isEvVar` to `isEvId`.
* I moved `inert_safehask` out of `InertCans` and into `InertSet` where it
more properly belongs.
Compiler-perf changes:
* There was a palpable bug (#26117) which this MR fixes in
newWantedEvVar, which bypassed all the subtle overlapping-Given
and shortcutting logic. (See the new `newWantedEvVar`.) Fixing this
but leads to extra dictionary bindings; they are optimised away quickly
but they made CoOpt_Read allocate 3.6% more.
* Hpapily T15164 improves.
* The net compiler-allocation change is 0.0%
Metric Decrease:
T15164
Metric Increase:
CoOpt_Read
T12425
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bdc68a92
by Simon Peyton Jones at 2025-07-21T10:54:13+01:00
Solve forall-constraints immediately, or not at all
Triggered by the new short-cut solver, I realised that it is nicer to solve
forall-constraints immediately, rather than emitting an implication constraint
to be solved later.
This is an un-forced refactoring, but it saves quite a bit of plumbing; e.g
- The `wl_implics` field of `WorkList` is gone,
- The types of `solveSimpleWanteds` and friends are simplified.
- An EvFun contains binding, rather than an EvBindsVar ref-cell that
will in the future contain bindings. That makes `evVarsOfTerm`
simpler
It also improves error messages a bit.
One tiresome point: in the tricky case of `inferConstraintsCoerceBased`
we make a forall-constraint. This we /do/ want to partially solve, so
we can infer a suitable context. (I'd be quite happy to force the user to
write a context, bt I don't want to change behavior.) So we want to generate
an /implication/ constraint in `emitPredSpecConstraints` rather than a
/forall-constraint/ as we were doing before.
Incidental refactoring
* `GHC.Tc.Deriv.Infer.inferConstraints` was consulting the state monad for
the DerivEnv that the caller had just consulted. Nicer to pass it as an
argument I think, so I have done that. No change in behaviour.
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521af58c
by Simon Peyton Jones at 2025-07-21T10:54:13+01:00
Remove duplicated code in Ast.hs for evTermFreeVars
This is just a tidy up.
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d433cb52
by Simon Peyton Jones at 2025-07-21T10:54:13+01:00
Small tc-tracing changes only