Match ergonomics 2024: Implement TC's match ergonomics proposal

compiler/rustc_ast/src/ast.rs

@@ -711,6 +711,7 @@ pub enum ByRef {
 }
 
 impl ByRef {
+    #[must_use]
     pub fn cap_ref_mutability(mut self, mutbl: Mutability) -> Self {
         if let ByRef::Yes(old_mutbl) = &mut self {
             *old_mutbl = cmp::min(*old_mutbl, mutbl);

compiler/rustc_feature/src/unstable.rs

@@ -573,6 +573,8 @@ declare_features! (
     (unstable, raw_ref_op, "1.41.0", Some(64490)),
     /// Makes `&` and `&mut` patterns eat only one layer of references in Rust 2024.
     (incomplete, ref_pat_eat_one_layer_2024, "1.79.0", Some(123076)),
+    /// Makes `&` and `&mut` patterns eat only one layer of references in Rust 2024—structural variant
+    (incomplete, ref_pat_eat_one_layer_2024_structural, "CURRENT_RUSTC_VERSION", Some(123076)),
     /// Allows using the `#[register_tool]` attribute.
     (unstable, register_tool, "1.41.0", Some(66079)),
     /// Allows the `#[repr(i128)]` attribute for enums.

compiler/rustc_hir_typeck/src/pat.rs

@@ -328,8 +328,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         adjust_mode: AdjustMode,
         max_ref_mutbl: MutblCap,
     ) -> (Ty<'tcx>, ByRef, MutblCap) {
-        if let ByRef::Yes(Mutability::Mut) = def_br {
-            debug_assert!(max_ref_mutbl == MutblCap::Mut);
+        #[cfg(debug_assertions)]
+        if def_br == ByRef::Yes(Mutability::Mut) && max_ref_mutbl != MutblCap::Mut {
+            span_bug!(pat.span, "Pattern mutability cap violated!");
         }
         match adjust_mode {
             AdjustMode::Pass => (expected, def_br, max_ref_mutbl),
@@ -437,7 +438,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             });
         }
 
-        if self.tcx.features().ref_pat_eat_one_layer_2024 {
+        let features = self.tcx.features();
+        if features.ref_pat_eat_one_layer_2024 || features.ref_pat_eat_one_layer_2024_structural {
             def_br = def_br.cap_ref_mutability(max_ref_mutbl.as_mutbl());
             if def_br == ByRef::Yes(Mutability::Not) {
                 max_ref_mutbl = MutblCap::Not;
@@ -669,7 +671,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // Determine the binding mode...
         let bm = match user_bind_annot {
             BindingMode(ByRef::No, Mutability::Mut) if matches!(def_br, ByRef::Yes(_)) => {
-                if pat.span.at_least_rust_2024() && self.tcx.features().ref_pat_eat_one_layer_2024 {
+                if pat.span.at_least_rust_2024()
+                    && (self.tcx.features().ref_pat_eat_one_layer_2024
+                        || self.tcx.features().ref_pat_eat_one_layer_2024_structural)
+                {
                     if !self.tcx.features().mut_ref {
                         feature_err(
                             &self.tcx.sess,
@@ -2122,7 +2127,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         mut expected: Ty<'tcx>,
         mut pat_info: PatInfo<'tcx, '_>,
     ) -> Ty<'tcx> {
-        let no_ref_mut_behind_and = self.tcx.features().ref_pat_eat_one_layer_2024;
+        let tcx = self.tcx;
+        let features = tcx.features();
+        let ref_pat_eat_one_layer_2024 = features.ref_pat_eat_one_layer_2024;
+        let ref_pat_eat_one_layer_2024_structural = features.ref_pat_eat_one_layer_2024_structural;
+
+        let no_ref_mut_behind_and =
+            ref_pat_eat_one_layer_2024 || ref_pat_eat_one_layer_2024_structural;
         let new_match_ergonomics = pat.span.at_least_rust_2024() && no_ref_mut_behind_and;
 
         let pat_prefix_span =
@@ -2137,32 +2148,49 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             pat_info.max_ref_mutbl = MutblCap::Mut;
         }
 
