Implement macro-based deref_pat!() syntax for deref patterns

Stop using `box PAT` syntax for deref patterns, as it's misleading and
also causes their semantics being tangled up.

compiler/rustc_ast/src/ast.rs

@@ -621,7 +621,9 @@ impl Pat {
             | PatKind::Or(s) => s.iter().for_each(|p| p.walk(it)),
 
             // Trivial wrappers over inner patterns.
-            PatKind::Box(s) | PatKind::Ref(s, _) | PatKind::Paren(s) => s.walk(it),
+            PatKind::Box(s) | PatKind::Deref(s) | PatKind::Ref(s, _) | PatKind::Paren(s) => {
+                s.walk(it)
+            }
 
             // These patterns do not contain subpatterns, skip.
             PatKind::Wild
@@ -792,6 +794,9 @@ pub enum PatKind {
     /// A `box` pattern.
     Box(P<Pat>),
 
+    /// A `deref` pattern (currently `deref_pat!()` macro-based syntax).
+    Deref(P<Pat>),
+
     /// A reference pattern (e.g., `&mut (a, b)`).
     Ref(P<Pat>, Mutability),
 

compiler/rustc_ast/src/mut_visit.rs

@@ -1295,6 +1295,7 @@ pub fn noop_visit_pat<T: MutVisitor>(pat: &mut P<Pat>, vis: &mut T) {
             fields.flat_map_in_place(|field| vis.flat_map_pat_field(field));
         }
         PatKind::Box(inner) => vis.visit_pat(inner),
+        PatKind::Deref(inner) => vis.visit_pat(inner),
         PatKind::Ref(inner, _mutbl) => vis.visit_pat(inner),
         PatKind::Range(e1, e2, Spanned { span: _, node: _ }) => {
             visit_opt(e1, |e| vis.visit_expr(e));

compiler/rustc_ast/src/visit.rs

@@ -576,7 +576,10 @@ pub fn walk_pat<'a, V: Visitor<'a>>(visitor: &mut V, pattern: &'a Pat) -> V::Res
             try_visit!(visitor.visit_path(path, pattern.id));
             walk_list!(visitor, visit_pat_field, fields);
         }
-        PatKind::Box(subpattern) | PatKind::Ref(subpattern, _) | PatKind::Paren(subpattern) => {
+        PatKind::Box(subpattern)
+        | PatKind::Deref(subpattern)
+        | PatKind::Ref(subpattern, _)
+        | PatKind::Paren(subpattern) => {
             try_visit!(visitor.visit_pat(subpattern));
         }
         PatKind::Ident(_, ident, optional_subpattern) => {

compiler/rustc_ast_lowering/src/pat.rs

@@ -91,6 +91,9 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     PatKind::Box(inner) => {
                         break hir::PatKind::Box(self.lower_pat(inner));
                     }
+                    PatKind::Deref(inner) => {
+                        break hir::PatKind::Deref(self.lower_pat(inner));
+                    }
                     PatKind::Ref(inner, mutbl) => {
                         break hir::PatKind::Ref(self.lower_pat(inner), *mutbl);
                     }

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -413,10 +413,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                 }
             }
             PatKind::Box(..) => {
-                if !self.features.deref_patterns {
-                    // Allow box patterns under `deref_patterns`.
-                    gate!(&self, box_patterns, pattern.span, "box pattern syntax is experimental");
-                }
+                gate!(&self, box_patterns, pattern.span, "box pattern syntax is experimental");
             }
             PatKind::Range(_, Some(_), Spanned { node: RangeEnd::Excluded, .. }) => {
                 gate!(
@@ -610,10 +607,7 @@ 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!(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

