Typeck break expr even if break is illegal

We were earlier returning immediately when encountering an illegal break. However, this caused problems later
when the expr the break was returning was evaluated during writeback. So now we allow the
typeck of break expr to happen even though we've encountered an illegal break.

Author: gurry

compiler/rustc_hir_typeck/src/expr.rs

@@ -626,18 +626,20 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     }
                 };
 
-                let coerce_to = match opt_coerce_to {
-                    Some(c) => c,
-                    None => {
-                        // If the loop context is not a `loop { }`, then break with
-                        // a value is illegal, and `opt_coerce_to` will be `None`.
-                        // Return error in that case (#114529).
-                        return Ty::new_misc_error(tcx);
-                    }
-                };
+                // If the loop context is not a `loop { }`, then break with
+                // a value is illegal, and `opt_coerce_to` will be `None`.
+                // Set expectation to error in that case.
+                let coerce_to = opt_coerce_to.unwrap_or_else(|| Ty::new_misc_error(tcx));
 
                 // Recurse without `enclosing_breakables` borrowed.
                 e_ty = self.check_expr_with_hint(e, coerce_to);
+
+                // If break with value was found illegal above,
+                // return error right here or we risk passing typeck
+                if let ty::Error(_) = coerce_to.kind() {
+                    return coerce_to;
+                }
+
                 cause = self.misc(e.span);
             } else {
                 // Otherwise, this is a break *without* a value. That's

tests/ui/typeck/issue-114529-illegal-break-with-value.rs

@@ -17,4 +17,10 @@ fn main() {
         };
         51
     }];
+
+    while true {
+        break (|| { //~ ERROR `break` with value from a `while` loop
+            let local = 9;
+        });
+    }
 }

tests/ui/typeck/issue-114529-illegal-break-with-value.stderr

@@ -24,6 +24,21 @@ help: use `break` on its own without a value inside this `while` loop
 LL |             break;
    |             ~~~~~
 
-error: aborting due to 2 previous errors
+error[E0571]: `break` with value from a `while` loop
+  --> $DIR/issue-114529-illegal-break-with-value.rs:22:9
+   |
+LL |       while true {
+   |       ---------- you can't `break` with a value in a `while` loop
+LL | /         break (|| {
+LL | |             let local = 9;
+LL | |         });
+   | |__________^ can only break with a value inside `loop` or breakable block
+   |
+help: use `break` on its own without a value inside this `while` loop
+   |
+LL |         break;
+   |         ~~~~~
+
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0571`.