Fail typeck for illegal break-with-value

This is fixes the issue wherein typeck was succeeding for break-with-value at illegal locations such as inside `while`, `while let` and `for` loops which eventually caused an ICE during MIR interpetation for const eval. Now we fail typeck for such code which prevents faulty MIR from being generated and interpreted, thus fixing the ICE.
rust-lang · bors · Oct 30, 2023 · Oct 30, 2023 · Oct 30, 2023 · 0c8bdd0bf33c2313ad9d0be495ffde8f4532237a
commit 0c8bdd0bf33c2313ad9d0be495ffde8f4532237a
diff --git a/compiler/rustc_hir_typeck/src/expr.rs b/compiler/rustc_hir_typeck/src/expr.rs
@@ -625,10 +625,15 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     }
                 };
 
-                // If the loop context is not a `loop { }`, then break with
-                // a value is illegal, and `opt_coerce_to` will be `None`.
-                // Just set expectation to error in that case.
-                let coerce_to = opt_coerce_to.unwrap_or_else(|| Ty::new_misc_error(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);
+                    }
+                };
 
                 // Recurse without `enclosing_breakables` borrowed.
                 e_ty = self.check_expr_with_hint(e, coerce_to);

diff --git a/tests/ui/typeck/issue-114529-illegal-break-with-value.rs b/tests/ui/typeck/issue-114529-illegal-break-with-value.rs
@@ -0,0 +1,20 @@
+// Regression test for issue #114529
+// Tests that we do not ICE during const eval for a
+// break-with-value in contexts where it is illegal
+
+#[allow(while_true)]
+fn main() {
+    [(); {
+        while true {
+            break 9; //~ ERROR `break` with value from a `while` loop
+        };
+        51
+    }];
+
+    [(); {
+        while let Some(v) = Some(9) {
+            break v; //~ ERROR `break` with value from a `while` loop
+        };
+        51
+    }];
+}
diff --git a/tests/ui/typeck/issue-114529-illegal-break-with-value.stderr b/tests/ui/typeck/issue-114529-illegal-break-with-value.stderr
@@ -0,0 +1,29 @@
+error[E0571]: `break` with value from a `while` loop
+  --> $DIR/issue-114529-illegal-break-with-value.rs:9:13
+   |
+LL |         while true {
+   |         ---------- you can't `break` with a value in a `while` loop
+LL |             break 9;
+   |             ^^^^^^^ 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[E0571]: `break` with value from a `while` loop
+  --> $DIR/issue-114529-illegal-break-with-value.rs:16:13
+   |
+LL |         while let Some(v) = Some(9) {
+   |         --------------------------- you can't `break` with a value in a `while` loop
+LL |             break v;
+   |             ^^^^^^^ 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 2 previous errors
+
+For more information about this error, try `rustc --explain E0571`.