resolve: disallow label use through closure/async

This commit modifies resolve to disallow `break`/`continue` to labels
through closures or async blocks. This doesn't make sense and should
have been prohibited anyway.

Signed-off-by: David Wood <david@davidtw.co>

1 files changed, 20 insertions, 0 deletions

diff --git a/src/librustc_error_codes/error_codes/E0767.md b/src/librustc_error_codes/error_codes/E0767.md
new file mode 100644
index 00000000000..679fe7e41e9
--- /dev/null
+++ b/src/librustc_error_codes/error_codes/E0767.md
@@ -0,0 +1,20 @@
+An unreachable label was used.
+
+Erroneous code example:
+
+```compile_fail,E0767
+'a: loop {
+    || {
+        loop { break 'a } // error: use of unreachable label `'a`
+    };
+}
+```
+
+Ensure that the label is within scope. Labels are not reachable through
+functions, closures, async blocks or modules. Example:
+
+```
+'a: loop {
+    break 'a; // ok!
+}
+```