Match ergonomics 2024: Implement eat-one-layer

compiler/rustc_ast/src/ast.rs

@@ -36,6 +36,7 @@ use rustc_macros::HashStable_Generic;
 use rustc_span::source_map::{respan, Spanned};
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::{ErrorGuaranteed, Span, DUMMY_SP};
+use std::cmp;
 use std::fmt;
 use std::mem;
 use thin_vec::{thin_vec, ThinVec};
@@ -731,6 +732,13 @@ impl BindingAnnotation {
             Self::MUT_REF_MUT => "mut ref mut ",
         }
     }
+
+    pub fn cap_ref_mutability(mut self, mutbl: Mutability) -> Self {
+        if let ByRef::Yes(old_mutbl) = &mut self.0 {
+            *old_mutbl = cmp::min(*old_mutbl, mutbl);
+        }
+        self
+    }
 }
 
 #[derive(Clone, Encodable, Decodable, Debug)]

compiler/rustc_feature/src/unstable.rs

@@ -571,6 +571,8 @@ declare_features! (
     (unstable, proc_macro_hygiene, "1.30.0", Some(54727)),
     /// Allows `&raw const $place_expr` and `&raw mut $place_expr` expressions.
     (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, "CURRENT_RUSTC_VERSION", Some(123076)),
     /// Allows `&` and `&mut` patterns to consume match-ergonomics-inserted references.
     (incomplete, ref_pat_everywhere, "CURRENT_RUSTC_VERSION", Some(123076)),
     /// Allows using the `#[register_tool]` attribute.

compiler/rustc_hir_typeck/src/pat.rs

@@ -79,6 +79,7 @@ struct TopInfo<'tcx> {
 #[derive(Copy, Clone)]
 struct PatInfo<'tcx, 'a> {
     binding_mode: BindingAnnotation,
+    max_ref_mutbl: Mutability,
     top_info: TopInfo<'tcx>,
     decl_origin: Option<DeclOrigin<'a>>,
 
@@ -160,8 +161,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         decl_origin: Option<DeclOrigin<'tcx>>,
     ) {
         let info = TopInfo { expected, origin_expr, span };
-        let pat_info =
-            PatInfo { binding_mode: INITIAL_BM, top_info: info, decl_origin, current_depth: 0 };
+        let pat_info = PatInfo {
+            binding_mode: INITIAL_BM,
+            max_ref_mutbl: Mutability::Mut,
+            top_info: info,
+            decl_origin,
+            current_depth: 0,
+        };
         self.check_pat(pat, expected, pat_info);
     }
 
@@ -172,7 +178,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// Conversely, inside this module, `check_pat_top` should never be used.
     #[instrument(level = "debug", skip(self, pat_info))]
     fn check_pat(&self, pat: &'tcx Pat<'tcx>, expected: Ty<'tcx>, pat_info: PatInfo<'tcx, '_>) {
-        let PatInfo { binding_mode: def_bm, top_info: ti, current_depth, .. } = pat_info;
+        let PatInfo { binding_mode: def_bm, max_ref_mutbl, top_info: ti, current_depth, .. } =
+            pat_info;
 
