Clarify the match ergonomics 2024 migration lint's output

compiler/rustc_hir_typeck/src/pat.rs

@@ -718,25 +718,25 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     BindingMode(def_br, Mutability::Mut)
                 } else {
                     // `mut` resets the binding mode on edition <= 2021
-                    *self
-                        .typeck_results
-                        .borrow_mut()
-                        .rust_2024_migration_desugared_pats_mut()
-                        .entry(pat_info.top_info.hir_id)
-                        .or_default() |= pat.span.at_least_rust_2024();
+                    self.add_rust_2024_migration_desugared_pat(
+                        pat_info.top_info.hir_id,
+                        pat.span,
+                        ident.span,
+                        "requires binding by-value, but the implicit default is by-reference",
+                    );
                     BindingMode(ByRef::No, Mutability::Mut)
                 }
             }
             BindingMode(ByRef::No, mutbl) => BindingMode(def_br, mutbl),
             BindingMode(ByRef::Yes(_), _) => {
                 if matches!(def_br, ByRef::Yes(_)) {
                     // `ref`/`ref mut` overrides the binding mode on edition <= 2021
-                    *self
-                        .typeck_results
-                        .borrow_mut()
-                        .rust_2024_migration_desugared_pats_mut()
-                        .entry(pat_info.top_info.hir_id)
-                        .or_default() |= pat.span.at_least_rust_2024();
+                    self.add_rust_2024_migration_desugared_pat(
+                        pat_info.top_info.hir_id,
+                        pat.span,
+                        ident.span,
+                        "cannot override to bind by-reference when that is the implicit default",
+                    );
                 }
                 user_bind_annot
             }
@@ -2266,12 +2266,12 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // Reset binding mode on old editions
             if pat_info.binding_mode != ByRef::No {
                 pat_info.binding_mode = ByRef::No;
-                *self
-                    .typeck_results
-                    .borrow_mut()
-                    .rust_2024_migration_desugared_pats_mut()
-                    .entry(pat_info.top_info.hir_id)
-                    .or_default() |= pat.span.at_least_rust_2024();
+                self.add_rust_2024_migration_desugared_pat(
+                    pat_info.top_info.hir_id,
+                    pat.span,
+                    inner.span,
+                    "cannot implicitly match against multiple layers of reference",
+                )
             }
         }
 
@@ -2630,4 +2630,39 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             _ => (false, ty),
         }
     }
+
+    /// Record a pattern that's invalid under Rust 2024 match ergonomics, along with a problematic
+    /// span, so that the pattern migration lint can desugar it during THIR construction.
+    fn add_rust_2024_migration_desugared_pat(
+        &self,
+        pat_id: HirId,
+        subpat_span: Span,
+        cutoff_span: Span,
+        detailed_label: &str,
+    ) {
+        // Try to trim the span we're labeling to just the `&` or binding mode that's an issue.
+        // If the subpattern's span is is from an expansion, the emitted label will not be trimmed.
+        let source_map = self.tcx.sess.source_map();
+        let cutoff_span = source_map
+            .span_extend_prev_while(cutoff_span, char::is_whitespace)
+            .unwrap_or(cutoff_span);
+        // Ensure we use the syntax context and thus edition of `subpat_span`; this will be a hard
+        // error if the subpattern is of edition >= 2024.
+        let trimmed_span = subpat_span.until(cutoff_span).with_ctxt(subpat_span.ctxt());
+
+        // Only provide a detailed label if the problematic subpattern isn't from an expansion.
+        // In the case that it's from a macro, we'll add a more detailed note in the emitter.
+        let desc = if subpat_span.from_expansion() {
+            "default binding mode is reset within expansion"
+        } else {
+            detailed_label
+        };
+
+        self.typeck_results
+            .borrow_mut()
+            .rust_2024_migration_desugared_pats_mut()
+            .entry(pat_id)
+            .or_default()
+            .push((trimmed_span, desc.to_owned()));
+    }
 }

compiler/rustc_middle/src/ty/typeck_results.rs

@@ -74,9 +74,8 @@ pub struct TypeckResults<'tcx> {
     pat_binding_modes: ItemLocalMap<BindingMode>,
 
     /// Top-level patterns whose match ergonomics need to be desugared by the Rust 2021 -> 2024
-    /// migration lint. The boolean indicates whether the emitted diagnostic should be a hard error
-    /// (if any of the incompatible pattern elements are in edition 2024).
-    rust_2024_migration_desugared_pats: ItemLocalMap<bool>,
+    /// migration lint. Problematic subpatterns are stored in the `Vec` for the lint to highlight.
+    rust_2024_migration_desugared_pats: ItemLocalMap<Vec<(Span, String)>>,
 
