Match ergonomics 2024: migration lint

Unfortunately, we can't always offer a machine-applicable suggestion when there are subpatterns from macro expansion.

Co-Authored-By: Guillaume Boisseau <[email protected]>

Author: Jules-Bertholet

Cargo.lock

@@ -4367,6 +4367,7 @@ dependencies = [
  "rustc_hir",
  "rustc_index",
  "rustc_infer",
+ "rustc_lint",
  "rustc_macros",
  "rustc_middle",
  "rustc_pattern_analysis",

compiler/rustc_hir_typeck/messages.ftl

@@ -46,10 +46,6 @@ hir_typeck_ctor_is_private = tuple struct constructor `{$def}` is private
 
 hir_typeck_deref_is_empty = this expression `Deref`s to `{$deref_ty}` which implements `is_empty`
 
-hir_typeck_dereferencing_mut_binding = dereferencing `mut` binding
-    .label = `mut` dereferences the type of this binding
-    .help = this will change in edition 2024
-
 hir_typeck_expected_default_return_type = expected `()` because of default return type
 
 hir_typeck_expected_return_type = expected `{$expected}` because of return type

compiler/rustc_hir_typeck/src/errors.rs

@@ -651,10 +651,3 @@ pub enum SuggestBoxingForReturnImplTrait {
         ends: Vec<Span>,
     },
 }
-#[derive(LintDiagnostic)]
-#[diag(hir_typeck_dereferencing_mut_binding)]
-pub struct DereferencingMutBinding {
-    #[label]
-    #[help]
-    pub span: Span,
-}

compiler/rustc_hir_typeck/src/pat.rs

@@ -73,21 +73,23 @@ struct TopInfo<'tcx> {
     ///              found type `std::result::Result<_, _>`
     /// ```
     span: Option<Span>,
+    /// The [`HirId`] of the top-level pattern.
+    hir_id: HirId,
 }
 
 #[derive(Copy, Clone)]
 struct PatInfo<'tcx, 'a> {
     binding_mode: ByRef,
     max_ref_mutbl: MutblCap,
-    top_info: TopInfo<'tcx>,
-    decl_origin: Option<DeclOrigin<'a>>,
+    top_info: &'a TopInfo<'tcx>,
+    decl_origin: Option<DeclOrigin<'tcx>>,
 