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -1626,6 +1626,12 @@ impl<'a> State<'a> {
                 self.word("box ");
                 self.print_pat(inner);
             }
+            PatKind::Deref(inner) => {
+                self.word("deref_pat!");
+                self.popen();
+                self.print_pat(inner);
+                self.pclose();
+            }
             PatKind::Ref(inner, mutbl) => {
                 self.word("&");
                 if mutbl.is_mut() {

compiler/rustc_builtin_macros/src/deref_pat.rs

@@ -0,0 +1,23 @@
+use rustc_ast as ast;
+use rustc_ast::tokenstream::TokenStream;
+use rustc_expand::base::{DummyResult, ExpandResult, ExtCtxt, MacEager, MacroExpanderResult};
+use rustc_parse::parser::{CommaRecoveryMode, RecoverColon, RecoverComma};
+use rustc_span::Span;
+
+pub fn expand_deref_pat(
+    cx: &mut ExtCtxt<'_>,
+    span: Span,
+    tts: TokenStream,
+) -> MacroExpanderResult<'static> {
+    ExpandResult::Ready(
+        match cx.new_parser_from_tts(tts).parse_pat_allow_top_alt(
+            None,
+            RecoverComma::Yes,
+            RecoverColon::Yes,
+            CommaRecoveryMode::LikelyTuple,
+        ) {
+            Ok(parsed) => MacEager::pat(cx.pat(span, ast::PatKind::Deref(parsed))),
+            Err(err) => DummyResult::any(span, err.emit()),
+        },
+    )
+}

compiler/rustc_builtin_macros/src/lib.rs

@@ -37,6 +37,7 @@ mod compile_error;
 mod concat;
 mod concat_bytes;
 mod concat_idents;
+mod deref_pat;
 mod derive;
 mod deriving;
 mod edition_panic;
@@ -84,6 +85,7 @@ pub fn register_builtin_macros(resolver: &mut dyn ResolverExpand) {
         concat_idents: concat_idents::expand_concat_idents,
         const_format_args: format::expand_format_args,
         core_panic: edition_panic::expand_panic,
+        deref_pat: deref_pat::expand_deref_pat,
         env: env::expand_env,
         file: source_util::expand_file,
         format_args: format::expand_format_args,

compiler/rustc_hir/src/hir.rs

@@ -1003,7 +1003,7 @@ impl<'hir> Pat<'hir> {
         use PatKind::*;
         match self.kind {
             Wild | Never | Lit(_) | Range(..) | Binding(.., None) | Path(_) | Err(_) => true,
-            Box(s) | Ref(s, _) | Binding(.., Some(s)) => s.walk_short_(it),
+            Box(s) | Deref(s) | Ref(s, _) | Binding(.., Some(s)) => s.walk_short_(it),
             Struct(_, fields, _) => fields.iter().all(|field| field.pat.walk_short_(it)),
             TupleStruct(_, s, _) | Tuple(s, _) | Or(s) => s.iter().all(|p| p.walk_short_(it)),
             Slice(before, slice, after) => {
@@ -1030,7 +1030,7 @@ impl<'hir> Pat<'hir> {
         use PatKind::*;
         match self.kind {
             Wild | Never | Lit(_) | Range(..) | Binding(.., None) | Path(_) | Err(_) => {}
-            Box(s) | Ref(s, _) | Binding(.., Some(s)) => s.walk_(it),
+            Box(s) | Deref(s) | Ref(s, _) | Binding(.., Some(s)) => s.walk_(it),
             Struct(_, fields, _) => fields.iter().for_each(|field| field.pat.walk_(it)),
             TupleStruct(_, s, _) | Tuple(s, _) | Or(s) => s.iter().for_each(|p| p.walk_(it)),
             Slice(before, slice, after) => {
@@ -1173,6 +1173,9 @@ pub enum PatKind<'hir> {
     /// A `box` pattern.
     Box(&'hir Pat<'hir>),
 
+    /// A `deref` pattern (currently `deref_pat!()` macro-based syntax).
+    Deref(&'hir Pat<'hir>),
+
     /// A reference pattern (e.g., `&mut (a, b)`).
     Ref(&'hir Pat<'hir>, Mutability),
 

compiler/rustc_hir/src/intravisit.rs

@@ -660,7 +660,9 @@ pub fn walk_pat<'v, V: Visitor<'v>>(visitor: &mut V, pattern: &'v Pat<'v>) -> V:
         PatKind::Tuple(tuple_elements, _) => {
             walk_list!(visitor, visit_pat, tuple_elements);
         }
-        PatKind::Box(ref subpattern) | PatKind::Ref(ref subpattern, _) => {
+        PatKind::Box(ref subpattern)
+        | PatKind::Deref(ref subpattern)
+        | PatKind::Ref(ref subpattern, _) => {
             try_visit!(visitor.visit_pat(subpattern));
         }
         PatKind::Binding(_, _hir_id, ident, ref optional_subpattern) => {

compiler/rustc_hir_analysis/src/check/region.rs

@@ -668,7 +668,7 @@ fn resolve_local<'tcx>(
             | PatKind::TupleStruct(_, subpats, _)
             | PatKind::Tuple(subpats, _) => subpats.iter().any(|p| is_binding_pat(p)),
 
