deref patterns: bare-bones feature gate and typechecking

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -413,7 +413,10 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                 }
             }
             PatKind::Box(..) => {
-                gate!(&self, box_patterns, pattern.span, "box pattern syntax is experimental");
+                if !self.features.deref_patterns {
+                    // Allow box patterns under `deref_patterns`.
+                    gate!(&self, box_patterns, pattern.span, "box pattern syntax is experimental");
+                }
             }
             PatKind::Range(_, Some(_), Spanned { node: RangeEnd::Excluded, .. }) => {
                 gate!(
@@ -607,13 +610,16 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
         };
     }
 
+    if !visitor.features.deref_patterns {
+        // Allow box patterns under `deref_patterns`.
+        gate_all_legacy_dont_use!(box_patterns, "box pattern syntax is experimental");
+    }
     gate_all_legacy_dont_use!(trait_alias, "trait aliases are experimental");
     // Despite being a new feature, `where T: Trait<Assoc(): Sized>`, which is RTN syntax now,
     // used to be gated under associated_type_bounds, which are right above, so RTN needs to
     // be too.
     gate_all_legacy_dont_use!(return_type_notation, "return type notation is experimental");
     gate_all_legacy_dont_use!(decl_macro, "`macro` is experimental");
-    gate_all_legacy_dont_use!(box_patterns, "box pattern syntax is experimental");
     gate_all_legacy_dont_use!(
         exclusive_range_pattern,
         "exclusive range pattern syntax is experimental"

compiler/rustc_feature/src/unstable.rs

@@ -436,6 +436,8 @@ declare_features! (
     (unstable, deprecated_safe, "1.61.0", Some(94978)),
     /// Allows having using `suggestion` in the `#[deprecated]` attribute.
     (unstable, deprecated_suggestion, "1.61.0", Some(94785)),
+    /// Allows deref patterns.
+    (incomplete, deref_patterns, "CURRENT_RUSTC_VERSION", Some(87121)),
     /// Controls errors in trait implementations.
     (unstable, do_not_recommend, "1.67.0", Some(51992)),
     /// Tells rustdoc to automatically generate `#[doc(cfg(...))]`.

compiler/rustc_hir_typeck/src/pat.rs

@@ -18,8 +18,7 @@ use rustc_span::edit_distance::find_best_match_for_name;
 use rustc_span::hygiene::DesugaringKind;
 use rustc_span::source_map::Spanned;
 use rustc_span::symbol::{kw, sym, Ident};
-use rustc_span::Span;
-use rustc_span::{BytePos, DUMMY_SP};
+use rustc_span::{BytePos, Span, DUMMY_SP};
 use rustc_target::abi::FieldIdx;
 use rustc_trait_selection::traits::{ObligationCause, Pattern};
 use ty::VariantDef;
@@ -211,6 +210,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             PatKind::Tuple(elements, ddpos) => {
                 self.check_pat_tuple(pat.span, elements, ddpos, expected, pat_info)
             }
+            PatKind::Box(inner) if self.tcx.features().deref_patterns => {
+                self.check_pat_deref(pat.span, inner, expected, pat_info)
+            }
             PatKind::Box(inner) => self.check_pat_box(pat.span, inner, expected, pat_info),
             PatKind::Ref(inner, mutbl) => self.check_pat_ref(pat, inner, mutbl, expected, pat_info),
             PatKind::Slice(before, slice, after) => {
@@ -1975,6 +1977,28 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         box_ty
     }
 
+    fn check_pat_deref(
+        &self,
+        span: Span,
+        inner: &'tcx Pat<'tcx>,
+        expected: Ty<'tcx>,
+        pat_info: PatInfo<'tcx, '_>,
+    ) -> Ty<'tcx> {
+        let tcx = self.tcx;
+        // FIXME(deref_patterns): use `DerefPure` for soundness
+        // FIXME(deref_patterns): use `DerefMut` when required
+        // <expected as Deref>::Target
+        let ty = Ty::new_projection(
+            tcx,
+            tcx.require_lang_item(hir::LangItem::DerefTarget, Some(span)),
+            [expected],
+        );
+        let ty = self.normalize(span, ty);
+        let ty = self.try_structurally_resolve_type(span, ty);
+        self.check_pat(inner, ty, pat_info);
+        expected
+    }
+
     // Precondition: Pat is Ref(inner)
     fn check_pat_ref(
         &self,

compiler/rustc_middle/src/thir.rs

@@ -647,6 +647,7 @@ impl<'tcx> Pat<'tcx> {
             AscribeUserType { subpattern, .. }
             | Binding { subpattern: Some(subpattern), .. }
             | Deref { subpattern }
+            | DerefPattern { subpattern }
             | InlineConstant { subpattern, .. } => subpattern.walk_(it),
             Leaf { subpatterns } | Variant { subpatterns, .. } => {
                 subpatterns.iter().for_each(|field| field.pattern.walk_(it))
@@ -762,6 +763,11 @@ pub enum PatKind<'tcx> {
         subpattern: Box<Pat<'tcx>>,
     },
 
