let deref patterns participate in usefulness/exhaustiveness

This does not yet handle the case of mixed deref patterns with normal
constructors; it'll ICE in `Constructor::is_covered_by`. That'll be
fixed in a later commit.

Author: dianne

compiler/rustc_pattern_analysis/src/constructor.rs

@@ -696,6 +696,10 @@ pub enum Constructor<Cx: PatCx> {
     F128Range(IeeeFloat<QuadS>, IeeeFloat<QuadS>, RangeEnd),
     /// String literals. Strings are not quite the same as `&[u8]` so we treat them separately.
     Str(Cx::StrLit),
+    /// Deref patterns (enabled by the `deref_patterns` feature) provide a way of matching on a
+    /// smart pointer ADT through its pointee. They don't directly correspond to ADT constructors,
+    /// and currently are not supported alongside them. Carries the type of the pointee.
+    DerefPattern(Cx::Ty),
     /// Constants that must not be matched structurally. They are treated as black boxes for the
     /// purposes of exhaustiveness: we must not inspect them, and they don't count towards making a
     /// match exhaustive.
@@ -740,6 +744,7 @@ impl<Cx: PatCx> Clone for Constructor<Cx> {
             Constructor::F64Range(lo, hi, end) => Constructor::F64Range(*lo, *hi, *end),
             Constructor::F128Range(lo, hi, end) => Constructor::F128Range(*lo, *hi, *end),
             Constructor::Str(value) => Constructor::Str(value.clone()),
+            Constructor::DerefPattern(ty) => Constructor::DerefPattern(ty.clone()),
             Constructor::Opaque(inner) => Constructor::Opaque(inner.clone()),
             Constructor::Or => Constructor::Or,
             Constructor::Never => Constructor::Never,
@@ -856,6 +861,10 @@ impl<Cx: PatCx> Constructor<Cx> {
             }
             (Slice(self_slice), Slice(other_slice)) => self_slice.is_covered_by(*other_slice),
 
+            // Deref patterns only interact with other deref patterns. Prior to usefulness analysis,
+            // we ensure they don't appear alongside any other non-wild non-opaque constructors.
+            (DerefPattern(_), DerefPattern(_)) => true,
+
             // Opaque constructors don't interact with anything unless they come from the
             // syntactically identical pattern.
             (Opaque(self_id), Opaque(other_id)) => self_id == other_id,
@@ -932,6 +941,7 @@ impl<Cx: PatCx> Constructor<Cx> {
             F64Range(lo, hi, end) => write!(f, "{lo}{end}{hi}")?,
             F128Range(lo, hi, end) => write!(f, "{lo}{end}{hi}")?,
             Str(value) => write!(f, "{value:?}")?,
+            DerefPattern(_) => write!(f, "deref!({:?})", fields.next().unwrap())?,
             Opaque(..) => write!(f, "<constant pattern>")?,
             Or => {
                 for pat in fields {
@@ -1039,15 +1049,27 @@ impl<Cx: PatCx> ConstructorSet<Cx> {
         let mut missing = Vec::new();
         // Constructors in `ctors`, except wildcards and opaques.
         let mut seen = Vec::new();
+        // If we see a deref pattern, it must be the only non-wildcard non-opaque constructor; we
+        // ensure this prior to analysis.
+        let mut deref_pat_present = false;
         for ctor in ctors.cloned() {
             match ctor {
+                DerefPattern(..) => {
+                    if !deref_pat_present {
+                        deref_pat_present = true;
+                        present.push(ctor);
+                    }
+                }
                 Opaque(..) => present.push(ctor),
                 Wildcard => {} // discard wildcards
                 _ => seen.push(ctor),
             }
         }
 
         match self {
+            _ if deref_pat_present => {
+                // Deref patterns are the only constructor; nothing is missing.
+            }
             ConstructorSet::Struct { empty } => {
                 if !seen.is_empty() {
                     present.push(Struct);