         let path_res = match &pat.kind {
             PatKind::Path(qpath) => Some(
@@ -181,10 +188,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             _ => None,
         };
         let adjust_mode = self.calc_adjust_mode(pat, path_res.map(|(res, ..)| res));
-        let (expected, def_bm, ref_pattern_already_consumed) =
-            self.calc_default_binding_mode(pat, expected, def_bm, adjust_mode);
+        let (expected, def_bm, max_ref_mutbl, ref_pattern_already_consumed) =
+            self.calc_default_binding_mode(pat, expected, def_bm, adjust_mode, max_ref_mutbl);
         let pat_info = PatInfo {
             binding_mode: def_bm,
+            max_ref_mutbl,
             top_info: ti,
             decl_origin: pat_info.decl_origin,
             current_depth: current_depth + 1,
@@ -289,16 +297,42 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         expected: Ty<'tcx>,
         def_bm: BindingAnnotation,
         adjust_mode: AdjustMode,
-    ) -> (Ty<'tcx>, BindingAnnotation, bool) {
+        max_ref_mutbl: Mutability,
+    ) -> (Ty<'tcx>, BindingAnnotation, Mutability, bool) {
+        if let ByRef::Yes(mutbl) = def_bm.0 {
+            debug_assert!(mutbl <= max_ref_mutbl);
+        }
         match adjust_mode {
-            AdjustMode::Pass => (expected, def_bm, false),
-            AdjustMode::Reset => (expected, INITIAL_BM, false),
-            AdjustMode::ResetAndConsumeRef(mutbl) => {
-                (expected, INITIAL_BM, def_bm.0 == ByRef::Yes(mutbl))
+            AdjustMode::Pass => (expected, def_bm, max_ref_mutbl, false),
+            AdjustMode::Reset => (expected, INITIAL_BM, Mutability::Mut, false),
+            AdjustMode::ResetAndConsumeRef(ref_pat_mutbl) => {
+                let mutbls_match = def_bm.0 == ByRef::Yes(ref_pat_mutbl);
+                if pat.span.at_least_rust_2024() && self.tcx.features().ref_pat_eat_one_layer_2024 {
+                    if mutbls_match {
+                        debug!("consuming inherited reference");
+                        (expected, INITIAL_BM, cmp::min(max_ref_mutbl, ref_pat_mutbl), true)
+                    } else {
+                        let (new_ty, new_bm, max_ref_mutbl) = if ref_pat_mutbl == Mutability::Mut {
+                            self.peel_off_references(
+                                pat,
+                                expected,
+                                def_bm,
+                                Mutability::Not,
+                                max_ref_mutbl,
+                            )
+                        } else {
+                            (expected, def_bm.cap_ref_mutability(Mutability::Not), Mutability::Not)
+                        };
+                        (new_ty, new_bm, max_ref_mutbl, false)
+                    }
+                } else {
+                    (expected, INITIAL_BM, max_ref_mutbl, mutbls_match)
+                }
             }
             AdjustMode::Peel => {
-                let peeled = self.peel_off_references(pat, expected, def_bm);
-                (peeled.0, peeled.1, false)
+                let peeled =
+                    self.peel_off_references(pat, expected, def_bm, Mutability::Mut, max_ref_mutbl);
+                (peeled.0, peeled.1, peeled.2, false)
             }
         }
     }
@@ -379,7 +413,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         pat: &'tcx Pat<'tcx>,
         expected: Ty<'tcx>,
         mut def_bm: BindingAnnotation,
-    ) -> (Ty<'tcx>, BindingAnnotation) {
+        max_peelable_mutability: Mutability,
+        mut max_ref_mutability: Mutability,
+    ) -> (Ty<'tcx>, BindingAnnotation, Mutability) {
         let mut expected = self.try_structurally_resolve_type(pat.span, expected);
         // Peel off as many `&` or `&mut` from the scrutinee type as possible. For example,
         // for `match &&&mut Some(5)` the loop runs three times, aborting when it reaches
@@ -390,7 +426,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         //
         // See the examples in `ui/match-defbm*.rs`.
         let mut pat_adjustments = vec![];
-        while let ty::Ref(_, inner_ty, inner_mutability) = *expected.kind() {
+        while let ty::Ref(_, inner_ty, inner_mutability) = *expected.kind()
+            && inner_mutability <= max_peelable_mutability
+        {
             debug!("inspecting {:?}", expected);
 
             debug!("current discriminant is Ref, inserting implicit deref");
@@ -410,6 +448,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             });
         }
 
+        if pat.span.at_least_rust_2024() && self.tcx.features().ref_pat_eat_one_layer_2024 {
+            def_bm = def_bm.cap_ref_mutability(max_ref_mutability);
+            if def_bm.0 == ByRef::Yes(Mutability::Not) {
+                max_ref_mutability = Mutability::Not;
+            }
+        }
+
         if !pat_adjustments.is_empty() {
             debug!("default binding mode is now {:?}", def_bm);
             self.typeck_results
@@ -418,7 +463,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 .insert(pat.hir_id, pat_adjustments);
         }
 