+        expected = self.try_structurally_resolve_type(pat.span, expected);
         if new_match_ergonomics {
             if let ByRef::Yes(inh_mut) = pat_info.binding_mode {
-                // ref pattern consumes inherited reference
-
-                if pat_mutbl > inh_mut {
-                    // Tried to match inherited `ref` with `&mut`, which is an error
-                    let err_msg = "cannot match inherited `&` with `&mut` pattern";
-                    let err = if let Some(span) = pat_prefix_span {
-                        let mut err = self.dcx().struct_span_err(span, err_msg);
-                        err.span_suggestion_verbose(
-                            span,
-                            "replace this `&mut` pattern with `&`",
-                            "&",
-                            Applicability::MachineApplicable,
-                        );
-                        err
+                if !ref_pat_eat_one_layer_2024 && let ty::Ref(_, _, r_mutbl) = *expected.kind() {
+                    // Don't attempt to consume inherited reference
+                    pat_info.binding_mode = pat_info.binding_mode.cap_ref_mutability(r_mutbl);
+                } else {
+                    // ref pattern attempts to consume inherited reference
+                    if pat_mutbl > inh_mut {
+                        // Tried to match inherited `ref` with `&mut`
+                        if !ref_pat_eat_one_layer_2024_structural {
+                            let err_msg = "mismatched types";
+                            let err = if let Some(span) = pat_prefix_span {
+                                let mut err = self.dcx().struct_span_err(span, err_msg);
+                                err.code(E0308);
+                                err.note("cannot match inherited `&` with `&mut` pattern");
+                                err.span_suggestion_verbose(
+                                    span,
+                                    "replace this `&mut` pattern with `&`",
+                                    "&",
+                                    Applicability::MachineApplicable,
+                                );
+                                err
+                            } else {
+                                self.dcx().struct_span_err(pat.span, err_msg)
+                            };
+                            err.emit();
+
+                            pat_info.binding_mode = ByRef::No;
+                            self.typeck_results
+                                .borrow_mut()
+                                .skipped_ref_pats_mut()
+                                .insert(pat.hir_id);
+                            self.check_pat(inner, expected, pat_info);
+                            return expected;
+                        }
                     } else {
-                        self.dcx().struct_span_err(pat.span, err_msg)
-                    };
-                    err.emit();
+                        pat_info.binding_mode = ByRef::No;
+                        self.typeck_results.borrow_mut().skipped_ref_pats_mut().insert(pat.hir_id);
+                        self.check_pat(inner, expected, pat_info);
+                        return expected;
+                    }
                 }
-
-                pat_info.binding_mode = ByRef::No;
-                self.typeck_results.borrow_mut().skipped_ref_pats_mut().insert(pat.hir_id);
-                self.check_pat(inner, expected, pat_info);
-                return expected;
             }
         } else {
             // Reset binding mode on old editions
@@ -2177,8 +2205,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             }
         }
 
-        let tcx = self.tcx;
-        expected = self.try_structurally_resolve_type(pat.span, expected);
         let (ref_ty, inner_ty) = match self.check_dereferenceable(pat.span, expected, inner) {
             Ok(()) => {
                 // `demand::subtype` would be good enough, but using `eqtype` turns

compiler/rustc_span/src/symbol.rs

@@ -1502,6 +1502,7 @@ symbols! {
         recursion_limit,
         reexport_test_harness_main,
         ref_pat_eat_one_layer_2024,
+        ref_pat_eat_one_layer_2024_structural,
         ref_pat_everywhere,
         ref_unwind_safe_trait,
         reference,

src/tools/tidy/src/features.rs

@@ -112,7 +112,7 @@ pub fn check(
             let file = entry.path();
             let filename = file.file_name().unwrap().to_string_lossy();
             let filen_underscore = filename.replace('-', "_").replace(".rs", "");
-            let filename_is_gate_test = test_filen_gate(&filen_underscore, &mut features);
+            let filename_gate = test_filen_gate(&filen_underscore, &mut features);
 
             for (i, line) in contents.lines().enumerate() {
                 let mut err = |msg: &str| {
@@ -128,7 +128,7 @@ pub fn check(
                 };
                 match features.get_mut(feature_name) {
                     Some(f) => {
-                        if filename_is_gate_test {
+                        if filename_gate == Some(feature_name) {
                             err(&format!(
                                 "The file is already marked as gate test \
                                       through its name, no need for a \
@@ -259,18 +259,18 @@ fn find_attr_val<'a>(line: &'a str, attr: &str) -> Option<&'a str> {
     r.captures(line).and_then(|c| c.get(1)).map(|m| m.as_str())
 }
 