+    /// Deref pattern, written `box P` for now.
+    DerefPattern {
+        subpattern: Box<Pat<'tcx>>,
+    },
+
     /// One of the following:
     /// * `&str` (represented as a valtree), which will be handled as a string pattern and thus
     ///   exhaustiveness checking will detect if you use the same string twice in different
@@ -1172,6 +1178,9 @@ impl<'tcx> fmt::Display for Pat<'tcx> {
                 }
                 write!(f, "{subpattern}")
             }
+            PatKind::DerefPattern { ref subpattern } => {
+                write!(f, "k#deref {subpattern}")
+            }
             PatKind::Constant { value } => write!(f, "{value}"),
             PatKind::InlineConstant { def: _, ref subpattern } => {
                 write!(f, "{} (from inline const)", subpattern)

compiler/rustc_middle/src/thir/visit.rs

@@ -229,6 +229,7 @@ pub fn walk_pat<'thir, 'tcx: 'thir, V: Visitor<'thir, 'tcx>>(
     match &pat.kind {
         AscribeUserType { subpattern, ascription: _ }
         | Deref { subpattern }
+        | DerefPattern { subpattern }
         | Binding {
             subpattern: Some(subpattern),
             mutability: _,

compiler/rustc_mir_build/src/build/matches/mod.rs

@@ -879,6 +879,10 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 self.visit_primary_bindings(subpattern, pattern_user_ty.deref(), f);
             }
 
+            PatKind::DerefPattern { ref subpattern } => {
+                self.visit_primary_bindings(subpattern, UserTypeProjections::none(), f);
+            }
+
             PatKind::AscribeUserType {
                 ref subpattern,
                 ascription: thir::Ascription { ref annotation, variance: _ },

compiler/rustc_mir_build/src/build/matches/util.rs

@@ -256,6 +256,12 @@ impl<'pat, 'tcx> MatchPair<'pat, 'tcx> {
                 subpairs.push(MatchPair::new(place_builder, subpattern, cx));
                 default_irrefutable()
             }
+
+            PatKind::DerefPattern { .. } => {
+                // FIXME(deref_patterns)
+                // Treat it like a wildcard for now.
+                default_irrefutable()
+            }
         };
 
         MatchPair { place, test_case, subpairs, pattern }

compiler/rustc_mir_build/src/check_unsafety.rs

@@ -250,6 +250,7 @@ impl<'a, 'tcx> Visitor<'a, 'tcx> for UnsafetyVisitor<'a, 'tcx> {
                 | PatKind::Variant { .. }
                 | PatKind::Leaf { .. }
                 | PatKind::Deref { .. }
+                | PatKind::DerefPattern { .. }
                 | PatKind::Range { .. }
                 | PatKind::Slice { .. }
                 | PatKind::Array { .. } => {
@@ -310,7 +311,7 @@ impl<'a, 'tcx> Visitor<'a, 'tcx> for UnsafetyVisitor<'a, 'tcx> {
                 }
                 visit::walk_pat(self, pat);
             }
-            PatKind::Deref { .. } => {
+            PatKind::Deref { .. } | PatKind::DerefPattern { .. } => {
                 let old_inside_adt = std::mem::replace(&mut self.inside_adt, false);
                 visit::walk_pat(self, pat);
                 self.inside_adt = old_inside_adt;

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

@@ -257,6 +257,9 @@ impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
                 return self.lower_path(qpath, pat.hir_id, pat.span);
             }
 
+            hir::PatKind::Box(subpattern) if self.tcx.features().deref_patterns => {
+                PatKind::DerefPattern { subpattern: self.lower_pattern(subpattern) }
+            }
             hir::PatKind::Ref(subpattern, _) | hir::PatKind::Box(subpattern) => {
                 PatKind::Deref { subpattern: self.lower_pattern(subpattern) }
             }

compiler/rustc_mir_build/src/thir/print.rs

@@ -688,6 +688,12 @@ impl<'a, 'tcx> ThirPrinter<'a, 'tcx> {
                 self.print_pat(subpattern, depth_lvl + 2);
                 print_indented!(self, "}", depth_lvl + 1);
             }
+            PatKind::DerefPattern { subpattern } => {
+                print_indented!(self, "DerefPattern { ", depth_lvl + 1);
+                print_indented!(self, "subpattern:", depth_lvl + 2);
+                self.print_pat(subpattern, depth_lvl + 2);
+                print_indented!(self, "}", depth_lvl + 1);
+            }
             PatKind::Constant { value } => {
                 print_indented!(self, "Constant {", depth_lvl + 1);
                 print_indented!(self, format!("value: {:?}", value), depth_lvl + 2);

