Rollup merge of #137269 - dianne:fix-ref-pat-label-span, r=davidtwco

Pattern Migration 2024: properly label `&` patterns whose subpatterns are from macro expansions

See the failing test output in the first commit for an example of what this going wrong looks like. The error/lint diagnostic tries to point to just the `&` or `&mut` of reference patterns when labeling the causes, to make the output clearer (#134394). The trimming there wasn't quite right though: it used the interior of the reference pattern as a cutoff and extended backwards to find where to trim the pattern's span, but this breaks if the `&` and the interior are from different sources. This PR instead trims by starting at the start of the pattern and ending at the final character of the `&` (or `&mut`, `ref`, `ref mut`, or `mut`, depending on what the error/lint is labeling); that way, there's no opportunity for failure from mixing sources.

I'm not 100% happy with this approach, but I'm also not sure what the best practices are as far as hacky `SourceMap` munching goes, so please let me know if something else would be preferred.

Since `SourceMap::span_through_char` can't change the syntax context of the span, I've also removed a call to `Span::with_ctxt` (we care about the edition of the span in question since this is a hard error in Rust 2024). If we want to be extra safe in case that changes, I can re-add it or track error hardness separately in the `rust_2024_migration_desugared_pats` table.

3 files changed, 31 insertions, 1 deletions

diff --git a/tests/ui/pattern/rfc-3627-match-ergonomics-2024/migration_lint.fixed b/tests/ui/pattern/rfc-3627-match-ergonomics-2024/migration_lint.fixed
index e35896f32ad..bb4ecc09063 100644
--- a/tests/ui/pattern/rfc-3627-match-ergonomics-2024/migration_lint.fixed
+++ b/tests/ui/pattern/rfc-3627-match-ergonomics-2024/migration_lint.fixed
@@ -244,4 +244,10 @@ fn main() {
     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);
+
+    // Test that we use the correct span when labeling a `&` whose subpattern is from an expansion.
+    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
+    //~| WARN: this changes meaning in Rust 2024
+    assert_type_eq(a, &0u32);
 }
diff --git a/tests/ui/pattern/rfc-3627-match-ergonomics-2024/migration_lint.rs b/tests/ui/pattern/rfc-3627-match-ergonomics-2024/migration_lint.rs
index 10a23e6f2fa..2837c8d81db 100644
--- a/tests/ui/pattern/rfc-3627-match-ergonomics-2024/migration_lint.rs
+++ b/tests/ui/pattern/rfc-3627-match-ergonomics-2024/migration_lint.rs
@@ -244,4 +244,10 @@ fn main() {
     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);
+
+    // Test that we use the correct span when labeling a `&` whose subpattern is from an expansion.
+    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
+    //~| WARN: this changes meaning in Rust 2024
+    assert_type_eq(a, &0u32);
 }
diff --git a/tests/ui/pattern/rfc-3627-match-ergonomics-2024/migration_lint.stderr b/tests/ui/pattern/rfc-3627-match-ergonomics-2024/migration_lint.stderr
index 3dd91c86a3b..6efda4f757f 100644
--- a/tests/ui/pattern/rfc-3627-match-ergonomics-2024/migration_lint.stderr
+++ b/tests/ui/pattern/rfc-3627-match-ergonomics-2024/migration_lint.stderr
@@ -580,5 +580,23 @@ help: make the implied reference pattern explicit
 LL |     let &[migration_lint_macros::bind_ref!(a)] = &[0];
    |         +
 
-error: aborting due to 30 previous errors
+error: reference patterns may only be written when the default binding mode is `move` in Rust 2024
+  --> $DIR/migration_lint.rs:249:10
+   |
+LL |     let [&migration_lint_macros::bind_ref!(a)] = &[&0];
+   |          ^ reference pattern not allowed under `ref` default binding mode
+   |
+   = warning: this changes meaning in Rust 2024
+   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2024/match-ergonomics.html>
+note: matching on a reference type with a non-reference pattern changes the default binding mode
+  --> $DIR/migration_lint.rs:249:9
+   |
+LL |     let [&migration_lint_macros::bind_ref!(a)] = &[&0];
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this matches on type `&_`
+help: make the implied reference pattern explicit
+   |
+LL |     let &[&migration_lint_macros::bind_ref!(a)] = &[&0];
+   |         +
+
+error: aborting due to 31 previous errors