lower implicit deref patterns to THIR

Since this uses `pat_adjustments`, I've also tweaked the documentation
to mention implicit deref patterns and made sure the pattern migration
diagnostic logic accounts for it. I'll adjust `ExprUseVisitor` in a
later commit and add some tests there for closure capture inference.

Author: dianne

compiler/rustc_middle/src/ty/typeck_results.rs

@@ -77,16 +77,24 @@ pub struct TypeckResults<'tcx> {
     /// to a form valid in all Editions, either as a lint diagnostic or hard error.
     rust_2024_migration_desugared_pats: ItemLocalMap<Rust2024IncompatiblePatInfo>,
 
-    /// Stores the types which were implicitly dereferenced in pattern binding modes
-    /// for later usage in THIR lowering. For example,
+    /// Stores the types which were implicitly dereferenced in pattern binding modes or deref
+    /// patterns for later usage in THIR lowering. For example,
     ///
     /// ```
     /// match &&Some(5i32) {
     ///     Some(n) => {},
     ///     _ => {},
     /// }
     /// ```
-    /// leads to a `vec![&&Option<i32>, &Option<i32>]`. Empty vectors are not stored.
+    /// leads to a `vec![&&Option<i32>, &Option<i32>]` and
+    ///
+    /// ```no_run
+    /// match &Box::new(Some(5i32)) {
+    ///     Some(n) => {},
+    ///     _ => {},
+    /// }
+    /// ```
+    /// leads to a `vec![&Box<Option<i32>>, Box<Option<i32>>]`. Empty vectors are not stored.
     ///
     /// See:
     /// <https://github.com/rust-lang/rfcs/blob/master/text/2005-match-ergonomics.md#definitions>

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

@@ -4,7 +4,6 @@ use rustc_data_structures::fx::FxIndexMap;
 use rustc_errors::MultiSpan;
 use rustc_hir::{BindingMode, ByRef, HirId, Mutability};
 use rustc_lint as lint;
-use rustc_middle::span_bug;
 use rustc_middle::ty::{self, Rust2024IncompatiblePatInfo, Ty, TyCtxt};
 use rustc_span::{Ident, Span};
 
@@ -95,11 +94,8 @@ impl<'a> PatMigration<'a> {
         pat_span: Span,
         adjustments: &[Ty<'tcx>],
     ) -> Option<(Span, Mutability)> {
-        let implicit_deref_mutbls = adjustments.iter().map(|ref_ty| {
-            let &ty::Ref(_, _, mutbl) = ref_ty.kind() else {
-                span_bug!(pat_span, "pattern implicitly dereferences a non-ref type");
-            };
-            mutbl
+        let implicit_deref_mutbls = adjustments.iter().filter_map(|ref_ty| {
+            if let &ty::Ref(_, _, mutbl) = ref_ty.kind() { Some(mutbl) } else { None }
         });
 
         if !self.info.suggest_eliding_modes {

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

@@ -66,9 +66,11 @@ impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
             self.typeck_results.pat_adjustments().get(pat.hir_id).map_or(&[], |v| &**v);
 
         // Track the default binding mode for the Rust 2024 migration suggestion.
+        // Implicitly dereferencing references changes the default binding mode, but implicit deref
+        // patterns do not. Only track binding mode changes if a ref type is in the adjustments.
         let mut opt_old_mode_span = None;
         if let Some(s) = &mut self.rust_2024_migration
-            && !adjustments.is_empty()
+            && adjustments.iter().any(|ty| ty.is_ref())
         {
             opt_old_mode_span = s.visit_implicit_derefs(pat.span, adjustments);
         }
@@ -103,15 +105,20 @@ impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
 
         let adjusted_pat = adjustments.iter().rev().fold(unadjusted_pat, |thir_pat, ref_ty| {
             debug!("{:?}: wrapping pattern with type {:?}", thir_pat, ref_ty);
-            Box::new(Pat {
-                span: thir_pat.span,
-                ty: *ref_ty,
-                kind: PatKind::Deref { subpattern: thir_pat },
-            })
+            let span = thir_pat.span;
+            // TODO: use an enum to distinguish between builtin and overloaded derefs
+            let kind = if ref_ty.is_ref() {
+                PatKind::Deref { subpattern: thir_pat }
+            } else {
+                let mutable = self.typeck_results.pat_has_ref_mut_binding(pat);
+                let mutability = if mutable { hir::Mutability::Mut } else { hir::Mutability::Not };
+                PatKind::DerefPattern { subpattern: thir_pat, mutability }
+            };
+            Box::new(Pat { span, ty: *ref_ty, kind })
         });
 
         if let Some(s) = &mut self.rust_2024_migration
-            && !adjustments.is_empty()
+            && adjustments.iter().any(|ty| ty.is_ref())
         {
             s.leave_ref(opt_old_mode_span);
         }