Rollup merge of #131381 - Nadrieril:min-match-ergonomics, r=pnkfelix

Implement edition 2024 match ergonomics restrictions

This implements the minimalest version of [match ergonomics for edition 2024](https://rust-lang.github.io/rfcs/3627-match-ergonomics-2024.html). This minimal version makes it an error to ever reset the default binding mode. The implemented proposal is described precisely [here](https://hackmd.io/zUqs2ISNQ0Wrnxsa9nhD0Q#RFC-3627-nano), where it is called "RFC 3627-nano".

Rules:
- Rule 1C: When the DBM (default binding mode) is not `move` (whether or not behind a reference), writing `mut`, `ref`, or `ref mut` on a binding is an error.
- Rule 2C: Reference patterns can only match against references in the scrutinee when the DBM is `move`.

This minimal version is forward-compatible with the main proposals for match ergonomics 2024: [RFC3627](https://rust-lang.github.io/rfcs/3627-match-ergonomics-2024.html) itself, the alternative [rule 4-early variant](https://rust-lang.github.io/rfcs/3627-match-ergonomics-2024.html), and [others](https://hackmd.io/zUqs2ISNQ0Wrnxsa9nhD0Q). The idea is to give us more time to iron out a final proposal.

This includes a migration lint that desugars any offending pattern into one that doesn't make use of match ergonomics. Such patterns have identical meaning across editions.

This PR insta-stabilizes the proposed behavior onto edition 2024.

r? `@ghost`

Tracking:

- #123076

compiler/rustc_hir_typeck/src/pat.rs

@@ -690,16 +690,29 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
                     BindingMode(def_br, Mutability::Mut)
                 } else {
-                    // `mut` resets binding mode on edition <= 2021
-                    self.typeck_results
+                    // `mut` resets the binding mode on edition <= 2021
+                    *self
+                        .typeck_results
                         .borrow_mut()
                         .rust_2024_migration_desugared_pats_mut()
-                        .insert(pat_info.top_info.hir_id);
+                        .entry(pat_info.top_info.hir_id)
+                        .or_default() |= pat.span.at_least_rust_2024();
                     BindingMode(ByRef::No, Mutability::Mut)
                 }
             }
             BindingMode(ByRef::No, mutbl) => BindingMode(def_br, mutbl),
-            BindingMode(ByRef::Yes(_), _) => user_bind_annot,
+            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();
+                }
+                user_bind_annot
+            }
         };
 
         if bm.0 == ByRef::Yes(Mutability::Mut)
@@ -2204,14 +2217,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             }
         } else {
             // Reset binding mode on old editions
-
             if pat_info.binding_mode != ByRef::No {
                 pat_info.binding_mode = ByRef::No;
-
-                self.typeck_results
+                *self
+                    .typeck_results
                     .borrow_mut()
                     .rust_2024_migration_desugared_pats_mut()
-                    .insert(pat_info.top_info.hir_id);
+                    .entry(pat_info.top_info.hir_id)
+                    .or_default() |= pat.span.at_least_rust_2024();
             }
         }
 
@@ -2262,6 +2275,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 (err, err)
             }
         };
+
         self.check_pat(inner, inner_ty, pat_info);
         ref_ty
     }

compiler/rustc_hir_typeck/src/writeback.rs

@@ -635,7 +635,7 @@ 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
+        if let Some(is_hard_error) = self
             .fcx
             .typeck_results
             .borrow_mut()
@@ -645,7 +645,9 @@ impl<'cx, 'tcx> WritebackCx<'cx, 'tcx> {
             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);
+            self.typeck_results
+                .rust_2024_migration_desugared_pats_mut()
+                .insert(hir_id, is_hard_error);
         }
     }
 