-        (expected, def_bm)
+        (expected, def_bm, max_ref_mutability)
     }
 
     fn check_pat_lit(
@@ -1094,15 +1139,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         expected: Ty<'tcx>,
         pat_info: PatInfo<'tcx, '_>,
     ) -> Ty<'tcx> {
-        let PatInfo { binding_mode: def_bm, top_info: ti, decl_origin, current_depth } = pat_info;
         let tcx = self.tcx;
         let on_error = |e| {
             for pat in subpats {
-                self.check_pat(
-                    pat,
-                    Ty::new_error(tcx, e),
-                    PatInfo { binding_mode: def_bm, top_info: ti, decl_origin, current_depth },
-                );
+                self.check_pat(pat, Ty::new_error(tcx, e), pat_info);
             }
         };
         let report_unexpected_res = |res: Res| {
@@ -1147,7 +1187,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let pat_ty = pat_ty.no_bound_vars().expect("expected fn type");
 
         // Type-check the tuple struct pattern against the expected type.
-        let diag = self.demand_eqtype_pat_diag(pat.span, expected, pat_ty, ti);
+        let diag = self.demand_eqtype_pat_diag(pat.span, expected, pat_ty, pat_info.top_info);
         let had_err = if let Some(err) = diag {
             err.emit();
             true
@@ -1165,11 +1205,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             for (i, subpat) in subpats.iter().enumerate_and_adjust(variant.fields.len(), ddpos) {
                 let field = &variant.fields[FieldIdx::from_usize(i)];
                 let field_ty = self.field_ty(subpat.span, field, args);
-                self.check_pat(
-                    subpat,
-                    field_ty,
-                    PatInfo { binding_mode: def_bm, top_info: ti, decl_origin, current_depth },
-                );
+                self.check_pat(subpat, field_ty, pat_info);
 
                 self.tcx.check_stability(
                     variant.fields[FieldIdx::from_usize(i)].did,
@@ -2056,61 +2092,70 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         pat_info: PatInfo<'tcx, '_>,
         consumed_inherited_ref: bool,
     ) -> Ty<'tcx> {
-        let tcx = self.tcx;
-        let expected = self.shallow_resolve(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
-                // out to be equally general. See (note_1) for details.
-
-                // Take region, inner-type from expected type if we can,
-                // to avoid creating needless variables. This also helps with
-                // the bad interactions of the given hack detailed in (note_1).
-                debug!("check_pat_ref: expected={:?}", expected);
-                match *expected.kind() {
-                    ty::Ref(_, r_ty, r_mutbl) if r_mutbl == mutbl => (expected, r_ty),
-                    _ => {
-                        if consumed_inherited_ref && self.tcx.features().ref_pat_everywhere {
-                            // We already matched against a match-ergonmics inserted reference,
-                            // so we don't need to match against a reference from the original type.
-                            // Save this infor for use in lowering later
-                            self.typeck_results
-                                .borrow_mut()
-                                .skipped_ref_pats_mut()
-                                .insert(pat.hir_id);
-                            (expected, expected)
-                        } else {
-                            let inner_ty = self.next_ty_var(TypeVariableOrigin {
-                                param_def_id: None,
-                                span: inner.span,
-                            });
-                            let ref_ty = self.new_ref_ty(pat.span, mutbl, inner_ty);
-                            debug!("check_pat_ref: demanding {:?} = {:?}", expected, ref_ty);
-                            let err = self.demand_eqtype_pat_diag(
-                                pat.span,
-                                expected,
-                                ref_ty,
-                                pat_info.top_info,
-                            );
+        if consumed_inherited_ref
+            && pat.span.at_least_rust_2024()
+            && self.tcx.features().ref_pat_eat_one_layer_2024
+        {
+            self.typeck_results.borrow_mut().skipped_ref_pats_mut().insert(pat.hir_id);
+            self.check_pat(inner, expected, pat_info);
+            expected
+        } else {
+            let tcx = self.tcx;
+            let expected = self.shallow_resolve(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
+                    // out to be equally general. See (note_1) for details.
+
+                    // Take region, inner-type from expected type if we can,
+                    // to avoid creating needless variables. This also helps with
+                    // the bad interactions of the given hack detailed in (note_1).
+                    debug!("check_pat_ref: expected={:?}", expected);
+                    match *expected.kind() {
+                        ty::Ref(_, r_ty, r_mutbl) if r_mutbl == mutbl => (expected, r_ty),
+                        _ => {
+                            if consumed_inherited_ref && self.tcx.features().ref_pat_everywhere {
+                                // We already matched against a match-ergonmics inserted reference,
+                                // so we don't need to match against a reference from the original type.
+                                // Save this infor for use in lowering later
+                                self.typeck_results
+                                    .borrow_mut()
+                                    .skipped_ref_pats_mut()
+                                    .insert(pat.hir_id);
+                                (expected, expected)
+                            } else {
+                                let inner_ty = self.next_ty_var(TypeVariableOrigin {
+                                    param_def_id: None,
+                                    span: inner.span,
+                                });
+                                let ref_ty = self.new_ref_ty(pat.span, mutbl, inner_ty);
+                                debug!("check_pat_ref: demanding {:?} = {:?}", expected, ref_ty);
+                                let err = self.demand_eqtype_pat_diag(
+                                    pat.span,
+                                    expected,
+                                    ref_ty,
+                                    pat_info.top_info,
+                                );
 