-            PatKind::Box(subpat) => is_binding_pat(subpat),
+            PatKind::Box(subpat) | PatKind::Deref(subpat) => is_binding_pat(subpat),
 
             PatKind::Ref(_, _)
             | PatKind::Binding(hir::BindingAnnotation(hir::ByRef::No, _), ..)

compiler/rustc_hir_pretty/src/lib.rs

@@ -1808,6 +1808,12 @@ impl<'a> State<'a> {
                     self.pclose();
                 }
             }
+            PatKind::Deref(inner) => {
+                self.word("deref_pat!");
+                self.popen();
+                self.print_pat(inner);
+                self.pclose();
+            }
             PatKind::Ref(inner, mutbl) => {
                 let is_range_inner = matches!(inner.kind, PatKind::Range(..));
                 self.word("&");

compiler/rustc_hir_typeck/src/expr_use_visitor.rs

@@ -463,6 +463,7 @@ impl<'a, 'tcx> ExprUseVisitor<'a, 'tcx> {
                     }
                     PatKind::Or(_)
                     | PatKind::Box(_)
+                    | PatKind::Deref(_)
                     | PatKind::Ref(..)
                     | PatKind::Wild
                     | PatKind::Err(_) => {

compiler/rustc_hir_typeck/src/mem_categorization.rs

@@ -719,7 +719,7 @@ impl<'a, 'tcx> MemCategorizationContext<'a, 'tcx> {
                 self.cat_pattern_(place_with_id, subpat, op)?;
             }
 
-            PatKind::Box(subpat) | PatKind::Ref(subpat, _) => {
+            PatKind::Box(subpat) | PatKind::Ref(subpat, _) | PatKind::Deref(subpat) => {
                 // box p1, &p1, &mut p1. we can ignore the mutability of
                 // PatKind::Ref since that information is already contained
                 // in the type.

compiler/rustc_hir_typeck/src/pat.rs

@@ -210,10 +210,8 @@ 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::Deref(inner) => self.check_pat_deref(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) => {
                 self.check_pat_slice(pat.span, before, slice, after, expected, pat_info)
@@ -297,6 +295,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             | PatKind::TupleStruct(..)
             | PatKind::Tuple(..)
             | PatKind::Box(_)
+            | PatKind::Deref(_)
             | PatKind::Range(..)
             | PatKind::Slice(..) => AdjustMode::Peel,
             // A never pattern behaves somewhat like a literal or unit variant.
@@ -762,6 +761,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         | PatKind::Binding(..)
                         | PatKind::Path(..)
                         | PatKind::Box(..)
+                        | PatKind::Deref(_)
                         | PatKind::Ref(..)
                         | PatKind::Lit(..)
                         | PatKind::Range(..)

compiler/rustc_lint/src/unused.rs

@@ -1181,7 +1181,7 @@ impl EarlyLintPass for UnusedParens {
                 self.check_unused_parens_pat(cx, &f.pat, false, false, keep_space);
             },
             // Avoid linting on `i @ (p0 | .. | pn)` and `box (p0 | .. | pn)`, #64106.
-            Ident(.., Some(p)) | Box(p) => self.check_unused_parens_pat(cx, p, true, false, keep_space),
+            Ident(.., Some(p)) | Box(p) | Deref(p) => self.check_unused_parens_pat(cx, p, true, false, keep_space),
             // Avoid linting on `&(mut x)` as `&mut x` has a different meaning, #55342.
             // Also avoid linting on `& mut? (p0 | .. | pn)`, #64106.
             Ref(p, m) => self.check_unused_parens_pat(cx, p, true, *m == Mutability::Not, keep_space),

compiler/rustc_middle/src/thir.rs

@@ -1179,7 +1179,7 @@ impl<'tcx> fmt::Display for Pat<'tcx> {
                 write!(f, "{subpattern}")
             }
             PatKind::DerefPattern { ref subpattern } => {
-                write!(f, "k#deref {subpattern}")
+                write!(f, "deref_pat!({subpattern})")
             }
             PatKind::Constant { value } => write!(f, "{value}"),
             PatKind::InlineConstant { def: _, ref subpattern } => {