-fn test_filen_gate(filen_underscore: &str, features: &mut Features) -> bool {
+fn test_filen_gate<'f>(filen_underscore: &'f str, features: &mut Features) -> Option<&'f str> {
     let prefix = "feature_gate_";
-    if filen_underscore.starts_with(prefix) {
+    if let Some(suffix) = filen_underscore.strip_prefix(prefix) {
         for (n, f) in features.iter_mut() {
             // Equivalent to filen_underscore == format!("feature_gate_{n}")
-            if &filen_underscore[prefix.len()..] == n {
+            if suffix == n {
                 f.has_gate_test = true;
-                return true;
+                return Some(suffix);
             }
         }
     }
-    false
+    None
 }
 
 pub fn collect_lang_features(base_compiler_path: &Path, bad: &mut bool) -> Features {

tests/ui/match/ref_pat_eat_one_layer_2024/feature-gate-ref_pat_eat_one_layer_2024.rs

@@ -1,5 +1,6 @@
 //@ edition: 2024
 //@ compile-flags: -Zunstable-options
+// gate-test-ref_pat_eat_one_layer_2024_structural
 
 pub fn main() {
     if let Some(Some(&x)) = &Some(&Some(0)) {

tests/ui/match/ref_pat_eat_one_layer_2024/feature-gate-ref_pat_eat_one_layer_2024.stderr

@@ -1,5 +1,5 @@
 error[E0308]: mismatched types
-  --> $DIR/feature-gate-ref_pat_eat_one_layer_2024.rs:5:22
+  --> $DIR/feature-gate-ref_pat_eat_one_layer_2024.rs:6:22
    |
 LL |     if let Some(Some(&x)) = &Some(&Some(0)) {
    |                      ^^     --------------- this expression has type `&Option<&Option<{integer}>>`
@@ -14,7 +14,7 @@ LL |     if let Some(Some(x)) = &Some(&Some(0)) {
    |                      ~
 
 error[E0308]: mismatched types
-  --> $DIR/feature-gate-ref_pat_eat_one_layer_2024.rs:10:23
+  --> $DIR/feature-gate-ref_pat_eat_one_layer_2024.rs:11:23
    |
 LL |         let _: &u32 = x;
    |                ----   ^ expected `&u32`, found integer
@@ -27,7 +27,7 @@ LL |         let _: &u32 = &x;
    |                       +
 
 error[E0308]: mismatched types
-  --> $DIR/feature-gate-ref_pat_eat_one_layer_2024.rs:13:23
+  --> $DIR/feature-gate-ref_pat_eat_one_layer_2024.rs:14:23
    |
 LL |     if let Some(Some(&&x)) = &Some(Some(&0)) {
    |                       ^^     --------------- this expression has type `&Option<Option<&{integer}>>`
@@ -43,7 +43,7 @@ LL +     if let Some(Some(&x)) = &Some(Some(&0)) {
    |
 
 error[E0308]: mismatched types
-  --> $DIR/feature-gate-ref_pat_eat_one_layer_2024.rs:17:17
+  --> $DIR/feature-gate-ref_pat_eat_one_layer_2024.rs:18:17
    |
 LL |     if let Some(&Some(x)) = &Some(Some(0)) {
    |                 ^^^^^^^^    -------------- this expression has type `&Option<Option<{integer}>>`
@@ -54,7 +54,7 @@ LL |     if let Some(&Some(x)) = &Some(Some(0)) {
            found reference `&_`
 