-                            // Look for a case like `fn foo(&foo: u32)` and suggest
-                            // `fn foo(foo: &u32)`
-                            if let Some(mut err) = err {
-                                self.borrow_pat_suggestion(&mut err, pat);
-                                err.emit();
+                                // Look for a case like `fn foo(&foo: u32)` and suggest
+                                // `fn foo(foo: &u32)`
+                                if let Some(mut err) = err {
+                                    self.borrow_pat_suggestion(&mut err, pat);
+                                    err.emit();
+                                }
+                                (ref_ty, inner_ty)
                             }
-                            (ref_ty, inner_ty)
                         }
                     }
                 }
-            }
-            Err(guar) => {
-                let err = Ty::new_error(tcx, guar);
-                (err, err)
-            }
-        };
-        self.check_pat(inner, inner_ty, pat_info);
-        ref_ty
+                Err(guar) => {
+                    let err = Ty::new_error(tcx, guar);
+                    (err, err)
+                }
+            };
+            self.check_pat(inner, inner_ty, pat_info);
+            ref_ty
+        }
     }
 
     /// Create a reference type with a fresh region variable.

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

@@ -69,7 +69,7 @@ impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
             hir::PatKind::Ref(inner, _)
                 if self.typeck_results.skipped_ref_pats().contains(pat.hir_id) =>
             {
-                self.lower_pattern_unadjusted(inner)
+                self.lower_pattern(inner)
             }
             _ => self.lower_pattern_unadjusted(pat),
         };

compiler/rustc_span/src/symbol.rs

@@ -1460,6 +1460,7 @@ symbols! {
         receiver,
         recursion_limit,
         reexport_test_harness_main,
+        ref_pat_eat_one_layer_2024,
         ref_pat_everywhere,
         ref_unwind_safe_trait,
         reference,

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

@@ -0,0 +1,37 @@
+//@ edition: 2024
+//@ compile-flags: -Zunstable-options
+
+pub fn main() {
+    if let Some(Some(&x)) = &Some(&Some(0)) {
+        //~^ ERROR: mismatched types
+        let _: u32 = x;
+    }
+    if let Some(Some(&x)) = &Some(Some(&0)) {
+        let _: &u32 = x;
+        //~^ ERROR: mismatched types
+    }
+    if let Some(Some(&&x)) = &Some(Some(&0)) {
+        //~^ ERROR: mismatched types
+        let _: u32 = x;
+    }
+    if let Some(&Some(x)) = &Some(Some(0)) {
+        //~^ ERROR: mismatched types
+        let _: u32 = x;
+    }
+    if let Some(Some(&mut x)) = &mut Some(&mut Some(0)) {
+        //~^ ERROR: mismatched types
+        let _: u32 = x;
+    }
+    if let Some(Some(&x)) = &Some(&Some(0)) {
+        //~^ ERROR: mismatched types
+        let _: u32 = x;
+    }
+    if let Some(&mut Some(&x)) = &Some(&mut Some(0)) {
+        //~^ ERROR: mismatched types
+        let _: u32 = x;
+    }
+    if let Some(&Some(&mut x)) = &mut Some(&Some(0)) {
+        //~^ ERROR: mismatched types
+        let _: u32 = x;
+    }
+}