compiler/rustc_lint_defs/src/builtin.rs

@@ -1651,7 +1651,6 @@ declare_lint! {
     /// ### Example
     ///
     /// ```rust,edition2021
-    /// #![feature(ref_pat_eat_one_layer_2024)]
     /// #![warn(rust_2024_incompatible_pat)]
     ///
     /// if let Some(&a) = &Some(&0u8) {
@@ -1672,12 +1671,10 @@ declare_lint! {
     pub RUST_2024_INCOMPATIBLE_PAT,
     Allow,
     "detects patterns whose meaning will change in Rust 2024",
-    @feature_gate = ref_pat_eat_one_layer_2024;
-    // FIXME uncomment below upon stabilization
-    /*@future_incompatible = FutureIncompatibleInfo {
+    @future_incompatible = FutureIncompatibleInfo {
         reason: FutureIncompatibilityReason::EditionSemanticsChange(Edition::Edition2024),
         reference: "123076",
-    };*/
+    };
 }
 
 declare_lint! {

compiler/rustc_middle/src/ty/typeck_results.rs

@@ -73,9 +73,10 @@ 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,
+    /// 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>,
 
     /// Stores the types which were implicitly dereferenced in pattern binding modes
     /// for later usage in THIR lowering. For example,
@@ -418,15 +419,15 @@ 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 {
+    pub fn rust_2024_migration_desugared_pats(&self) -> LocalTableInContext<'_, bool> {
+        LocalTableInContext {
             hir_owner: self.hir_owner,
             data: &self.rust_2024_migration_desugared_pats,
         }
     }
 
-    pub fn rust_2024_migration_desugared_pats_mut(&mut self) -> LocalSetInContextMut<'_> {
-        LocalSetInContextMut {
+    pub fn rust_2024_migration_desugared_pats_mut(&mut self) -> LocalTableInContextMut<'_, bool> {
+        LocalTableInContextMut {
             hir_owner: self.hir_owner,
             data: &mut self.rust_2024_migration_desugared_pats,
         }

compiler/rustc_mir_build/messages.ftl

@@ -265,7 +265,7 @@ 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 = the semantics of this pattern will change in edition 2024
+mir_build_rust_2024_incompatible_pat = patterns are not allowed to reset the default binding mode 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

@@ -983,6 +983,8 @@ pub(crate) struct Rust2024IncompatiblePat {
 
 pub(crate) struct Rust2024IncompatiblePatSugg {
     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,
 }
 
 impl Subdiagnostic for Rust2024IncompatiblePatSugg {

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

@@ -25,6 +25,7 @@ use tracing::{debug, instrument};
 
 pub(crate) use self::check_match::check_match;
 use crate::errors::*;
+use crate::fluent_generated as fluent;
 use crate::thir::util::UserAnnotatedTyHelpers;
 
 struct PatCtxt<'a, 'tcx> {
@@ -48,18 +49,28 @@ pub(super) fn pat_from_hir<'a, 'tcx>(
         typeck_results,
         rust_2024_migration_suggestion: typeck_results
             .rust_2024_migration_desugared_pats()
-            .contains(pat.hir_id)
-            .then_some(Rust2024IncompatiblePatSugg { suggestion: Vec::new() }),
+            .get(pat.hir_id)
+            .map(|&is_hard_error| Rust2024IncompatiblePatSugg {
+                suggestion: Vec::new(),
+                is_hard_error,
+            }),
     };
     let result = pcx.lower_pattern(pat);
     debug!("pat_from_hir({:?}) = {:?}", pat, result);
     if let Some(sugg) = pcx.rust_2024_migration_suggestion {
-        tcx.emit_node_span_lint(
-            lint::builtin::RUST_2024_INCOMPATIBLE_PAT,
-            pat.hir_id,
-            pat.span,
-            Rust2024IncompatiblePat { sugg },
-        );
+        if sugg.is_hard_error {
+            let mut err =
+                tcx.dcx().struct_span_err(pat.span, fluent::mir_build_rust_2024_incompatible_pat);
+            err.subdiagnostic(sugg);
+            err.emit();
+        } else {
+            tcx.emit_node_span_lint(
+                lint::builtin::RUST_2024_INCOMPATIBLE_PAT,
+                pat.hir_id,
+                pat.span,
+                Rust2024IncompatiblePat { sugg },
+            );
+        }
     }
     result
 }