Match Ergonomics 2024: update old-edition behavior of feature gates

compiler/rustc_hir_typeck/src/pat.rs

@@ -230,10 +230,19 @@ enum InheritedRefMatchRule {
     /// underlying type is not a reference type, the inherited reference will be consumed.
     EatInner,
     /// When the underlying type is a reference type, reference patterns consume both layers of
-    /// reference, i.e. they both reset the binding mode and consume the reference type. Reference
-    /// patterns are not permitted when there is no underlying reference type, i.e. they can't eat
-    /// only an inherited reference. This is the current stable Rust behavior.
-    EatBoth,
+    /// reference, i.e. they both reset the binding mode and consume the reference type.
+    EatBoth {
+        /// If `true`, an inherited reference will be considered when determining whether a reference
+        /// pattern matches a given type:
+        /// - If the underlying type is not a reference, a reference pattern may eat the inherited reference;
+        /// - If the underlying type is a reference, a reference pattern matches if it can eat either one
+        ///    of the underlying and inherited references. E.g. a `&mut` pattern is allowed if either the
+        ///    underlying type is `&mut` or the inherited reference is `&mut`.
+        /// If `false`, a reference pattern is only matched against the underlying type.
+        /// This is `false` for stable Rust and `true` for both the `ref_pat_eat_one_layer_2024` and
+        /// `ref_pat_eat_one_layer_2024_structural` feature gates.
+        consider_inherited_ref: bool,
+    },
 }
 
 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
@@ -259,10 +268,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             } else {
                 // Currently, matching against an inherited ref on edition 2024 is an error.
                 // Use `EatBoth` as a fallback to be similar to stable Rust.
-                InheritedRefMatchRule::EatBoth
+                InheritedRefMatchRule::EatBoth { consider_inherited_ref: false }
             }
         } else {
-            InheritedRefMatchRule::EatBoth
+            InheritedRefMatchRule::EatBoth {
+                consider_inherited_ref: self.tcx.features().ref_pat_eat_one_layer_2024()
+                    || self.tcx.features().ref_pat_eat_one_layer_2024_structural(),
+            }
         }
     }
 
@@ -2371,6 +2383,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             // NB: This assumes that `&` patterns can match against mutable
                             // references (RFC 3627, Rule 5). If we implement a pattern typing
                             // ruleset with Rule 4 but not Rule 5, we'll need to check that here.
+                            // FIXME(ref_pat_eat_one_layer_2024_structural): If we already tried
+                            // matching the real reference, the error message should explain that
+                            // falling back to the inherited reference didn't work. This should be
+                            // the same error as the old-Edition version below.
                             debug_assert!(ref_pat_matches_mut_ref);
                             self.error_inherited_ref_mutability_mismatch(pat, pat_prefix_span);
                         }
@@ -2381,9 +2397,46 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         return expected;
                     }
                 }
-                InheritedRefMatchRule::EatBoth => {
+                InheritedRefMatchRule::EatBoth { consider_inherited_ref: true } => {
                     // Reset binding mode on old editions
                     pat_info.binding_mode = ByRef::No;
+
+                    if let ty::Ref(_, inner_ty, _) = *expected.kind() {
+                        // Consume both the inherited and inner references.
+                        if pat_mutbl.is_mut() && inh_mut.is_mut() {
+                            // As a special case, a `&mut` reference pattern will be able to match
+                            // against a reference type of any mutability if the inherited ref is
+                            // mutable. Since this allows us to match against a shared reference
+                            // type, we refer to this as "falling back" to matching the inherited
+                            // reference, though we consume the real reference as well. We handle
+                            // this here to avoid adding this case to the common logic below.
+                            self.check_pat(inner, inner_ty, pat_info);
+                            return expected;
+                        } else {
+                            // Otherwise, use the common logic below for matching the inner
+                            // reference type.
+                            // FIXME(ref_pat_eat_one_layer_2024_structural): If this results in a
+                            // mutability mismatch, the error message should explain that falling
+                            // back to the inherited reference didn't work. This should be the same
+                            // error as the Edition 2024 version above.
+                        }
+                    } else {
+                        // The expected type isn't a reference type, so only match against the
+                        // inherited reference.
+                        if pat_mutbl > inh_mut {
+                            // We can't match a lone inherited shared reference with `&mut`.
+                            self.error_inherited_ref_mutability_mismatch(pat, pat_prefix_span);
+                        }
+
+                        self.typeck_results.borrow_mut().skipped_ref_pats_mut().insert(pat.hir_id);
+                        self.check_pat(inner, expected, pat_info);
+                        return expected;
+                    }
+                }
+                InheritedRefMatchRule::EatBoth { consider_inherited_ref: false } => {
+                    // Reset binding mode on stable Rust. This will be a type error below if
+                    // `expected` is not a reference type.
+                    pat_info.binding_mode = ByRef::No;
                     self.add_rust_2024_migration_desugared_pat(
                         pat_info.top_info.hir_id,
                         pat,

src/doc/unstable-book/src/language-features/ref-pat-eat-one-layer-2024-structural.md

@@ -10,10 +10,11 @@ This feature is incomplete and not yet intended for general use.
 
