Require DerefMut and DerefPure on deref!() patterns when appropriate

compiler/rustc_hir/src/lang_items.rs

@@ -199,6 +199,7 @@ language_item_table! {
 
     Deref,                   sym::deref,               deref_trait,                Target::Trait,          GenericRequirement::Exact(0);
     DerefMut,                sym::deref_mut,           deref_mut_trait,            Target::Trait,          GenericRequirement::Exact(0);
+    DerefPure,               sym::deref_pure,          deref_pure_trait,           Target::Trait,          GenericRequirement::Exact(0);
     DerefTarget,             sym::deref_target,        deref_target,               Target::AssocTy,        GenericRequirement::None;
     Receiver,                sym::receiver,            receiver_trait,             Target::Trait,          GenericRequirement::None;
 

compiler/rustc_hir_typeck/src/pat.rs

@@ -2002,8 +2002,12 @@ impl<'a, 'tcx> FnCtxt<'a, '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
+        // Register a `DerefPure` bound, which is required by all `deref!()` pats.
+        self.register_bound(
+            expected,
+            tcx.require_lang_item(hir::LangItem::DerefPure, Some(span)),
+            self.misc(span),
+        );
         // <expected as Deref>::Target
         let ty = Ty::new_projection(
             tcx,
@@ -2013,6 +2017,19 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let ty = self.normalize(span, ty);
         let ty = self.try_structurally_resolve_type(span, ty);
         self.check_pat(inner, ty, pat_info);
+
+        // Check if the pattern has any `ref mut` bindings, which would require
+        // `DerefMut` to be emitted in MIR building instead of just `Deref`.
+        // We do this *after* checking the inner pattern, since we want to make
+        // sure to apply any match-ergonomics adjustments.
+        if self.typeck_results.borrow().pat_has_ref_mut_binding(inner) {
+            self.register_bound(
+                expected,
+                tcx.require_lang_item(hir::LangItem::DerefMut, Some(span)),
+                self.misc(span),
+            );
+        }
+
         expected
     }
 

compiler/rustc_middle/src/ty/typeck_results.rs

@@ -430,6 +430,31 @@ impl<'tcx> TypeckResults<'tcx> {
         LocalTableInContextMut { hir_owner: self.hir_owner, data: &mut self.pat_adjustments }
     }
 
+    /// Does the pattern recursively contain a `ref mut` binding in it?
+    ///
+    /// This is used to determined whether a `deref` pattern should emit a `Deref`
+    /// or `DerefMut` call for its pattern scrutinee.
+    ///
+    /// This is computed from the typeck results since we want to make
+    /// sure to apply any match-ergonomics adjustments, which we cannot
+    /// determine from the HIR alone.
+    pub fn pat_has_ref_mut_binding(&self, pat: &'tcx hir::Pat<'tcx>) -> bool {
+        let mut has_ref_mut = false;
+        pat.walk(|pat| {
+            if let hir::PatKind::Binding(_, id, _, _) = pat.kind
+                && let Some(ty::BindByReference(ty::Mutability::Mut)) =
+                    self.pat_binding_modes().get(id)
+            {
+                has_ref_mut = true;
+                // No need to continue recursing
+                false
+            } else {
+                true
+            }
+        });
+        has_ref_mut
+    }
+
     /// For a given closure, returns the iterator of `ty::CapturedPlace`s that are captured
     /// by the closure.
     pub fn closure_min_captures_flattened(

compiler/rustc_span/src/symbol.rs

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

library/alloc/src/boxed.rs

@@ -161,7 +161,7 @@ use core::marker::Unsize;
 use core::mem::{self, SizedTypeProperties};
 use core::ops::{AsyncFn, AsyncFnMut, AsyncFnOnce};
 use core::ops::{
-    CoerceUnsized, Coroutine, CoroutineState, Deref, DerefMut, DispatchFromDyn, Receiver,
+    CoerceUnsized, Coroutine, CoroutineState, Deref, DerefMut, DerefPure, DispatchFromDyn, Receiver,
 };
 use core::pin::Pin;
 use core::ptr::{self, addr_of_mut, NonNull, Unique};
@@ -1939,6 +1939,9 @@ impl<T: ?Sized, A: Allocator> DerefMut for Box<T, A> {
     }
 }
 
+#[unstable(feature = "deref_pure_trait", issue = "87121")]
+unsafe impl<T: ?Sized, A: Allocator> DerefPure for Box<T, A> {}
+
 #[unstable(feature = "receiver_trait", issue = "none")]
 impl<T: ?Sized, A: Allocator> Receiver for Box<T, A> {}
 

library/alloc/src/lib.rs

@@ -122,6 +122,7 @@
 #![feature(const_waker)]
 #![feature(core_intrinsics)]
 #![feature(deprecated_suggestion)]
+#![feature(deref_pure_trait)]
 #![feature(dispatch_from_dyn)]
 #![feature(error_generic_member_access)]
 #![feature(error_in_core)]

library/alloc/src/rc.rs

@@ -260,7 +260,7 @@ use core::marker::{PhantomData, Unsize};
 #[cfg(not(no_global_oom_handling))]
 use core::mem::size_of_val;
 use core::mem::{self, align_of_val_raw, forget, ManuallyDrop};
-use core::ops::{CoerceUnsized, Deref, DerefMut, DispatchFromDyn, Receiver};
+use core::ops::{CoerceUnsized, Deref, DerefMut, DerefPure, DispatchFromDyn, Receiver};
 use core::panic::{RefUnwindSafe, UnwindSafe};
 #[cfg(not(no_global_oom_handling))]
 use core::pin::Pin;
@@ -2126,6 +2126,9 @@ impl<T: ?Sized, A: Allocator> Deref for Rc<T, A> {
     }
 }
 