compiler/rustc_pattern_analysis/src/rustc.rs

@@ -269,6 +269,7 @@ impl<'p, 'tcx: 'p> RustcPatCtxt<'p, 'tcx> {
                 }
                 _ => bug!("bad slice pattern {:?} {:?}", ctor, ty),
             },
+            DerefPattern(pointee_ty) => reveal_and_alloc(cx, once(pointee_ty.inner())),
             Bool(..) | IntRange(..) | F16Range(..) | F32Range(..) | F64Range(..)
             | F128Range(..) | Str(..) | Opaque(..) | Never | NonExhaustive | Hidden | Missing
             | PrivateUninhabited | Wildcard => &[],
@@ -296,7 +297,7 @@ impl<'p, 'tcx: 'p> RustcPatCtxt<'p, 'tcx> {
                 }
                 _ => bug!("Unexpected type for constructor `{ctor:?}`: {ty:?}"),
             },
-            Ref => 1,
+            Ref | DerefPattern(_) => 1,
             Slice(slice) => slice.arity(),
             Bool(..) | IntRange(..) | F16Range(..) | F32Range(..) | F64Range(..)
             | F128Range(..) | Str(..) | Opaque(..) | Never | NonExhaustive | Hidden | Missing
@@ -493,11 +494,15 @@ 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::DerefPattern { subpattern, .. } => {
+                // NB(deref_patterns): This assumes the deref pattern is matching on a trusted
+                // `DerefPure` type. If the `Deref` impl isn't trusted, exhaustiveness must take
+                // into account that multiple calls to deref may return different results. Hence
+                // multiple deref! patterns cannot be exhaustive together unless each is exhaustive
+                // by itself.
+                fields = vec![self.lower_pat(subpattern).at_index(0)];
+                arity = 1;
+                ctor = DerefPattern(cx.reveal_opaque_ty(subpattern.ty));
             }
             PatKind::Leaf { subpatterns } | PatKind::Variant { subpatterns, .. } => {
                 match ty.kind() {
@@ -874,6 +879,7 @@ impl<'p, 'tcx: 'p> RustcPatCtxt<'p, 'tcx> {
                 print::write_ref_like(&mut s, pat.ty().inner(), &print(&pat.fields[0])).unwrap();
                 s
             }
+            DerefPattern(_) => format!("deref!({})", print(&pat.fields[0])),
             Slice(slice) => {
                 let (prefix_len, has_dot_dot) = match slice.kind {
                     SliceKind::FixedLen(len) => (len, false),

compiler/rustc_pattern_analysis/src/usefulness.rs

@@ -702,6 +702,7 @@
 //!   - `ui/consts/const_in_pattern`
 //!   - `ui/rfc-2008-non-exhaustive`
 //!   - `ui/half-open-range-patterns`
+//!   - `ui/pattern/deref-patterns`
 //!   - probably many others
 //!
 //! I (Nadrieril) prefer to put new tests in `ui/pattern/usefulness` unless there's a specific
@@ -866,7 +867,8 @@ impl PlaceValidity {
     /// inside `&` and union fields where validity is reset to `MaybeInvalid`.
     fn specialize<Cx: PatCx>(self, ctor: &Constructor<Cx>) -> Self {
         // We preserve validity except when we go inside a reference or a union field.
-        if matches!(ctor, Constructor::Ref | Constructor::UnionField) {
+        if matches!(ctor, Constructor::Ref | Constructor::DerefPattern(_) | Constructor::UnionField)
+        {
             // Validity of `x: &T` does not imply validity of `*x: T`.
             MaybeInvalid
         } else {