 This implements experimental, Edition-dependent match ergonomics under consideration for inclusion
 in Rust.
-For more information, see the corresponding typing rules for [Editions 2024 and later].
-On earlier Editions, the current behavior is unspecified.
+For more information, see the corresponding typing rules for [Editions 2021 and earlier] and for
+[Editions 2024 and later].
 
 For alternative experimental match ergonomics, see the feature
 [`ref_pat_eat_one_layer_2024`](./ref-pat-eat-one-layer-2024.md).
 
+[Editions 2021 and earlier]: https://nadrieril.github.io/typing-rust-patterns/?compare=false&opts1=AQEBAQIBAQEBAAAAAAAAAAAAAAAAAAA%3D&mode=rules&do_cmp=false
 [Editions 2024 and later]: https://nadrieril.github.io/typing-rust-patterns/?compare=false&opts1=AQEBAgEBAQEBAgIAAAAAAAAAAAAAAAA%3D&mode=rules&do_cmp=false

src/doc/unstable-book/src/language-features/ref-pat-eat-one-layer-2024.md

@@ -10,10 +10,11 @@ This feature is incomplete and not yet intended for general use.
 
 This implements experimental, Edition-dependent match ergonomics under consideration for inclusion
 in Rust.
-For more information, see the corresponding typing rules for [Editions 2024 and later].
-On earlier Editions, the current behavior is unspecified.
+For more information, see the corresponding typing rules for [Editions 2021 and earlier] and for
+[Editions 2024 and later].
 
 For alternative experimental match ergonomics, see the feature
 [`ref_pat_eat_one_layer_2024_structural`](./ref-pat-eat-one-layer-2024-structural.md).
 
+[Editions 2021 and earlier]: https://nadrieril.github.io/typing-rust-patterns/?compare=false&opts1=AQEBAQIBAQABAAAAAQEBAAEBAAABAAA%3D&mode=rules&do_cmp=false
 [Editions 2024 and later]: https://nadrieril.github.io/typing-rust-patterns/?compare=false&opts1=AQEBAAABAQABAgIAAQEBAAEBAAABAAA%3D&mode=rules&do_cmp=false

tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/auxiliary/mixed-editions-macros.rs

@@ -0,0 +1,48 @@
+//@[classic2021] edition: 2024
+//@[structural2021] edition: 2024
+//@[classic2024] edition: 2021
+//@[structural2024] edition: 2021
+//! This contains macros in an edition *different* to the one used in `../mixed-editions.rs`, in
+//! order to test typing mixed-edition patterns.
+
+#[macro_export]
+macro_rules! match_ctor {
+    ($p:pat) => {
+        [$p]
+    };
+}
+
+#[macro_export]
+macro_rules! match_ref {
+    ($p:pat) => {
+        &$p
+    };
+}
+
+#[macro_export]
+macro_rules! bind {
+    ($i:ident) => {
+        $i
+    }
+}
+
+#[macro_export]
+macro_rules! bind_ref {
+    ($i:ident) => {
+        ref $i
+    }
+}
+
+#[macro_export]
+macro_rules! bind_mut {
+    ($i:ident) => {
+        mut $i
+    }
+}
+
+#[macro_export]
+macro_rules! bind_ref_mut {
+    ($i:ident) => {
+        ref mut $i
+    }
+}

tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/borrowck-errors.classic2021.stderr

@@ -0,0 +1,37 @@
+error[E0507]: cannot move out of a shared reference
+  --> $DIR/borrowck-errors.rs:31:29
+   |
+LL |     if let Some(&Some(x)) = Some(&Some(&mut 0)) {
+   |                       -     ^^^^^^^^^^^^^^^^^^^
+   |                       |
+   |                       data moved here
+   |                       move occurs because `x` has type `&mut u32`, which does not implement the `Copy` trait
+   |
+help: consider removing the borrow
+   |
+LL -     if let Some(&Some(x)) = Some(&Some(&mut 0)) {
+LL +     if let Some(Some(x)) = Some(&Some(&mut 0)) {
+   |
+
+error[E0596]: cannot borrow data in a `&` reference as mutable
+  --> $DIR/borrowck-errors.rs:36:10
+   |
+LL |     let &ref mut x = &0;
+   |          ^^^^^^^^^ cannot borrow as mutable
+
+error[E0596]: cannot borrow data in a `&` reference as mutable
+  --> $DIR/borrowck-errors.rs:41:23
+   |
+LL |     if let &Some(Some(x)) = &Some(&mut Some(0)) {
+   |                       ^ cannot borrow as mutable
+
+error[E0596]: cannot borrow data in a `&` reference as mutable
+  --> $DIR/borrowck-errors.rs:46:11
+   |
+LL |     let &[x] = &&mut [0];
+   |           ^ cannot borrow as mutable
+
+error: aborting due to 4 previous errors
+
+Some errors have detailed explanations: E0507, E0596.
+For more information about an error, try `rustc --explain E0507`.

tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/borrowck-errors.classic2024.stderr

@@ -1,33 +1,33 @@
-error[E0508]: cannot move out of type `[&mut u32; 1]`, a non-copy array
-  --> $DIR/borrowck-errors.rs:13:16
+error[E0508]: cannot move out of type `[&mut i32; 1]`, a non-copy array
+  --> $DIR/borrowck-errors.rs:15:16
    |
 LL |     let [&x] = &[&mut 0];
    |           -    ^^^^^^^^^ cannot move out of here
    |           |
    |           data moved here
-   |           move occurs because `x` has type `&mut u32`, which does not implement the `Copy` trait
+   |           move occurs because `x` has type `&mut i32`, which does not implement the `Copy` trait
    |
 help: consider borrowing the pattern binding
    |
 LL |     let [&ref x] = &[&mut 0];
    |           +++
 
-error[E0508]: cannot move out of type `[&mut u32; 1]`, a non-copy array
-  --> $DIR/borrowck-errors.rs:19:16
+error[E0508]: cannot move out of type `[&mut i32; 1]`, a non-copy array
+  --> $DIR/borrowck-errors.rs:22:16
    |
 LL |     let [&x] = &mut [&mut 0];
    |           -    ^^^^^^^^^^^^^ cannot move out of here
    |           |
    |           data moved here
-   |           move occurs because `x` has type `&mut u32`, which does not implement the `Copy` trait
+   |           move occurs because `x` has type `&mut i32`, which does not implement the `Copy` trait
    |
 help: consider borrowing the pattern binding
    |
 LL |     let [&ref x] = &mut [&mut 0];
    |           +++
 
 error[E0507]: cannot move out of a shared reference
-  --> $DIR/borrowck-errors.rs:27:29
+  --> $DIR/borrowck-errors.rs:31:29
    |
 LL |     if let Some(&Some(x)) = Some(&Some(&mut 0)) {
    |                       -     ^^^^^^^^^^^^^^^^^^^
@@ -42,25 +42,25 @@ LL +     if let Some(Some(x)) = Some(&Some(&mut 0)) {
    |
 
 error[E0596]: cannot borrow data in a `&` reference as mutable
-  --> $DIR/borrowck-errors.rs:32:10
+  --> $DIR/borrowck-errors.rs:36:10
    |
 LL |     let &ref mut x = &0;
    |          ^^^^^^^^^ cannot borrow as mutable
 
 error[E0596]: cannot borrow data in a `&` reference as mutable
-  --> $DIR/borrowck-errors.rs:37:23
+  --> $DIR/borrowck-errors.rs:41:23
    |
 LL |     if let &Some(Some(x)) = &Some(&mut Some(0)) {
    |                       ^ cannot borrow as mutable
 
 error[E0596]: cannot borrow data in a `&` reference as mutable
-  --> $DIR/borrowck-errors.rs:42:11
+  --> $DIR/borrowck-errors.rs:46:11
    |
 LL |     let &[x] = &&mut [0];
    |           ^ cannot borrow as mutable
 
 error[E0508]: cannot move out of type `[&mut i32; 1]`, a non-copy array
-  --> $DIR/borrowck-errors.rs:46:20
+  --> $DIR/borrowck-errors.rs:50:20
    |
 LL |     let [&mut x] = &mut [&mut 0];
    |               -    ^^^^^^^^^^^^^ cannot move out of here

tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/borrowck-errors.rs

@@ -1,11 +1,13 @@
-//@ revisions: stable2021 classic2024 structural2024
+//@ revisions: stable2021 classic2021 structural2021 classic2024 structural2024
 //@[stable2021] edition: 2021
+//@[classic2021] edition: 2021
+//@[structural2021] edition: 2021
 //@[classic2024] edition: 2024
 //@[structural2024] edition: 2024
 //! Tests for pattern errors not handled by the pattern typing rules, but by borrowck.
 #![allow(incomplete_features)]
-#![cfg_attr(classic2024, feature(ref_pat_eat_one_layer_2024))]
-#![cfg_attr(structural2024, feature(ref_pat_eat_one_layer_2024_structural))]
+#![cfg_attr(any(classic2021, classic2024), feature(ref_pat_eat_one_layer_2024))]
+#![cfg_attr(any(structural2021, structural2024), feature(ref_pat_eat_one_layer_2024_structural))]
 