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

@@ -257,7 +257,7 @@ 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 => {
+            hir::PatKind::Deref(subpattern) => {
                 PatKind::DerefPattern { subpattern: self.lower_pattern(subpattern) }
             }
             hir::PatKind::Ref(subpattern, _) | hir::PatKind::Box(subpattern) => {

compiler/rustc_passes/src/hir_stats.rs

@@ -300,6 +300,7 @@ impl<'v> hir_visit::Visitor<'v> for StatCollector<'v> {
                 Path,
                 Tuple,
                 Box,
+                Deref,
                 Ref,
                 Lit,
                 Range,
@@ -566,6 +567,7 @@ impl<'v> ast_visit::Visitor<'v> for StatCollector<'v> {
                 Path,
                 Tuple,
                 Box,
+                Deref,
                 Ref,
                 Lit,
                 Range,

compiler/rustc_pattern_analysis/src/rustc.rs

@@ -465,6 +465,7 @@ impl<'p, 'tcx: 'p> RustcPatCtxt<'p, 'tcx> {
             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, .. } => {

compiler/rustc_span/src/symbol.rs

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

library/core/src/macros/mod.rs

@@ -1714,6 +1714,18 @@ pub(crate) mod builtin {
         /* compiler built-in */
     }
 
+    #[cfg(not(bootstrap))]
+    /// Unstable placeholder for type ascription.
+    #[rustc_builtin_macro]
+    #[unstable(
+        feature = "deref_patterns",
+        issue = "87121",
+        reason = "placeholder syntax for deref patterns"
+    )]
+    pub macro deref_pat($pat:pat) {
+        /* compiler built-in */
+    }
+
     /// Unstable implementation detail of the `rustc` compiler, do not use.
     #[rustc_builtin_macro]
     #[stable(feature = "rust1", since = "1.0.0")]

library/core/src/prelude/v1.rs

@@ -103,3 +103,11 @@ pub use crate::macros::builtin::cfg_eval;
     reason = "placeholder syntax for type ascription"
 )]
 pub use crate::macros::builtin::type_ascribe;
+
+#[cfg(not(bootstrap))]
+#[unstable(
+    feature = "deref_patterns",
+    issue = "87121",
+    reason = "placeholder syntax for deref patterns"
+)]
+pub use crate::macros::builtin::deref_pat;

library/std/src/prelude/v1.rs

@@ -91,6 +91,15 @@ pub use core::prelude::v1::cfg_eval;
 )]
 pub use core::prelude::v1::type_ascribe;
 
+#[cfg(not(bootstrap))]
+// Do not `doc(no_inline)` either.
+#[unstable(
+    feature = "deref_patterns",
+    issue = "87121",
+    reason = "placeholder syntax for deref patterns"
+)]
+pub use core::prelude::v1::deref_pat;
+
 // The file so far is equivalent to core/src/prelude/v1.rs. It is duplicated
 // rather than glob imported because we want docs to show these re-exports as
 // pointing to within `std`.

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

@@ -7,19 +7,19 @@ use std::rc::Rc;
 fn main() {
     let vec: Vec<u32> = Vec::new();
     match vec {
-        box [..] => {}
+        deref_pat!([..]) => {}
         _ => {}
     }
     match Box::new(true) {
-        box true => {}
+        deref_pat!(true) => {}
         _ => {}
     }
     match &Box::new(true) {
-        box true => {}
+        deref_pat!(true) => {}
         _ => {}
     }
     match &Rc::new(0) {
-        box (1..) => {}
+        deref_pat!(1..) => {}
         _ => {}
     }
     // FIXME(deref_patterns): fails to typecheck because `"foo"` has type &str but deref creates a