use more specific wording for subpatterns from macro expansions

Author: dianne

compiler/rustc_hir_typeck/src/pat.rs

@@ -2787,7 +2787,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // Only provide a detailed label if the problematic subpattern isn't from an expansion.
         // In the case that it's from a macro, we'll add a more detailed note in the emitter.
         let desc = if subpat.span.from_expansion() {
-            "occurs within expansion"
+            // NB: This wording assumes the only expansions that can produce problematic reference
+            // patterns and bindings are macros. If a desugaring or AST pass is added that can do
+            // so, we may want to inspect the span's source callee or macro backtrace.
+            "occurs within macro expansion"
         } else {
             match def_br_mutbl {
                 Mutability::Not => "default binding mode is `ref`",

tests/ui/pattern/rfc-3627-match-ergonomics-2024/migration_lint.stderr

@@ -206,7 +206,7 @@ error: binding modifiers may only be written when the default binding mode is `m
   --> $DIR/migration_lint.rs:137:15
    |
 LL |         (Some(mut x), migration_lint_macros::mixed_edition_pat!(y)) => {
-   |               ^^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ occurs within expansion
+   |               ^^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ occurs within macro expansion
    |               |
    |               default binding mode is `ref`
    |