Don't name variables from external macros in borrow errors.

Co-authored-by: Mara Bos <m-ou.se@m-ou.se>

Author: jdonszelmann

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);

library/stdarch

@@ -9,4 +9,3 @@ macro_rules! foo {
         }
     };
 }
-

src/doc/nomicon

@@ -1,27 +1,29 @@
-error[E0515]: cannot return reference to local variable `args`
+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
+   |             ^^^^^^^^^^^^^^^^^^^^
+   |             |
+   |             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[E0597]: `args` does not live long enough
+error[E0716]: temporary value dropped while borrowed
   --> $DIR/return_from_external_macro.rs:10:5
    |
 LL |     ret_from_ext::foo!()
    |     ^^^^^^^^^^^^^^^^^^^^
    |     |
-   |     borrowed value does not live long enough
-   |     binding `args` declared here
-   |     opaque type requires that `args` is borrowed for `'static`
+   |     creates a temporary value which is freed while still in use
+   |     opaque type requires that borrow lasts for `'static`
 LL |
 LL | }
-   | - `args` dropped here while still borrowed
+   | - 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, E0597.
+Some errors have detailed explanations: E0515, E0716.
 For more information about an error, try `rustc --explain E0515`.