Test that names of variables in external macros are not shown on a borrow error

compiler/rustc_borrowck/src/diagnostics/mod.rs

@@ -317,6 +317,14 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
         opt: DescribePlaceOpt,
     ) -> Option<String> {
         let local = place.local;
+        if self.body.local_decls[local]
+            .source_info
+            .span
+            .in_external_macro(self.infcx.tcx.sess.source_map())
+        {
+            return None;
+        }
+
         let mut autoderef_index = None;
         let mut buf = String::new();
         let mut ok = self.append_local_to_string(local, &mut buf);

tests/ui/macros/auxiliary/return_from_external_macro.rs

@@ -0,0 +1,11 @@
+#![feature(super_let)]
+
+#[macro_export]
+macro_rules! foo {
+    () => {
+        {
+            super let args = 1;
+            &args
+        }
+    };
+}

tests/ui/macros/return_from_external_macro.rs

@@ -0,0 +1,17 @@
+//@ aux-crate: ret_from_ext=return_from_external_macro.rs
+
+#![feature(super_let)]
+extern crate ret_from_ext;
+
+fn foo() -> impl Sized {
+    drop(|| ret_from_ext::foo!());
+    //~^ ERROR cannot return reference to local binding
+
+    ret_from_ext::foo!()
+    //~^ ERROR temporary value dropped while borrowed
+}
+//~^ NOTE temporary value is freed at the end of this statement
+
+fn main() {
+    foo();
+}

tests/ui/macros/return_from_external_macro.stderr

@@ -0,0 +1,29 @@
+error[E0515]: cannot return reference to local binding
+  --> $DIR/return_from_external_macro.rs:7:13
+   |
+LL |     drop(|| ret_from_ext::foo!());
+   |             ^^^^^^^^^^^^^^^^^^^^
+   |             |
+   |             returns a reference to data owned by the current function
+   |             local binding introduced here
+   |
+   = note: this error originates in the macro `ret_from_ext::foo` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/return_from_external_macro.rs:10:5
+   |
+LL |     ret_from_ext::foo!()
+   |     ^^^^^^^^^^^^^^^^^^^^
+   |     |
+   |     creates a temporary value which is freed while still in use
+   |     opaque type requires that borrow lasts for `'static`
+LL |
+LL | }
+   | - temporary value is freed at the end of this statement
+   |
+   = note: this error originates in the macro `ret_from_ext::foo` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0515, E0716.
+For more information about an error, try `rustc --explain E0515`.