     /// The depth of current pattern
     current_depth: u32,
 }
 
 impl<'tcx> FnCtxt<'_, 'tcx> {
-    fn pattern_cause(&self, ti: TopInfo<'tcx>, cause_span: Span) -> ObligationCause<'tcx> {
+    fn pattern_cause(&self, ti: &TopInfo<'tcx>, cause_span: Span) -> ObligationCause<'tcx> {
         let code = ObligationCauseCode::Pattern {
             span: ti.span,
             root_ty: ti.expected,
@@ -101,7 +103,7 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
         cause_span: Span,
         expected: Ty<'tcx>,
         actual: Ty<'tcx>,
-        ti: TopInfo<'tcx>,
+        ti: &TopInfo<'tcx>,
     ) -> Option<Diag<'tcx>> {
         let mut diag =
             self.demand_eqtype_with_origin(&self.pattern_cause(ti, cause_span), expected, actual)?;
@@ -118,7 +120,7 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
         cause_span: Span,
         expected: Ty<'tcx>,
         actual: Ty<'tcx>,
-        ti: TopInfo<'tcx>,
+        ti: &TopInfo<'tcx>,
     ) {
         if let Some(err) = self.demand_eqtype_pat_diag(cause_span, expected, actual, ti) {
             err.emit();
@@ -199,11 +201,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         origin_expr: Option<&'tcx hir::Expr<'tcx>>,
         decl_origin: Option<DeclOrigin<'tcx>>,
     ) {
-        let info = TopInfo { expected, origin_expr, span };
+        let info = TopInfo { expected, origin_expr, span, hir_id: pat.hir_id };
         let pat_info = PatInfo {
             binding_mode: ByRef::No,
             max_ref_mutbl: MutblCap::Mut,
-            top_info: info,
+            top_info: &info,
             decl_origin,
             current_depth: 0,
         };
@@ -463,7 +465,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         span: Span,
         lt: &hir::Expr<'tcx>,
         expected: Ty<'tcx>,
-        ti: TopInfo<'tcx>,
+        ti: &TopInfo<'tcx>,
     ) -> Ty<'tcx> {
         // We've already computed the type above (when checking for a non-ref pat),
         // so avoid computing it again.
@@ -533,7 +535,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         lhs: Option<&'tcx hir::Expr<'tcx>>,
         rhs: Option<&'tcx hir::Expr<'tcx>>,
         expected: Ty<'tcx>,
-        ti: TopInfo<'tcx>,
+        ti: &TopInfo<'tcx>,
     ) -> Ty<'tcx> {
         let calc_side = |opt_expr: Option<&'tcx hir::Expr<'tcx>>| match opt_expr {
             None => None,
@@ -671,18 +673,16 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         // Determine the binding mode...
         let bm = match user_bind_annot {
+            // `mut` resets binding mode on edition <= 2021
             BindingMode(ByRef::No, Mutability::Mut)
                 if !(pat.span.at_least_rust_2024()
                     && self.tcx.features().mut_preserve_binding_mode_2024)
                     && matches!(def_br, ByRef::Yes(_)) =>
             {
-                // `mut x` resets the binding mode in edition <= 2021.
-                self.tcx.emit_node_span_lint(
-                    rustc_lint::builtin::DEREFERENCING_MUT_BINDING,
-                    pat.hir_id,
-                    pat.span,
-                    errors::DereferencingMutBinding { span: pat.span },
-                );
+                self.typeck_results
+                    .borrow_mut()
+                    .rust_2024_migration_desugared_pats_mut()
+                    .insert(pat_info.top_info.hir_id);
                 BindingMode(ByRef::No, Mutability::Mut)
             }
             BindingMode(ByRef::No, mutbl) => BindingMode(def_br, mutbl),
@@ -754,7 +754,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         span: Span,
         var_id: HirId,
         ty: Ty<'tcx>,
-        ti: TopInfo<'tcx>,
+        ti: &TopInfo<'tcx>,
     ) {
         let var_ty = self.local_ty(span, var_id);
         if let Some(mut err) = self.demand_eqtype_pat_diag(span, var_ty, ty, ti) {
@@ -996,7 +996,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         qpath: &hir::QPath<'_>,
         path_resolution: (Res, Option<LoweredTy<'tcx>>, &'tcx [hir::PathSegment<'tcx>]),
         expected: Ty<'tcx>,
-        ti: TopInfo<'tcx>,
+        ti: &TopInfo<'tcx>,
     ) -> Ty<'tcx> {
         let tcx = self.tcx;
 
@@ -2178,8 +2178,17 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             }
         } else {
             // Reset binding mode on old editions
-            pat_info.binding_mode = ByRef::No;
-            pat_info.max_ref_mutbl = MutblCap::Mut
+
+            if pat_info.binding_mode != ByRef::No {
+                pat_info.binding_mode = ByRef::No;
+
+                self.typeck_results
+                    .borrow_mut()
+                    .rust_2024_migration_desugared_pats_mut()
+                    .insert(pat_info.top_info.hir_id);
+            }
+
+            pat_info.max_ref_mutbl = MutblCap::Mut;
         }
 
         let tcx = self.tcx;

compiler/rustc_hir_typeck/src/writeback.rs

@@ -346,6 +346,7 @@ impl<'cx, 'tcx> Visitor<'tcx> for WritebackCx<'cx, 'tcx> {
             _ => {}
         };
 
+        self.visit_rust_2024_migration_desugared_pats(p.hir_id);
         self.visit_skipped_ref_pats(p.hir_id);
         self.visit_pat_adjustments(p.span, p.hir_id);
 