     /// Stores the types which were implicitly dereferenced in pattern binding modes
     /// for later usage in THIR lowering. For example,
@@ -419,14 +418,18 @@ impl<'tcx> TypeckResults<'tcx> {
         LocalTableInContextMut { hir_owner: self.hir_owner, data: &mut self.pat_adjustments }
     }
 
-    pub fn rust_2024_migration_desugared_pats(&self) -> LocalTableInContext<'_, bool> {
+    pub fn rust_2024_migration_desugared_pats(
+        &self,
+    ) -> LocalTableInContext<'_, Vec<(Span, String)>> {
         LocalTableInContext {
             hir_owner: self.hir_owner,
             data: &self.rust_2024_migration_desugared_pats,
         }
     }
 
-    pub fn rust_2024_migration_desugared_pats_mut(&mut self) -> LocalTableInContextMut<'_, bool> {
+    pub fn rust_2024_migration_desugared_pats_mut(
+        &mut self,
+    ) -> LocalTableInContextMut<'_, Vec<(Span, String)>> {
         LocalTableInContextMut {
             hir_owner: self.hir_owner,
             data: &mut self.rust_2024_migration_desugared_pats,

compiler/rustc_mir_build/messages.ftl

@@ -285,7 +285,7 @@ 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 = patterns are not allowed to reset the default binding mode in edition 2024
+mir_build_rust_2024_incompatible_pat = this pattern relies on behavior which may change in edition 2024
 
 mir_build_rustc_box_attribute_error = `#[rustc_box]` attribute used incorrectly
     .attributes = no other attributes may be applied

compiler/rustc_mir_build/src/errors.rs

@@ -1,7 +1,7 @@
 use rustc_errors::codes::*;
 use rustc_errors::{
     Applicability, Diag, DiagArgValue, DiagCtxtHandle, Diagnostic, EmissionGuarantee, Level,
-    MultiSpan, SubdiagMessageOp, Subdiagnostic,
+    MultiSpan, SubdiagMessageOp, Subdiagnostic, pluralize,
 };
 use rustc_macros::{Diagnostic, LintDiagnostic, Subdiagnostic};
 use rustc_middle::ty::{self, Ty};
@@ -1089,18 +1089,20 @@ pub(crate) enum RustcBoxAttrReason {
 
 #[derive(LintDiagnostic)]
 #[diag(mir_build_rust_2024_incompatible_pat)]
-pub(crate) struct Rust2024IncompatiblePat {
+pub(crate) struct Rust2024IncompatiblePat<'a> {
     #[subdiagnostic]
-    pub(crate) sugg: Rust2024IncompatiblePatSugg,
+    pub(crate) sugg: Rust2024IncompatiblePatSugg<'a>,
 }
 
-pub(crate) struct Rust2024IncompatiblePatSugg {
+pub(crate) struct Rust2024IncompatiblePatSugg<'a> {
     pub(crate) suggestion: Vec<(Span, String)>,
-    /// Whether the incompatibility is a hard error because a relevant span is in edition 2024.
-    pub(crate) is_hard_error: bool,
+    pub(crate) ref_pattern_count: usize,
+    pub(crate) binding_mode_count: usize,
+    /// Labeled spans for subpatterns invalid in Rust 2024.
+    pub(crate) labels: &'a [(Span, String)],
 }
 
-impl Subdiagnostic for Rust2024IncompatiblePatSugg {
+impl<'a> Subdiagnostic for Rust2024IncompatiblePatSugg<'a> {
     fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
         self,
         diag: &mut Diag<'_, G>,
@@ -1112,6 +1114,16 @@ impl Subdiagnostic for Rust2024IncompatiblePatSugg {
             } else {
                 Applicability::MaybeIncorrect
             };
-        diag.multipart_suggestion("desugar the match ergonomics", self.suggestion, applicability);
+        let plural_derefs = pluralize!(self.ref_pattern_count);
+        let and_modes = if self.binding_mode_count > 0 {
+            format!(" and variable binding mode{}", pluralize!(self.binding_mode_count))
+        } else {
+            String::new()
+        };
+        diag.multipart_suggestion_verbose(
+            format!("make the implied reference pattern{plural_derefs}{and_modes} explicit"),
+            self.suggestion,
+            applicability,
+        );
     }
 }

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

@@ -6,6 +6,7 @@ mod const_to_pat;
 use std::cmp::Ordering;
 
 use rustc_abi::{FieldIdx, Integer};
+use rustc_errors::MultiSpan;
 use rustc_errors::codes::*;
 use rustc_hir::def::{CtorOf, DefKind, Res};
 use rustc_hir::pat_util::EnumerateAndAdjustIterator;
@@ -34,7 +35,7 @@ struct PatCtxt<'a, 'tcx> {
     typeck_results: &'a ty::TypeckResults<'tcx>,
 
