clarify wording of match ergonomics diagnostics

Author: dianne

compiler/rustc_hir_typeck/src/pat.rs

@@ -3145,11 +3145,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // so, we may want to inspect the span's source callee or macro backtrace.
             "occurs within macro expansion".to_owned()
         } else {
-            let dbm_str = match def_br_mutbl {
-                Mutability::Not => "ref",
-                Mutability::Mut => "ref mut",
-            };
-            format!("{pat_kind} not allowed under `{dbm_str}` default binding mode")
+            format!("explicit {pat_kind} not allowed within elided reference pattern")
         };
         info.primary_labels.push((trimmed_span, primary_label));
     }

compiler/rustc_lint_defs/src/builtin.rs

@@ -1599,7 +1599,7 @@ declare_lint! {
     "detects patterns whose meaning will change in Rust 2024",
     @future_incompatible = FutureIncompatibleInfo {
         reason: FutureIncompatibilityReason::EditionSemanticsChange(Edition::Edition2024),
-        reference: "<https://doc.rust-lang.org/nightly/edition-guide/rust-2024/match-ergonomics.html>",
+        reference: "<https://doc.rust-lang.org/edition-guide/rust-2024/match-ergonomics.html>",
     };
 }
 

compiler/rustc_mir_build/messages.ftl

@@ -318,13 +318,13 @@ mir_build_pointer_pattern = function pointers and raw pointers not derived from
 
 mir_build_privately_uninhabited = pattern `{$witness_1}` is currently uninhabited, but this variant contains private fields which may become inhabited in the future
 
-mir_build_rust_2024_incompatible_pat = {$bad_modifiers ->
+mir_build_rust_2024_incompatible_pat = explicit {$bad_modifiers ->
         *[true] binding modifiers{$bad_ref_pats ->
             *[true] {" "}and reference patterns
             [false] {""}
         }
         [false] reference patterns
-    } may only be written when the default binding mode is `move`{$is_hard_error ->
+    } may not be written within elided reference patterns{$is_hard_error ->
         *[true] {""}
         [false] {" "}in Rust 2024
     }

compiler/rustc_mir_build/src/errors.rs

@@ -1113,6 +1113,7 @@ pub(crate) struct Rust2024IncompatiblePatSugg {
     pub(crate) suggestion: Vec<(Span, String)>,
     pub(crate) ref_pattern_count: usize,
     pub(crate) binding_mode_count: usize,
+    pub(crate) bad_modifiers: bool,
     /// Labels for where incompatibility-causing by-ref default binding modes were introduced.
     pub(crate) default_mode_labels: FxIndexMap<Span, ty::Mutability>,
 }
@@ -1124,12 +1125,19 @@ impl Subdiagnostic for Rust2024IncompatiblePatSugg {
         for (span, def_br_mutbl) in self.default_mode_labels.into_iter().rev() {
             // Don't point to a macro call site.
             if !span.from_expansion() {
-                let note_msg = "matching on a reference type with a non-reference pattern changes the default binding mode";
-                let label_msg =
-                    format!("this matches on type `{}_`", def_br_mutbl.ref_prefix_str());
-                let mut label = MultiSpan::from(span);
-                label.push_span_label(span, label_msg);
-                diag.span_note(label, note_msg);
+                let label_msg = format!(
+                    "this matches on a reference type `{}_` without a reference pattern",
+                    def_br_mutbl.ref_prefix_str()
+                );
+                if self.bad_modifiers {
+                    // Only explain default binding modes if pointing to redundant/conflicting modifiers.
+                    let note_msg = "matching on a reference type with a non-reference pattern makes variables within bind by reference";
+                    let mut label = MultiSpan::from(span);
+                    label.push_span_label(span, label_msg);
+                    diag.span_note(label, note_msg);
+                } else {
+                    diag.span_label(span, label_msg);
+                }
             }
         }
 
@@ -1150,7 +1158,7 @@ impl Subdiagnostic for Rust2024IncompatiblePatSugg {
             } else {
                 String::new()
             };
-            format!("make the implied reference pattern{plural_derefs}{and_modes} explicit")
+            format!("make the elided reference pattern{plural_derefs}{and_modes} explicit")
         };
         // FIXME(dianne): for peace of mind, don't risk emitting a 0-part suggestion (that panics!)
         if !self.suggestion.is_empty() {

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

@@ -54,6 +54,7 @@ impl<'a> PatMigration<'a> {
             suggestion: self.suggestion,
             ref_pattern_count: self.ref_pattern_count,
             binding_mode_count: self.binding_mode_count,
+            bad_modifiers: self.info.bad_modifiers,
             default_mode_labels: self.default_mode_labels,
         };
         // If a relevant span is from at least edition 2024, this is a hard error.

tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/mixed-editions.classic2021.stderr

@@ -59,20 +59,20 @@ LL |     let [&bind_ref_mut!(x)] = &mut [0];
    |          |
    |          help: replace this `&` with `&mut`: `&mut`
 
