Alan Zimmerman pushed to branch wip/az/exactprint-annotation-rationalisation at Glasgow Haskell Compiler / GHC Commits: 67d41299 by Sebastian Graf at 2026-06-18T05:18:24-04:00 Desugar a `case` scrutinee only once (#27383, #20251) In `dsExpr` for `HsCase` we desugared the scrutinee /twice/: once to build the Core `case` itself, and again inside `matchWrapper`, which re-desugared the source scrutinee (via `addHsScrutTmCs`) purely to record long-distance information for the pattern-match checker. For a single `case` that is merely wasteful. But for nested cases it is catastrophic. Consider case (case (case e of ... ) of ... ) of ... Desugaring the outer scrutinee desugars the middle `case` twice, each of which desugars the inner `case` twice, and so on. The work doubles at every level, so desugaring takes O(2^n) time in the nesting depth. That is the blowup reported in #27383; it is also what makes the machine-generated program in #20251 take an age to compile. The fix is simple. `matchWrapper` is handed the scrutinee anyway, so we give it the Core expression we have /already/ desugared, and record the long-distance term constraint with `addCoreScrutTmCs` instead of re-desugaring from source. This is just what `matchSinglePatVar` already does for single-pattern matches. So: * `matchWrapper` now takes `Maybe [CoreExpr]` rather than `Maybe [LHsExpr GhcTc]`. * The `HsCase` equation of `dsExpr` passes the already-desugared `core_discrim`; the arrow desugarer passes its match variables. * `addHsScrutTmCs` had no other use, so it is gone. Desugaring is now linear in the nesting depth. (The coverage checker still runs `simpleOptExpr` over each scrutinee, which leaves the total at O(n^2); that is ample.) The long-distance information itself is unchanged: the checker sees precisely the Core that backs the generated code. Test: deSugar/should_compile/T27383 - - - - - fa5defde by Rodrigo Mesquita at 2026-06-18T05:19:11-04:00 fix: Save FastStrings in the PMC There is no point in adding the unique to the occurrence FastString we create, since it is part of the Id anyway. Adding it to the FastString, meant each FastString was unique unnecessarily! In a separate branch, running the compiler on test `InstanceMatching` observed 30000 `FastString`s created by this code path. Plus, `fsLit "pm"` follows the existing pattern in `mkPmId`. - - - - - 4efb4a66 by Alan Zimmerman at 2026-06-18T14:41:14-04:00 TTG: Add extension points to HsConDetails Extend HsConDetails as data HsConDetails p arg rec = PrefixCon !(XPrefixCon p) [arg] -- C @t1 @t2 p1 p2 p3 | RecCon !(XRecCon p) rec -- C { x = p1, y = p2 } | InfixCon !(XInfixCon p) arg arg -- p1 `C` p2 | XHsConDetails !(XXHsConDetails p) type family XPrefixCon p type family XRecCon p type family XInfixCon p type family XXHsConDetails p - - - - - c8d27dd4 by Simon Jakobi at 2026-06-18T14:41:59-04:00 CI: quiet submodule clean output in after_script and setup The clean and cleanup_submodules functions ran 'git submodule foreach git clean -xdf', flooding the job log with 'Entering ...' and 'Removing ...' lines. Pass --quiet to 'git submodule' and -q to 'git clean' to drop the success output; errors are still reported. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com> - - - - - 91f24930 by Alan Zimmerman at 2026-06-18T22:20:59+01:00 EPA: remove LocatedL / SrcSpanAnnL and LocatedLI / SrcSpanAnnLI This is part of a refactor towards only having LocatedA / SrcSpanAnnA It removes the stated items, but has to add back one for BooleanFormula, LocatedBF / SrcSpanAnnBF This commit also use the HsConDetails RecCon extension point to capture the braces in a record constructor - - - - - e5091eb3 by Alan Zimmerman at 2026-06-18T22:23:28+01:00 EPA: Remove LocatedC / SrcSpanAnnC Used for contexts - - - - - 063683fd by Alan Zimmerman at 2026-06-18T22:23:28+01:00 EPA: Harmonise HsQual/HsQualTy TTG extension annotations - - - - - 09cebf4b by Alan Zimmerman at 2026-06-18T22:26:56+01:00 EPA Remove LocatedLC / LocatedLS LocatedLC/LocatedLS were unused - - - - - 32e1e58c by Alan Zimmerman at 2026-06-18T22:26:56+01:00 EPA: Remove LocatedLW from LStmtLR - - - - - 17f74912 by Alan Zimmerman at 2026-06-18T22:26:56+01:00 EPA: Remove LocatedLW from MatchGroup This is the last usage of LocatedLW / SrcSpanAnnLW - - - - - d87c5fd7 by Alan Zimmerman at 2026-06-18T22:26:56+01:00 EPA: Move the 'where' annotation for PatSynBind This allows us to move it out of the MatchGroup exact print annotation too - - - - - db6b172a by Alan Zimmerman at 2026-06-18T22:26:56+01:00 EPA: Replace AnnListItem with simply [TrailingAnn] Remove the unnecessary wrapper around a single field. - - - - - 342e27ce by Alan Zimmerman at 2026-06-18T22:26:56+01:00 Keep binds and sigs together in HsValBindsLR TBD - - - - - 113 changed files: - .gitlab/ci.sh - + changelog.d/fix-exponential-case-desugar-27383 - compiler/GHC/Data/BooleanFormula.hs - compiler/GHC/Hs.hs - compiler/GHC/Hs/Binds.hs - compiler/GHC/Hs/Decls.hs - compiler/GHC/Hs/Dump.hs - compiler/GHC/Hs/Expr.hs - compiler/GHC/Hs/ImpExp.hs - compiler/GHC/Hs/Instances.hs - compiler/GHC/Hs/Pat.hs - compiler/GHC/Hs/Stats.hs - compiler/GHC/Hs/Type.hs - compiler/GHC/Hs/Utils.hs - compiler/GHC/HsToCore/Arrows.hs - compiler/GHC/HsToCore/Docs.hs - compiler/GHC/HsToCore/Expr.hs - compiler/GHC/HsToCore/Match.hs - compiler/GHC/HsToCore/Match.hs-boot - compiler/GHC/HsToCore/Match/Constructor.hs - compiler/GHC/HsToCore/Pmc.hs - compiler/GHC/HsToCore/Pmc/Desugar.hs - compiler/GHC/HsToCore/Pmc/Solver.hs - compiler/GHC/HsToCore/Quote.hs - compiler/GHC/HsToCore/Ticks.hs - compiler/GHC/Iface/Ext/Ast.hs - compiler/GHC/Parser.y - compiler/GHC/Parser/Annotation.hs - compiler/GHC/Parser/PostProcess.hs - compiler/GHC/Parser/PostProcess/Haddock.hs - compiler/GHC/Parser/Types.hs - compiler/GHC/Rename/Bind.hs - compiler/GHC/Rename/Expr.hs - compiler/GHC/Rename/HsType.hs - compiler/GHC/Rename/Module.hs - compiler/GHC/Rename/Names.hs - compiler/GHC/Rename/Pat.hs - compiler/GHC/Rename/Utils.hs - compiler/GHC/Runtime/Eval.hs - compiler/GHC/Tc/Deriv.hs - compiler/GHC/Tc/Deriv/Generate.hs - compiler/GHC/Tc/Gen/App.hs - compiler/GHC/Tc/Gen/Arrow.hs - compiler/GHC/Tc/Gen/Do.hs - compiler/GHC/Tc/Gen/Export.hs - compiler/GHC/Tc/Gen/Expr.hs - compiler/GHC/Tc/Gen/Head.hs - compiler/GHC/Tc/Gen/HsType.hs - compiler/GHC/Tc/Gen/Match.hs - compiler/GHC/Tc/Gen/Pat.hs - compiler/GHC/Tc/Gen/Sig.hs - compiler/GHC/Tc/Module.hs - compiler/GHC/Tc/TyCl.hs - compiler/GHC/Tc/TyCl/PatSyn.hs - compiler/GHC/Tc/TyCl/Utils.hs - compiler/GHC/Tc/Utils/Backpack.hs - compiler/GHC/Tc/Zonk/Type.hs - compiler/GHC/ThToHs.hs - compiler/Language/Haskell/Syntax.hs - compiler/Language/Haskell/Syntax/Binds.hs - compiler/Language/Haskell/Syntax/Decls.hs - compiler/Language/Haskell/Syntax/Expr.hs - compiler/Language/Haskell/Syntax/Extension.hs - compiler/Language/Haskell/Syntax/ImpExp.hs - compiler/Language/Haskell/Syntax/Pat.hs - compiler/Language/Haskell/Syntax/Type.hs - ghc/GHCi/UI.hs - + testsuite/tests/deSugar/should_compile/T27383.hs - testsuite/tests/deSugar/should_compile/all.T - testsuite/tests/ghc-api/T25121_status.stdout - testsuite/tests/ghc-api/exactprint/T22919.stderr - testsuite/tests/ghc-api/exactprint/Test20239.stderr - testsuite/tests/ghc-api/exactprint/ZeroWidthSemi.stderr - testsuite/tests/haddock/should_compile_flag_haddock/T17544.stderr - testsuite/tests/haddock/should_compile_flag_haddock/T17544_kw.stderr - testsuite/tests/haddock/should_compile_flag_haddock/T24221.stderr - testsuite/tests/module/mod185.stderr - testsuite/tests/parser/should_compile/DumpParsedAst.stderr - testsuite/tests/parser/should_compile/DumpParsedAstComments.stderr - testsuite/tests/parser/should_compile/DumpRenamedAst.stderr - testsuite/tests/parser/should_compile/DumpSemis.stderr - testsuite/tests/parser/should_compile/DumpTypecheckedAst.stderr - testsuite/tests/parser/should_compile/KindSigs.stderr - testsuite/tests/parser/should_compile/T14189.stderr - testsuite/tests/parser/should_compile/T15279.stderr - testsuite/tests/parser/should_compile/T15323.stderr - testsuite/tests/parser/should_compile/T20452.stderr - testsuite/tests/parser/should_compile/T20718.stderr - testsuite/tests/parser/should_compile/T20718b.stderr - testsuite/tests/parser/should_compile/T20846.stderr - testsuite/tests/parser/should_compile/T23315/T23315.stderr - testsuite/tests/parser/should_compile/all.T - testsuite/tests/printer/AnnotationNoListTuplePuns.stdout - testsuite/tests/printer/Makefile - testsuite/tests/printer/T18791.stderr - testsuite/tests/printer/Test20297.stdout - testsuite/tests/printer/Test24533.stdout - testsuite/tests/typecheck/should_compile/T15242.stderr - testsuite/tests/typecheck/should_compile/all.T - utils/check-exact/ExactPrint.hs - utils/check-exact/Main.hs - utils/check-exact/Transform.hs - utils/check-exact/Utils.hs - utils/haddock/haddock-api/src/Haddock/Backends/Hoogle.hs - utils/haddock/haddock-api/src/Haddock/Backends/LaTeX.hs - utils/haddock/haddock-api/src/Haddock/Backends/Xhtml/Decl.hs - utils/haddock/haddock-api/src/Haddock/Convert.hs - utils/haddock/haddock-api/src/Haddock/GhcUtils.hs - utils/haddock/haddock-api/src/Haddock/Interface/Create.hs - utils/haddock/haddock-api/src/Haddock/Interface/LexParseRn.hs - utils/haddock/haddock-api/src/Haddock/Interface/Rename.hs - utils/haddock/haddock-api/src/Haddock/Interface/RenameType.hs - utils/haddock/haddock-api/src/Haddock/Types.hs The diff was not included because it is too large. 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