Run linear Lint on the desugarer output (part of #19165)

This addresses points (1a) and (1b) of #19165.

- Move mkFailExpr to HsToCore/Utils, as it can be shared
- Desugar incomplete patterns and holes to an empty case,
  as in Note [Incompleteness and linearity]
- Enable linear linting of desugarer output
- Mark MultConstructor as broken. It fails Lint, but I'd like to fix this
  separately.

Metric Decrease:
    T6048

1 files changed, 12 insertions, 4 deletions

diff --git a/compiler/GHC/Core/Lint.hs b/compiler/GHC/Core/Lint.hs
index 382851a1e5..40de306802 100644
--- a/compiler/GHC/Core/Lint.hs
+++ b/compiler/GHC/Core/Lint.hs
@@ -488,7 +488,8 @@ lintCoreBindings dflags pass local_in_scope binds
     flags = (defaultLintFlags dflags)
                { lf_check_global_ids = check_globals
                , lf_check_inline_loop_breakers = check_lbs
-               , lf_check_static_ptrs = check_static_ptrs }
+               , lf_check_static_ptrs = check_static_ptrs
+               , lf_check_linearity = check_linearity }
 
     -- See Note [Checking for global Ids]
     check_globals = case pass of
@@ -510,6 +511,12 @@ lintCoreBindings dflags pass local_in_scope binds
                           CorePrep              -> AllowAtTopLevel
                           _                     -> AllowAnywhere
 
+    -- See Note [Linting linearity]
+    check_linearity = gopt Opt_DoLinearCoreLinting dflags || (
+                        case pass of
+                          CoreDesugar -> True
+                          _ -> False)
+
     (_, dups) = removeDups compare binders
 
     -- dups_ext checks for names with different uniques
@@ -2641,11 +2648,12 @@ to work with Linear Lint:
     in f True
   uses 'x' linearly, but this is not seen by the linter.
   Plan: make let-bound variables remember the usage environment.
-  See test LinearLetRec and https://github.com/tweag/ghc/issues/405.
+  See ticket #18694.
 
 We plan to fix both of the issues in the very near future.
-For now, linear Lint is disabled by default and
-has to be enabled manually with -dlinear-core-lint.
+For now, -dcore-lint enables only linting output of the desugarer,
+and full Linear Lint has to be enabled separately with -dlinear-core-lint.
+Ticket #19165 concerns enabling Linear Lint with -dcore-lint.
 -}
 
 instance Applicative LintM where