src/doc/unstable-book/src/language-features/deref-patterns.md

@@ -60,8 +60,7 @@ Like [`box_patterns`], deref patterns may move out of boxes:
 # #![feature(deref_patterns)]
 # #![allow(incomplete_features)]
 struct NoCopy;
-// Match exhaustiveness analysis is not yet implemented.
-let deref!(x) = Box::new(NoCopy) else { unreachable!() };
+let deref!(x) = Box::new(NoCopy);
 drop::<NoCopy>(x);
 ```
 

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

@@ -13,7 +13,6 @@ fn simple_vec(vec: Vec<u32>) -> u32 {
         deref!([x]) => x,
         deref!([1, x]) => x + 200,
         deref!(ref slice) => slice.iter().sum(),
-        _ => 2000,
     }
 }
 
@@ -25,7 +24,6 @@ fn simple_vec(vec: Vec<u32>) -> u32 {
         [x] => x,
         [1, x] => x + 200,
         deref!(ref slice) => slice.iter().sum(),
-        _ => 2000,
     }
 }
 

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

@@ -9,7 +9,7 @@ struct NoCopy;
 fn main() {
     let b = Rc::new("aaa".to_string());
     let f = || {
-        let deref!(ref s) = b else { unreachable!() };
+        let deref!(ref s) = b;
         assert_eq!(s.len(), 3);
     };
     assert_eq!(b.len(), 3);
@@ -26,7 +26,7 @@ fn main() {
 
     let mut b = "aaa".to_string();
     let mut f = || {
-        let deref!(ref mut s) = b else { unreachable!() };
+        let deref!(ref mut s) = b;
         s.make_ascii_uppercase();
     };
     f();
@@ -53,15 +53,15 @@ fn main() {
     let b = Box::new(NoCopy);
     let f = || {
         // this should move out of the box rather than borrow.
-        let deref!(x) = b else { unreachable!() };
+        let deref!(x) = b;
         drop::<NoCopy>(x);
     };
     f();
 
     let b = Box::new((NoCopy,));
     let f = || {
         // this should move out of the box rather than borrow.
-        let (x,) = b else { unreachable!() };
+        let (x,) = b;
         drop::<NoCopy>(x);
     };
     f();

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

@@ -6,18 +6,18 @@
 #![expect(incomplete_features)]
 
 fn unbox_1<T>(b: Box<T>) -> T {
-    let deref!(x) = b else { unreachable!() };
+    let deref!(x) = b;
     x
 }
 
 fn unbox_2<T>(b: Box<(T,)>) -> T {
-    let (x,) = b else { unreachable!() };
+    let (x,) = b;
     x
 }
 
 fn unbox_separately<T>(b: Box<(T, T)>) -> (T, T) {
-    let (x, _) = b else { unreachable!() };
-    let (_, y) = b else { unreachable!() };
+    let (x, _) = b;
+    let (_, y) = b;
     (x, y)
 }
 
@@ -31,7 +31,7 @@ fn main() {
 
     // test that borrowing from a box also works
     let mut b = "hi".to_owned().into_boxed_str();
-    let deref!(ref mut s) = b else { unreachable!() };
+    let deref!(ref mut s) = b;
     s.make_ascii_uppercase();
     assert_eq!(&*b, "HI");
 }

tests/ui/pattern/deref-patterns/implicit-cow-deref.rs

@@ -11,7 +11,6 @@ fn main() {
 
     match cow {
         [..] => {}
-        _ => unreachable!(),
     }
 
     match cow {
@@ -22,14 +21,12 @@ fn main() {
     match Rc::new(&cow) {
         Cow::Borrowed { 0: _ } => {}
         Cow::Owned { 0: _ } => unreachable!(),
-        _ => unreachable!(),
     }
 
     let cow_of_cow: Cow<'_, Cow<'static, [u8]>> = Cow::Owned(cow);
 
     match cow_of_cow {
         [..] => {}
-        _ => unreachable!(),
     }
 
     // This matches on the outer `Cow` (the owned one).
@@ -41,6 +38,5 @@ fn main() {
     match Rc::new(&cow_of_cow) {
         Cow::Borrowed { 0: _ } => unreachable!(),
         Cow::Owned { 0: _ } => {}
-        _ => unreachable!(),
     }
 }