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.

Author: estebank

compiler/rustc_hir_typeck/src/_match.rs

@@ -139,7 +139,30 @@ 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| {
+                    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)
+                },
                 false,
             );
 

compiler/rustc_hir_typeck/src/coercion.rs

@@ -1738,6 +1738,7 @@ 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) = (

compiler/rustc_hir_typeck/src/expr.rs

@@ -669,6 +669,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         self,
                         &cause,
                         |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,
                             );

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.

tests/ui/match/match-tail-expr-never-type-error.rs

@@ -0,0 +1,16 @@
+fn never() -> ! {
+    loop {}
+}
+
+fn bar(a: bool) {
+    match a {
+        true => 1,
+        false => {
+            never() //~ ERROR `match` arms have incompatible types
+        }
+    }
+}
+fn main() {
+    bar(true);
+    bar(false);
+}

tests/ui/match/match-tail-expr-never-type-error.stderr

@@ -0,0 +1,21 @@
+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 is coerced to `()` due to its surrounding expression
+LL | |         }
+LL | |     }
+   | |_____- `match` arms have incompatible types
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.