Add regression test for CONST_ERR lints in extern macros

Author: memoryruins

Diff for: src/test/ui/consts/auxiliary/external_macro.rs

@@ -0,0 +1,14 @@
+#![feature(allow_internal_unstable)]
+
+// Macro to help ensure CONST_ERR lint errors
+// are not silenced in external macros.
+// https://github.com/rust-lang/rust/issues/65300
+
+#[macro_export]
+#[allow_internal_unstable(type_ascription)]
+macro_rules! static_assert {
+    ($test:expr) => {
+        #[allow(dead_code)]
+        const _: () = [()][!($test: bool) as usize];
+    }
+}

Diff for: src/test/ui/consts/const-external-macro-const-err.rs

@@ -0,0 +1,13 @@
+// edition:2018
+// aux-build:external_macro.rs
+
+// Ensure that CONST_ERR lint errors
+// are not silenced in external macros.
+// https://github.com/rust-lang/rust/issues/65300
+
+extern crate external_macro;
+use external_macro::static_assert;
+
+fn main() {
+    static_assert!(2 + 2 == 5); //~ ERROR
+}

Diff for: src/test/ui/consts/const-external-macro-const-err.stderr

@@ -0,0 +1,11 @@
+error: any use of this value will cause an error
+  --> $DIR/const-external-macro-const-err.rs:12:5
+   |
+LL |     static_assert!(2 + 2 == 5);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ index out of bounds: the len is 1 but the index is 1
+   |
+   = note: `#[deny(const_err)]` on by default
+   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
+
+error: aborting due to previous error
+