-error: binding modifiers may only be written when the default binding mode is `move`
+error: explicit binding modifiers may not be written within elided reference patterns
   --> $DIR/mixed-editions.rs:30:10
    |
 LL |     let [bind_ref!(y)] = &[0];
    |          ^^^^^^^^^^^^ occurs within macro expansion
    |
-   = 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
+   = note: for more information, see <https://doc.rust-lang.org/edition-guide/rust-2024/match-ergonomics.html>
+note: matching on a reference type with a non-reference pattern makes variables within bind by reference
   --> $DIR/mixed-editions.rs:30:9
    |
 LL |     let [bind_ref!(y)] = &[0];
-   |         ^^^^^^^^^^^^^^ this matches on type `&_`
+   |         ^^^^^^^^^^^^^^ this matches on a reference type `&_` without a reference pattern
    = note: this error originates in the macro `bind_ref` (in Nightly builds, run with -Z macro-backtrace for more info)
-help: make the implied reference pattern explicit
+help: make the elided reference pattern explicit
    |
 LL |     let &[bind_ref!(y)] = &[0];
    |         +

tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/mixed-editions.classic2024.stderr

@@ -58,14 +58,14 @@ LL |     let [&bind_ref_mut!(x)] = &mut [0];
    |          |
    |          help: replace this `&` with `&mut`: `&mut`
 
-error: binding modifiers may only be written when the default binding mode is `move`
+error: explicit binding modifiers may not be written within elided reference patterns
   --> $DIR/mixed-editions.rs:26:21
    |
 LL |     let match_ctor!(ref x) = &[0];
-   |                     ^^^ binding modifier not allowed under `ref` default binding mode
+   |                     ^^^ explicit binding modifier not allowed within elided reference pattern
    |
-   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2024/match-ergonomics.html>
-help: make the implied reference pattern explicit
+   = note: for more information, see <https://doc.rust-lang.org/edition-guide/rust-2024/match-ergonomics.html>
+help: make the elided reference pattern explicit
   --> $DIR/auxiliary/mixed-editions-macros.rs:11:9
    |
 LL |         &[$p]

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

@@ -24,11 +24,11 @@ fn assert_type_eq<T, U: Eq<T>>(_: T, _: U) {}
 /// only when the binding is from edition 2024.
 fn ref_binding_tests() {
     let match_ctor!(ref x) = &[0];
-    //[classic2024,structural2024]~^ ERROR: binding modifiers may only be written when the default binding mode is `move`
+    //[classic2024,structural2024]~^ ERROR: explicit binding modifiers may not be written within elided reference patterns
     #[cfg(any(classic2021, structural2021))] assert_type_eq(x, &0u32);
 
     let [bind_ref!(y)] = &[0];
-    //[classic2021,structural2021]~^ ERROR: binding modifiers may only be written when the default binding mode is `move`
+    //[classic2021,structural2021]~^ ERROR: explicit binding modifiers may not be written within elided reference patterns
     #[cfg(any(classic2024, structural2024))] assert_type_eq(y, &0u32);
 }
 

tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/mixed-editions.structural2021.stderr

@@ -37,20 +37,20 @@ LL |     let [&bind_ref_mut!(x)] = &mut [0];
    |          |
    |          help: replace this `&` with `&mut`: `&mut`
 
-error: binding modifiers may only be written when the default binding mode is `move`
+error: explicit binding modifiers may not be written within elided reference patterns
   --> $DIR/mixed-editions.rs:30:10
    |
 LL |     let [bind_ref!(y)] = &[0];
    |          ^^^^^^^^^^^^ occurs within macro expansion
    |
-   = 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
+   = note: for more information, see <https://doc.rust-lang.org/edition-guide/rust-2024/match-ergonomics.html>
+note: matching on a reference type with a non-reference pattern makes variables within bind by reference
   --> $DIR/mixed-editions.rs:30:9
    |
 LL |     let [bind_ref!(y)] = &[0];
-   |         ^^^^^^^^^^^^^^ this matches on type `&_`
+   |         ^^^^^^^^^^^^^^ this matches on a reference type `&_` without a reference pattern
    = note: this error originates in the macro `bind_ref` (in Nightly builds, run with -Z macro-backtrace for more info)
-help: make the implied reference pattern explicit
+help: make the elided reference pattern explicit
    |
 LL |     let &[bind_ref!(y)] = &[0];
    |         +

tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/mixed-editions.structural2024.stderr

@@ -36,14 +36,14 @@ LL |     let [&bind_ref_mut!(x)] = &mut [0];
    |          |
    |          help: replace this `&` with `&mut`: `&mut`
 
-error: binding modifiers may only be written when the default binding mode is `move`
+error: explicit binding modifiers may not be written within elided reference patterns
   --> $DIR/mixed-editions.rs:26:21
    |
 LL |     let match_ctor!(ref x) = &[0];
-   |                     ^^^ binding modifier not allowed under `ref` default binding mode
+   |                     ^^^ explicit binding modifier not allowed within elided reference pattern
    |
-   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2024/match-ergonomics.html>
-help: make the implied reference pattern explicit
+   = note: for more information, see <https://doc.rust-lang.org/edition-guide/rust-2024/match-ergonomics.html>
+help: make the elided reference pattern explicit
   --> $DIR/auxiliary/mixed-editions-macros.rs:11:9
    |
 LL |         &[$p]