compiler/rustc_pattern_analysis/src/rustc.rs

@@ -462,6 +462,12 @@ impl<'p, 'tcx: 'p> RustcPatCtxt<'p, 'tcx> {
                     _ => bug!("pattern has unexpected type: pat: {:?}, ty: {:?}", pat, ty),
                 };
             }
+            PatKind::DerefPattern { .. } => {
+                // FIXME(deref_patterns): At least detect that `box _` is irrefutable.
+                fields = vec![];
+                arity = 0;
+                ctor = Opaque(OpaqueId::new());
+            }
             PatKind::Leaf { subpatterns } | PatKind::Variant { subpatterns, .. } => {
                 match ty.kind() {
                     ty::Tuple(fs) => {

compiler/rustc_span/src/symbol.rs

@@ -674,6 +674,7 @@ symbols! {
         deref_method,
         deref_mut,
         deref_mut_method,
+        deref_patterns,
         deref_target,
         derive,
         derive_const,

tests/ui/cfg/cfg-false-feature.stderr

@@ -1,15 +1,3 @@
-warning: trait aliases are experimental
-  --> $DIR/cfg-false-feature.rs:12:1
-   |
-LL | trait A = Clone;
-   | ^^^^^^^^^^^^^^^^
-   |
-   = note: see issue #41517 <https://github.com/rust-lang/rust/issues/41517> for more information
-   = help: add `#![feature(trait_alias)]` to the crate attributes to enable
-   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-   = warning: unstable syntax can change at any point in the future, causing a hard error!
-   = note: for more information, see issue #65860 <https://github.com/rust-lang/rust/issues/65860>
-
 warning: box pattern syntax is experimental
   --> $DIR/cfg-false-feature.rs:16:9
    |
@@ -22,5 +10,17 @@ LL |     let box _ = Box::new(0);
    = warning: unstable syntax can change at any point in the future, causing a hard error!
    = note: for more information, see issue #65860 <https://github.com/rust-lang/rust/issues/65860>
 
+warning: trait aliases are experimental
+  --> $DIR/cfg-false-feature.rs:12:1
+   |
+LL | trait A = Clone;
+   | ^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #41517 <https://github.com/rust-lang/rust/issues/41517> for more information
+   = help: add `#![feature(trait_alias)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = warning: unstable syntax can change at any point in the future, causing a hard error!
+   = note: for more information, see issue #65860 <https://github.com/rust-lang/rust/issues/65860>
+
 warning: 2 warnings emitted
 

tests/ui/feature-gates/feature-gate-deref_patterns.rs

@@ -0,0 +1,9 @@
+fn main() {
+    // We reuse the `box` syntax so this doesn't actually test the feature gate but eh.
+    let box x = Box::new('c'); //~ ERROR box pattern syntax is experimental
+    println!("x: {}", x);
+
+    // `box` syntax is allowed to be cfg-ed out for historical reasons (#65742).
+    #[cfg(FALSE)]
+    let box _x = Box::new('c');
+}

tests/ui/feature-gates/feature-gate-deref_patterns.stderr

@@ -0,0 +1,13 @@
+error[E0658]: box pattern syntax is experimental
+  --> $DIR/feature-gate-deref_patterns.rs:3:9
+   |
+LL |     let box x = Box::new('c');
+   |         ^^^^^
+   |
+   = note: see issue #29641 <https://github.com/rust-lang/rust/issues/29641> for more information
+   = help: add `#![feature(box_patterns)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.

tests/ui/pattern/deref-patterns/typeck.rs

@@ -0,0 +1,31 @@
+//@ check-pass
+#![feature(deref_patterns)]
+#![allow(incomplete_features)]
+
+use std::rc::Rc;
+
+fn main() {
+    let vec: Vec<u32> = Vec::new();
+    match vec {
+        box [..] => {}
+        _ => {}
+    }
+    match Box::new(true) {
+        box true => {}
+        _ => {}
+    }
+    match &Box::new(true) {
+        box true => {}
+        _ => {}
+    }
+    match &Rc::new(0) {
+        box (1..) => {}
+        _ => {}
+    }
+    // FIXME(deref_patterns): fails to typecheck because `"foo"` has type &str but deref creates a
+    // place of type `str`.
+    // match "foo".to_string() {
+    //     box "foo" => {}
+    //     _ => {}
+    // }
+}