From 8221f9c837f46461d2dd6f29a4468b3a0ee4b508 Mon Sep 17 00:00:00 2001
From: Esteban Küber <esteban@kuber.com.ar>
Date: Thu, 2 Nov 2023 21:22:42 +0000
Subject: Account for `!` arm in tail `match` expr

On functions with a default return type that influences the coerced type
of `match` arms, check if the failing arm is actually of type `!`. If
so, suggest changing the return type so the coercion against the prior
arms is successful.

```
error[E0308]: `match` arms have incompatible types
  --> $DIR/match-tail-expr-never-type-error.rs:9:13
   |
LL |   fn bar(a: bool) {
   |                  - help: try adding a return type: `-> i32`
LL | /     match a {
LL | |         true => 1,
   | |                 - this is found to be of type `{integer}`
LL | |         false => {
LL | |             never()
   | |             ^^^^^^^
   | |             |
   | |             expected integer, found `()`
   | |             this expression is of type `!`, but it get's coerced to `()` due to its surrounding expression
LL | |         }
LL | |     }
   | |_____- `match` arms have incompatible types
```

Fix #24157.
---
 compiler/rustc_hir_typeck/src/_match.rs            | 36 +++++++++++++++++++++-
 compiler/rustc_hir_typeck/src/coercion.rs          |  1 +
 compiler/rustc_hir_typeck/src/expr.rs              |  7 +++--
 .../rustc_hir_typeck/src/fn_ctxt/suggestions.rs    |  1 -
 4 files changed, 41 insertions(+), 4 deletions(-)

(limited to 'compiler')

diff --git a/compiler/rustc_hir_typeck/src/_match.rs b/compiler/rustc_hir_typeck/src/_match.rs
index a29d9c95e5e..e7cbbc71335 100644
--- a/compiler/rustc_hir_typeck/src/_match.rs
+++ b/compiler/rustc_hir_typeck/src/_match.rs
@@ -139,7 +139,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 &cause,
                 Some(arm.body),
                 arm_ty,
-                |err| self.suggest_removing_semicolon_for_coerce(err, expr, arm_ty, prior_arm),
+                |err| {
+                    self.explain_never_type_coerced_to_unit(err, arm, arm_ty, prior_arm, expr);
+                },
                 false,
             );
 
@@ -177,6 +179,38 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         coercion.complete(self)
     }
 
+    fn explain_never_type_coerced_to_unit(
+        &self,
+        err: &mut Diagnostic,
+        arm: &hir::Arm<'tcx>,
+        arm_ty: Ty<'tcx>,
+        prior_arm: Option<(Option<hir::HirId>, Ty<'tcx>, Span)>,
+        expr: &hir::Expr<'tcx>,
+    ) {
+        if let hir::ExprKind::Block(block, _) = arm.body.kind
+            && let Some(expr) = block.expr
+            && let arm_tail_ty = self.node_ty(expr.hir_id)
+            && arm_tail_ty.is_never()
+            && !arm_ty.is_never()
+        {
+            err.span_label(
+                expr.span,
+                format!(
+                    "this expression is of type `!`, but it is coerced to `{arm_ty}` due to its \
+                     surrounding expression",
+                ),
+            );
+            self.suggest_mismatched_types_on_tail(
+                err,
+                expr,
+                arm_ty,
+                prior_arm.map_or(arm_tail_ty, |(_, ty, _)| ty),
+                expr.hir_id,
+            );
+        }
+        self.suggest_removing_semicolon_for_coerce(err, expr, arm_ty, prior_arm)
+    }
+
     fn suggest_removing_semicolon_for_coerce(
         &self,
         diag: &mut Diagnostic,
diff --git a/compiler/rustc_hir_typeck/src/coercion.rs b/compiler/rustc_hir_typeck/src/coercion.rs
index ae2a4a9504c..587038d57bd 100644
--- a/compiler/rustc_hir_typeck/src/coercion.rs
+++ b/compiler/rustc_hir_typeck/src/coercion.rs
@@ -1715,6 +1715,7 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
         // label pointing out the cause for the type coercion will be wrong
         // as prior return coercions would not be relevant (#57664).
         let fn_decl = if let (Some(expr), Some(blk_id)) = (expression, blk_id) {
+            fcx.suggest_missing_semicolon(&mut err, expr, expected, false);
             let pointing_at_return_type =
                 fcx.suggest_mismatched_types_on_tail(&mut err, expr, expected, found, blk_id);
             if let (Some(cond_expr), true, false) = (
diff --git a/compiler/rustc_hir_typeck/src/expr.rs b/compiler/rustc_hir_typeck/src/expr.rs
index f16269795e9..7f15478349e 100644
--- a/compiler/rustc_hir_typeck/src/expr.rs
+++ b/compiler/rustc_hir_typeck/src/expr.rs
@@ -663,8 +663,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     coerce.coerce_forced_unit(
                         self,
                         &cause,
-                        |err| {
-                            self.suggest_mismatched_types_on_tail(err, expr, ty, e_ty, target_id);
+                        |mut err| {
+                            self.suggest_missing_semicolon(&mut err, expr, e_ty, false);
+                            self.suggest_mismatched_types_on_tail(
+                                &mut err, expr, ty, e_ty, target_id,
+                            );
                             let error = Some(Sorts(ExpectedFound { expected: ty, found: e_ty }));
                             self.annotate_loop_expected_due_to_inference(err, expr, error);
                             if let Some(val) = ty_kind_suggestion(ty) {
diff --git a/compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs b/compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs
index 6660bea03d8..c74ef8f2713 100644
--- a/compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs
+++ b/compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs
@@ -72,7 +72,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         blk_id: hir::HirId,
     ) -> bool {
         let expr = expr.peel_drop_temps();
-        self.suggest_missing_semicolon(err, expr, expected, false);
         let mut pointing_at_return_type = false;
         if let hir::ExprKind::Break(..) = expr.kind {
             // `break` type mismatches provide better context for tail `loop` expressions.
-- 
cgit 1.4.1-3-g733a5