     /// Used by the Rust 2024 migration lint.
-    rust_2024_migration_suggestion: Option<Rust2024IncompatiblePatSugg>,
+    rust_2024_migration_suggestion: Option<Rust2024IncompatiblePatSugg<'a>>,
 }
 
 pub(super) fn pat_from_hir<'a, 'tcx>(
@@ -50,24 +51,36 @@ pub(super) fn pat_from_hir<'a, 'tcx>(
         rust_2024_migration_suggestion: typeck_results
             .rust_2024_migration_desugared_pats()
             .get(pat.hir_id)
-            .map(|&is_hard_error| Rust2024IncompatiblePatSugg {
+            .map(|labels| Rust2024IncompatiblePatSugg {
                 suggestion: Vec::new(),
-                is_hard_error,
+                ref_pattern_count: 0,
+                binding_mode_count: 0,
+                labels: labels.as_slice(),
             }),
     };
     let result = pcx.lower_pattern(pat);
     debug!("pat_from_hir({:?}) = {:?}", pat, result);
     if let Some(sugg) = pcx.rust_2024_migration_suggestion {
-        if sugg.is_hard_error {
+        let mut spans = MultiSpan::from_spans(sugg.labels.iter().map(|(span, _)| *span).collect());
+        for (span, label) in sugg.labels {
+            spans.push_span_label(*span, label.clone());
+        }
+        // If a relevant span is from at least edition 2024, this is a hard error.
+        let is_hard_error = spans.primary_spans().iter().any(|span| span.at_least_rust_2024());
+        if is_hard_error {
             let mut err =
-                tcx.dcx().struct_span_err(pat.span, fluent::mir_build_rust_2024_incompatible_pat);
+                tcx.dcx().struct_span_err(spans, fluent::mir_build_rust_2024_incompatible_pat);
+            if let Some(info) = lint::builtin::RUST_2024_INCOMPATIBLE_PAT.future_incompatible {
+                // provide the same reference link as the lint
+                err.note(format!("for more information, see {}", info.reference));
+            }
             err.subdiagnostic(sugg);
             err.emit();
         } else {
             tcx.emit_node_span_lint(
                 lint::builtin::RUST_2024_INCOMPATIBLE_PAT,
                 pat.hir_id,
-                pat.span,
+                spans,
                 Rust2024IncompatiblePat { sugg },
             );
         }
@@ -133,6 +146,7 @@ impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
                 })
                 .collect();
             s.suggestion.push((pat.span.shrink_to_lo(), suggestion_str));
+            s.ref_pattern_count += adjustments.len();
         };
 
         adjusted_pat
@@ -371,7 +385,8 @@ impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
                     s.suggestion.push((
                         pat.span.with_lo(ident.span.lo()).shrink_to_lo(),
                         sugg_str.to_owned(),
-                    ))
+                    ));
+                    s.binding_mode_count += 1;
                 }
 
                 // A ref x pattern is the same node used for x, and as such it has

tests/ui/pattern/rfc-3627-match-ergonomics-2024/migration_lint.fixed

@@ -23,22 +23,22 @@ fn main() {
     assert_type_eq(x, &mut 0u8);
 
     let &Foo(mut x) = &Foo(0);
-    //~^ ERROR: patterns are not allowed to reset the default binding mode
+    //~^ ERROR: this pattern relies on behavior which may change in edition 2024
     //~| WARN: this changes meaning in Rust 2024
     assert_type_eq(x, 0u8);
 
     let &mut Foo(mut x) = &mut Foo(0);
-    //~^ ERROR: patterns are not allowed to reset the default binding mode
+    //~^ ERROR: this pattern relies on behavior which may change in edition 2024
     //~| WARN: this changes meaning in Rust 2024
     assert_type_eq(x, 0u8);
 
     let &Foo(ref x) = &Foo(0);
-    //~^ ERROR: patterns are not allowed to reset the default binding mode
+    //~^ ERROR: this pattern relies on behavior which may change in edition 2024
     //~| WARN: this changes meaning in Rust 2024
     assert_type_eq(x, &0u8);
 
     let &mut Foo(ref x) = &mut Foo(0);
-    //~^ ERROR: patterns are not allowed to reset the default binding mode
+    //~^ ERROR: this pattern relies on behavior which may change in edition 2024
     //~| WARN: this changes meaning in Rust 2024
     assert_type_eq(x, &0u8);
 