@@ -655,6 +656,22 @@ impl<'cx, 'tcx> WritebackCx<'cx, 'tcx> {
         }
     }
 
+    #[instrument(skip(self), level = "debug")]
+    fn visit_rust_2024_migration_desugared_pats(&mut self, hir_id: hir::HirId) {
+        if self
+            .fcx
+            .typeck_results
+            .borrow_mut()
+            .rust_2024_migration_desugared_pats_mut()
+            .remove(hir_id)
+        {
+            debug!(
+                "node is a pat whose match ergonomics are desugared by the Rust 2024 migration lint"
+            );
+            self.typeck_results.rust_2024_migration_desugared_pats_mut().insert(hir_id);
+        }
+    }
+
     #[instrument(skip(self, span), level = "debug")]
     fn visit_pat_adjustments(&mut self, span: Span, hir_id: HirId) {
         let adjustment = self.fcx.typeck_results.borrow_mut().pat_adjustments_mut().remove(hir_id);

compiler/rustc_lint_defs/src/builtin.rs

@@ -38,7 +38,6 @@ declare_lint_pass! {
         DEPRECATED_CFG_ATTR_CRATE_TYPE_NAME,
         DEPRECATED_IN_FUTURE,
         DEPRECATED_WHERE_CLAUSE_LOCATION,
-        DEREFERENCING_MUT_BINDING,
         DUPLICATE_MACRO_ATTRIBUTES,
         ELIDED_LIFETIMES_IN_ASSOCIATED_CONSTANT,
         ELIDED_LIFETIMES_IN_PATHS,
@@ -90,6 +89,7 @@ declare_lint_pass! {
         RUST_2021_INCOMPATIBLE_OR_PATTERNS,
         RUST_2021_PREFIXES_INCOMPATIBLE_SYNTAX,
         RUST_2021_PRELUDE_COLLISIONS,
+        RUST_2024_INCOMPATIBLE_PAT,
         SEMICOLON_IN_EXPRESSIONS_FROM_MACROS,
         SINGLE_USE_LIFETIMES,
         SOFT_UNSTABLE,
@@ -1630,34 +1630,34 @@ declare_lint! {
 }
 
 declare_lint! {
-    /// The `dereferencing_mut_binding` lint detects a `mut x` pattern that resets the binding mode,
-    /// as this behavior will change in rust 2024.
+    /// The `rust_2024_incompatible_pat` lint
+    /// detects patterns whose meaning will change in the Rust 2024 edition.
     ///
     /// ### Example
     ///
-    /// ```rust
-    /// # #![warn(dereferencing_mut_binding)]
-    /// let x = Some(123u32);
-    /// let _y = match &x {
-    ///     Some(mut x) => {
-    ///         x += 1;
-    ///         x
-    ///     }
-    ///     None => 0,
-    /// };
+    /// ```rust,edition2021
+    /// #![feature(ref_pat_eat_one_layer_2024)]
+    /// #![warn(rust_2024_incompatible_pat)]
+    ///
+    /// if let Some(&a) = &Some(&0u8) {
+    ///     let _: u8 = a;
+    /// }
+    /// if let Some(mut _a) = &mut Some(0u8) {
+    ///     _a = 7u8;
+    /// }
     /// ```
     ///
     /// {{produces}}
     ///
     /// ### Explanation
     ///
-    /// Without the `mut`, `x` would have type `&u32`. Pre-2024, adding `mut` makes `x` have type
-    /// `u32`, which was deemed surprising. After edition 2024, adding `mut` will not change the
-    /// type of `x`. This lint warns users of editions before 2024 to update their code.
-    pub DEREFERENCING_MUT_BINDING,
+    /// In Rust 2024 and above, the `mut` keyword does not reset the pattern binding mode,
+    /// and nor do `&` or `&mut` patterns. The lint will suggest code that
+    /// has the same meaning in all editions.
+    pub RUST_2024_INCOMPATIBLE_PAT,
     Allow,
-    "detects `mut x` bindings that change the type of `x`",
-    @feature_gate = sym::mut_preserve_binding_mode_2024;
+    "detects patterns whose meaning will change in Rust 2024",
+    @feature_gate = sym::ref_pat_eat_one_layer_2024;
     // FIXME uncomment below upon stabilization
     /*@future_incompatible = FutureIncompatibleInfo {
         reason: FutureIncompatibilityReason::EditionSemanticsChange(Edition::Edition2024),

compiler/rustc_middle/src/ty/typeck_results.rs

@@ -79,6 +79,10 @@ pub struct TypeckResults<'tcx> {
     /// Stores the actual binding mode for all instances of [`BindingMode`].
     pat_binding_modes: ItemLocalMap<BindingMode>,
 