 /// These patterns additionally use `&` to match a `&mut` reference type, which causes compilation
 /// to fail in HIR typeck on stable. As such, they need to be separate from the other tests.
@@ -14,13 +16,15 @@ fn errors_caught_in_hir_typeck_on_stable() {
     //[stable2021]~^ mismatched types
     //[stable2021]~| types differ in mutability
     //[classic2024]~^^^ ERROR: cannot move out of type
-    let _: &u32 = x;
+    #[cfg(any(classic2021, structural2021))] let _: u32 = x;
+    #[cfg(structural2024)] let _: &u32 = x;
 
     let [&x] = &mut [&mut 0];
     //[stable2021]~^ mismatched types
     //[stable2021]~| types differ in mutability
     //[classic2024]~^^^ ERROR: cannot move out of type
-    let _: &u32 = x;
+    #[cfg(any(classic2021, structural2021))] let _: u32 = x;
+    #[cfg(structural2024)] let _: &u32 = x;
 }
 
 pub fn main() {
@@ -35,16 +39,16 @@ pub fn main() {
     // For 2021 edition, this is also a regression test for #136223
     // since the maximum mutability is downgraded during the pattern check process.
     if let &Some(Some(x)) = &Some(&mut Some(0)) {
-        //[stable2021,classic2024]~^ ERROR: cannot borrow data in a `&` reference as mutable
-        let _: &u32 = x;
+        //[stable2021,classic2021,classic2024]~^ ERROR: cannot borrow data in a `&` reference as mutable
+        #[cfg(any(structural2021, structural2024))] let _: &u32 = x;
     }
 
     let &[x] = &&mut [0];
-    //[stable2021,classic2024]~^ ERROR: cannot borrow data in a `&` reference as mutable
-    let _: &u32 = x;
+    //[stable2021,classic2021,classic2024]~^ ERROR: cannot borrow data in a `&` reference as mutable
+    #[cfg(any(structural2021, structural2024))] let _: &u32 = x;
 
     let [&mut x] = &mut [&mut 0];
     //[classic2024]~^ ERROR: cannot move out of type
-    #[cfg(stable2021)] let _: u32 = x;
+    #[cfg(any(stable2021, classic2021, structural2021))] let _: u32 = x;
     #[cfg(structural2024)] let _: &mut u32 = x;
 }

tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/borrowck-errors.stable2021.stderr

@@ -1,5 +1,5 @@
 error[E0308]: mismatched types
-  --> $DIR/borrowck-errors.rs:13:10
+  --> $DIR/borrowck-errors.rs:15:10
    |
 LL |     let [&x] = &[&mut 0];
    |          ^^    --------- this expression has type `&[&mut {integer}; 1]`
@@ -15,7 +15,7 @@ LL +     let [x] = &[&mut 0];
    |
 
 error[E0308]: mismatched types
-  --> $DIR/borrowck-errors.rs:19:10
+  --> $DIR/borrowck-errors.rs:22:10
    |
 LL |     let [&x] = &mut [&mut 0];
    |          ^^    ------------- this expression has type `&mut [&mut {integer}; 1]`
@@ -31,7 +31,7 @@ LL +     let [x] = &mut [&mut 0];
    |
 
 error[E0507]: cannot move out of a shared reference
-  --> $DIR/borrowck-errors.rs:27:29
+  --> $DIR/borrowck-errors.rs:31:29
    |
 LL |     if let Some(&Some(x)) = Some(&Some(&mut 0)) {
    |                       -     ^^^^^^^^^^^^^^^^^^^
@@ -46,19 +46,19 @@ LL +     if let Some(Some(x)) = Some(&Some(&mut 0)) {
    |
 
 error[E0596]: cannot borrow data in a `&` reference as mutable
-  --> $DIR/borrowck-errors.rs:32:10
+  --> $DIR/borrowck-errors.rs:36:10
    |
 LL |     let &ref mut x = &0;
    |          ^^^^^^^^^ cannot borrow as mutable
 
 error[E0596]: cannot borrow data in a `&` reference as mutable
-  --> $DIR/borrowck-errors.rs:37:23
+  --> $DIR/borrowck-errors.rs:41:23
    |
 LL |     if let &Some(Some(x)) = &Some(&mut Some(0)) {
    |                       ^ cannot borrow as mutable
 
 error[E0596]: cannot borrow data in a `&` reference as mutable
-  --> $DIR/borrowck-errors.rs:42:11
+  --> $DIR/borrowck-errors.rs:46:11
    |
 LL |     let &[x] = &&mut [0];
    |           ^ cannot borrow as mutable