don't suggest removals inside macro expansions

This fixes the regression in the previous commit.

Author: dianne

compiler/rustc_mir_build/src/thir/pattern/migration.rs

@@ -293,9 +293,14 @@ impl<'a, 'tcx> PatMigration<'a, 'tcx> {
         self.add_default_mode_label_if_needed();
         // This sets the default binding mode to by-value in the user's pattern, but we'll try to
         // suggest removing it.
-        // TODO: if this is inside a macro expansion, we won't be able to remove it.
         self.push_deref(pat_span, mutbl, PatDerefKind::Explicit { inner_span: subpat.span });
 
+        // If this is inside a macro expansion, we won't be able to remove it.
+        if pat_span.from_expansion() {
+            self.add_derefs_to_suggestion(self.innermost_deref);
+            return;
+        }
+
         // If the immediate subpattern is a binding, removing this reference pattern would change
         // its type. To avoid that, we include it and all its parents in that case.
         // FIXME(ref_pat_eat_one_layer_2024): This assumes ref pats can't eat the binding mode
@@ -378,8 +383,11 @@ impl<'a, 'tcx> PatMigration<'a, 'tcx> {
 
         // If `mode` doesn't match the default, we'll need to specify its binding modifiers
         // explicitly, which in turn necessitates a by-move default binding mode.
-        // TODO: if this is inside a macro expansion, we won't be able to change it.
-        let suggest = mode != BindingMode(self.sugg_default_mode(), Mutability::Not);
+        // Additionally, if this is inside a macro expansion, we won't be able to change it. If a
+        // binding modifier is missing inside the expansion, there's not much we can do, but we can
+        // avoid suggestions to elide binding modifiers that are explicit within expansions.
+        let suggest = mode != BindingMode(self.sugg_default_mode(), Mutability::Not)
+            || pat_span.from_expansion() && explicit_ba != BindingMode::NONE;
 
         // Track the binding
         let span = if explicit_ba == BindingMode::NONE {

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

@@ -239,7 +239,7 @@ fn main() {
     assert_type_eq(d, 0u32);
 
     // Test that we use the correct message and suggestion style when pointing inside expansions.
-    let [migration_lint_macros::bind_ref!(a)] = &[0];
+    let &[migration_lint_macros::bind_ref!(a)] = &[0];
     //~^ ERROR: binding modifiers may only be written when the default binding mode is `move`
     assert_type_eq(a, &0u32);
 

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

@@ -580,6 +580,10 @@ note: matching on a reference type with a non-reference pattern changes the defa
 LL |     let [migration_lint_macros::bind_ref!(a)] = &[0];
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this matches on type `&_`
    = note: this error originates in the macro `migration_lint_macros::bind_ref` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: make the implied reference pattern explicit
+   |
+LL |     let &[migration_lint_macros::bind_ref!(a)] = &[0];
+   |         +
 
 error: reference patterns may only be written when the default binding mode is `move` in Rust 2024
   --> $DIR/migration_lint.rs:249:10