+    /// Top-level patterns whose match ergonomics need to be desugared
+    /// by the Rust 2021 -> 2024 migration lint.
+    rust_2024_migration_desugared_pats: ItemLocalSet,
+
     /// Stores the types which were implicitly dereferenced in pattern binding modes
     /// for later usage in THIR lowering. For example,
     ///
@@ -229,6 +233,7 @@ impl<'tcx> TypeckResults<'tcx> {
             adjustments: Default::default(),
             pat_binding_modes: Default::default(),
             pat_adjustments: Default::default(),
+            rust_2024_migration_desugared_pats: Default::default(),
             skipped_ref_pats: Default::default(),
             closure_kind_origins: Default::default(),
             liberated_fn_sigs: Default::default(),
@@ -432,6 +437,20 @@ impl<'tcx> TypeckResults<'tcx> {
         LocalTableInContextMut { hir_owner: self.hir_owner, data: &mut self.pat_adjustments }
     }
 
+    pub fn rust_2024_migration_desugared_pats(&self) -> LocalSetInContext<'_> {
+        LocalSetInContext {
+            hir_owner: self.hir_owner,
+            data: &self.rust_2024_migration_desugared_pats,
+        }
+    }
+
+    pub fn rust_2024_migration_desugared_pats_mut(&mut self) -> LocalSetInContextMut<'_> {
+        LocalSetInContextMut {
+            hir_owner: self.hir_owner,
+            data: &mut self.rust_2024_migration_desugared_pats,
+        }
+    }
+
     pub fn skipped_ref_pats(&self) -> LocalSetInContext<'_> {
         LocalSetInContext { hir_owner: self.hir_owner, data: &self.skipped_ref_pats }
     }

compiler/rustc_mir_build/Cargo.toml

@@ -16,6 +16,7 @@ rustc_fluent_macro = { path = "../rustc_fluent_macro" }
 rustc_hir = { path = "../rustc_hir" }
 rustc_index = { path = "../rustc_index" }
 rustc_infer = { path = "../rustc_infer" }
+rustc_lint = { path = "../rustc_lint" }
 rustc_macros = { path = "../rustc_macros" }
 rustc_middle = { path = "../rustc_middle" }
 rustc_pattern_analysis = { path = "../rustc_pattern_analysis" }

compiler/rustc_mir_build/messages.ftl

@@ -267,6 +267,8 @@ 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 = the semantics of this pattern will change in edition 2024
+
 mir_build_rustc_box_attribute_error = `#[rustc_box]` attribute used incorrectly
     .attributes = no other attributes may be applied
     .not_box = `#[rustc_box]` may only be applied to a `Box::new()` call

compiler/rustc_mir_build/src/errors.rs

@@ -950,3 +950,30 @@ pub enum RustcBoxAttrReason {
     #[note(mir_build_missing_box)]
     MissingBox,
 }
+
+#[derive(LintDiagnostic)]
+#[diag(mir_build_rust_2024_incompatible_pat)]
+pub struct Rust2024IncompatiblePat {
+    #[subdiagnostic]
+    pub sugg: Rust2024IncompatiblePatSugg,
+}
+
+pub struct Rust2024IncompatiblePatSugg {
+    pub suggestion: Vec<(Span, String)>,
+}
+
+impl Subdiagnostic for Rust2024IncompatiblePatSugg {
+    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
+        self,
+        diag: &mut Diag<'_, G>,
+        _f: &F,
+    ) {
+        let applicability =
+            if self.suggestion.iter().all(|(span, _)| span.can_be_used_for_suggestions()) {
+                Applicability::MachineApplicable
+            } else {
+                Applicability::MaybeIncorrect
+            };
+        diag.multipart_suggestion("desugar the match ergonomics", self.suggestion, applicability);
+    }
+}