 error[E0308]: mismatched types
-  --> $DIR/feature-gate-ref_pat_eat_one_layer_2024.rs:21:22
+  --> $DIR/feature-gate-ref_pat_eat_one_layer_2024.rs:22:22
    |
 LL |     if let Some(Some(&mut x)) = &mut Some(&mut Some(0)) {
    |                      ^^^^^^     ----------------------- this expression has type `&mut Option<&mut Option<{integer}>>`
@@ -64,7 +64,7 @@ LL |     if let Some(Some(&mut x)) = &mut Some(&mut Some(0)) {
    = note:           expected type `{integer}`
            found mutable reference `&mut _`
 note: to declare a mutable binding use: `mut x`
-  --> $DIR/feature-gate-ref_pat_eat_one_layer_2024.rs:21:22
+  --> $DIR/feature-gate-ref_pat_eat_one_layer_2024.rs:22:22
    |
 LL |     if let Some(Some(&mut x)) = &mut Some(&mut Some(0)) {
    |                      ^^^^^^
@@ -74,7 +74,7 @@ LL |     if let Some(Some(x)) = &mut Some(&mut Some(0)) {
    |                      ~
 
 error[E0308]: mismatched types
-  --> $DIR/feature-gate-ref_pat_eat_one_layer_2024.rs:25:22
+  --> $DIR/feature-gate-ref_pat_eat_one_layer_2024.rs:26:22
    |
 LL |     if let Some(Some(&x)) = &Some(&Some(0)) {
    |                      ^^     --------------- this expression has type `&Option<&Option<{integer}>>`
@@ -89,7 +89,7 @@ LL |     if let Some(Some(x)) = &Some(&Some(0)) {
    |                      ~
 
 error[E0308]: mismatched types
-  --> $DIR/feature-gate-ref_pat_eat_one_layer_2024.rs:29:27
+  --> $DIR/feature-gate-ref_pat_eat_one_layer_2024.rs:30:27
    |
 LL |     if let Some(&mut Some(&x)) = &Some(&mut Some(0)) {
    |                           ^^     ------------------- this expression has type `&Option<&mut Option<{integer}>>`
@@ -104,7 +104,7 @@ LL |     if let Some(&mut Some(x)) = &Some(&mut Some(0)) {
    |                           ~
 
 error[E0308]: mismatched types
-  --> $DIR/feature-gate-ref_pat_eat_one_layer_2024.rs:33:23
+  --> $DIR/feature-gate-ref_pat_eat_one_layer_2024.rs:34:23
    |
 LL |     if let Some(&Some(&mut x)) = &mut Some(&Some(0)) {
    |                       ^^^^^^     ------------------- this expression has type `&mut Option<&Option<{integer}>>`
@@ -114,7 +114,7 @@ LL |     if let Some(&Some(&mut x)) = &mut Some(&Some(0)) {
    = note:           expected type `{integer}`
            found mutable reference `&mut _`
 note: to declare a mutable binding use: `mut x`
-  --> $DIR/feature-gate-ref_pat_eat_one_layer_2024.rs:33:23
+  --> $DIR/feature-gate-ref_pat_eat_one_layer_2024.rs:34:23
    |
 LL |     if let Some(&Some(&mut x)) = &mut Some(&Some(0)) {
    |                       ^^^^^^

tests/ui/match/ref_pat_eat_one_layer_2024/ref_pat_eat_one_layer_2024.rs

@@ -1,8 +1,10 @@
 //@ run-pass
 //@ edition: 2024
 //@ compile-flags: -Zunstable-options
+//@ revisions: classic structural both
 #![allow(incomplete_features)]
-#![feature(ref_pat_eat_one_layer_2024)]
+#![cfg_attr(any(classic, both), feature(ref_pat_eat_one_layer_2024))]
+#![cfg_attr(any(structural, both), feature(ref_pat_eat_one_layer_2024_structural))]
 
 pub fn main() {
     if let Some(Some(&x)) = &Some(&Some(0)) {
@@ -53,4 +55,12 @@ pub fn main() {
     if let Some(&Some(x)) = &mut Some(Some(0)) {
         let _: u32 = x;
     }
+    #[cfg(any(classic, both))]
+    if let Some(&mut x) = &mut Some(&0) {
+        let _: &u32 = x;
+    }
+    #[cfg(any(structural, both))]
+    if let Some(&mut x) = &Some(&mut 0) {
+        let _: &u32 = x;
+    }
 }