@@ -55,22 +55,22 @@ fn main() {
     assert_type_eq(x, &0u8);
 
     let &Foo(&x) = &Foo(&0);
-    //~^ ERROR: patterns are not allowed to reset the default binding mode
+    //~^ ERROR: this pattern relies on behavior which may change in edition 2024
     //~| WARN: this changes meaning in Rust 2024
     assert_type_eq(x, 0u8);
 
     let &Foo(&mut x) = &Foo(&mut 0);
-    //~^ ERROR: patterns are not allowed to reset the default binding mode
+    //~^ ERROR: this pattern relies on behavior which may change in edition 2024
     //~| WARN: this changes meaning in Rust 2024
     assert_type_eq(x, 0u8);
 
     let &mut Foo(&x) = &mut Foo(&0);
-    //~^ ERROR: patterns are not allowed to reset the default binding mode
+    //~^ ERROR: this pattern relies on behavior which may change in edition 2024
     //~| WARN: this changes meaning in Rust 2024
     assert_type_eq(x, 0u8);
 
     let &mut Foo(&mut x) = &mut Foo(&mut 0);
-    //~^ ERROR: patterns are not allowed to reset the default binding mode
+    //~^ ERROR: this pattern relies on behavior which may change in edition 2024
     //~| WARN: this changes meaning in Rust 2024
     assert_type_eq(x, 0u8);
 
@@ -79,25 +79,25 @@ fn main() {
     }
 
     if let &&&&&Some(&x) = &&&&&Some(&0u8) {
-        //~^ ERROR: patterns are not allowed to reset the default binding mode
+        //~^ ERROR: this pattern relies on behavior which may change in edition 2024
         //~| WARN: this changes meaning in Rust 2024
         assert_type_eq(x, 0u8);
     }
 
     if let &&&&&Some(&mut x) = &&&&&Some(&mut 0u8) {
-        //~^ ERROR: patterns are not allowed to reset the default binding mode
+        //~^ ERROR: this pattern relies on behavior which may change in edition 2024
         //~| WARN: this changes meaning in Rust 2024
         assert_type_eq(x, 0u8);
     }
 
     if let &&&&&mut Some(&x) = &&&&&mut Some(&0u8) {
-        //~^ ERROR: patterns are not allowed to reset the default binding mode
+        //~^ ERROR: this pattern relies on behavior which may change in edition 2024
         //~| WARN: this changes meaning in Rust 2024
         assert_type_eq(x, 0u8);
     }
 
     if let &mut Some(&mut Some(&mut Some(ref mut x))) = &mut Some(&mut Some(&mut Some(0u8))) {
-        //~^ ERROR: patterns are not allowed to reset the default binding mode
+        //~^ ERROR: this pattern relies on behavior which may change in edition 2024
         //~| WARN: this changes meaning in Rust 2024
         assert_type_eq(x, &mut 0u8);
     }
@@ -109,20 +109,20 @@ fn main() {
     }
 
     let &Struct { ref a, mut b, ref c } = &Struct { a: 0, b: 0, c: 0 };
-    //~^ ERROR: patterns are not allowed to reset the default binding mode
+    //~^ ERROR: this pattern relies on behavior which may change in edition 2024
     //~| WARN: this changes meaning in Rust 2024
     assert_type_eq(a, &0u32);
     assert_type_eq(b, 0u32);
 
     let &Struct { a: &a, ref b, ref c } = &Struct { a: &0, b: &0, c: &0 };
-    //~^ ERROR: patterns are not allowed to reset the default binding mode
+    //~^ ERROR: this pattern relies on behavior which may change in edition 2024
     //~| WARN: this changes meaning in Rust 2024
     assert_type_eq(a, 0u32);
     assert_type_eq(b, &&0u32);
     assert_type_eq(c, &&0u32);
 
     if let &Struct { a: &Some(a), b: &Some(&b), c: &Some(ref c) } =
-        //~^ ERROR: patterns are not allowed to reset the default binding mode
+        //~^ ERROR: this pattern relies on behavior which may change in edition 2024
         //~| WARN: this changes meaning in Rust 2024
         &(Struct { a: &Some(&0), b: &Some(&0), c: &Some(&0) })
     {
@@ -135,7 +135,7 @@ fn main() {
         // The two patterns are the same syntactically, but because they're defined in different
         // editions they don't mean the same thing.
         &(Some(mut x), migration_lint_macros::mixed_edition_pat!(y)) => {
-            //~^ ERROR: patterns are not allowed to reset the default binding mode
+            //~^ ERROR: this pattern relies on behavior which may change in edition 2024
             assert_type_eq(x, 0u32);
             assert_type_eq(y, 0u32);
         }