+#[unstable(feature = "deref_pure_trait", issue = "87121")]
+unsafe impl<T: ?Sized, A: Allocator> DerefPure for Rc<T, A> {}
+
 #[unstable(feature = "receiver_trait", issue = "none")]
 impl<T: ?Sized> Receiver for Rc<T> {}
 

library/alloc/src/string.rs

@@ -2479,6 +2479,9 @@ impl ops::Deref for String {
     }
 }
 
+#[unstable(feature = "deref_pure_trait", issue = "87121")]
+unsafe impl ops::DerefPure for String {}
+
 #[stable(feature = "derefmut_for_string", since = "1.3.0")]
 impl ops::DerefMut for String {
     #[inline]

library/alloc/src/sync.rs

@@ -21,7 +21,7 @@ use core::marker::{PhantomData, Unsize};
 #[cfg(not(no_global_oom_handling))]
 use core::mem::size_of_val;
 use core::mem::{self, align_of_val_raw};
-use core::ops::{CoerceUnsized, Deref, DispatchFromDyn, Receiver};
+use core::ops::{CoerceUnsized, Deref, DerefPure, DispatchFromDyn, Receiver};
 use core::panic::{RefUnwindSafe, UnwindSafe};
 use core::pin::Pin;
 use core::ptr::{self, NonNull};
@@ -2107,6 +2107,9 @@ impl<T: ?Sized, A: Allocator> Deref for Arc<T, A> {
     }
 }
 
+#[unstable(feature = "deref_pure_trait", issue = "87121")]
+unsafe impl<T: ?Sized, A: Allocator> DerefPure for Arc<T, A> {}
+
 #[unstable(feature = "receiver_trait", issue = "none")]
 impl<T: ?Sized> Receiver for Arc<T> {}
 

library/alloc/src/vec/mod.rs

@@ -2772,6 +2772,9 @@ impl<T, A: Allocator> ops::DerefMut for Vec<T, A> {
     }
 }
 
+#[unstable(feature = "deref_pure_trait", issue = "87121")]
+unsafe impl<T, A: Allocator> ops::DerefPure for Vec<T, A> {}
+
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Clone, A: Allocator + Clone> Clone for Vec<T, A> {

library/core/src/ops/deref.rs

@@ -275,6 +275,25 @@ impl<T: ?Sized> DerefMut for &mut T {
     }
 }
 
+/// Perma-unstable marker trait. Indicates that the type has a well-behaved [`Deref`]
+/// (and, if applicable, [`DerefMut`]) implementation. This is relied on for soundness
+/// of deref patterns.
+///
+/// FIXME(deref_patterns): The precise semantics are undecided; the rough idea is that
+/// successive calls to `deref`/`deref_mut` without intermediate mutation should be
+/// idempotent, in the sense that they return the same value as far as pattern-matching
+/// is concerned. Calls to `deref`/`deref_mut`` must leave the pointer itself likewise
+/// unchanged.
+#[unstable(feature = "deref_pure_trait", issue = "87121")]
+#[cfg_attr(not(bootstrap), lang = "deref_pure")]
+pub unsafe trait DerefPure {}
+
+#[unstable(feature = "deref_pure_trait", issue = "87121")]
+unsafe impl<T: ?Sized> DerefPure for &T {}
+
+#[unstable(feature = "deref_pure_trait", issue = "87121")]
+unsafe impl<T: ?Sized> DerefPure for &mut T {}
+
 /// Indicates that a struct can be used as a method receiver, without the
 /// `arbitrary_self_types` feature. This is implemented by stdlib pointer types like `Box<T>`,
 /// `Rc<T>`, `&T`, and `Pin<P>`.

library/core/src/ops/mod.rs

@@ -165,6 +165,9 @@ pub use self::bit::{BitAndAssign, BitOrAssign, BitXorAssign, ShlAssign, ShrAssig
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::deref::{Deref, DerefMut};
 
+#[unstable(feature = "deref_pure_trait", issue = "87121")]
+pub use self::deref::DerefPure;
+
 #[unstable(feature = "receiver_trait", issue = "none")]
 pub use self::deref::Receiver;
 

tests/ui/pattern/deref-patterns/ref-mut.rs

@@ -0,0 +1,17 @@
+#![feature(deref_patterns)]
+//~^ WARN the feature `deref_patterns` is incomplete
+
+use std::rc::Rc;
+
+fn main() {
+    match &mut vec![1] {
+        deref!(x) => {}
+        _ => {}
+    }
+
+    match &mut Rc::new(1) {
+        deref!(x) => {}
+        //~^ ERROR the trait bound `Rc<{integer}>: DerefMut` is not satisfied
+        _ => {}
+    }
+}

tests/ui/pattern/deref-patterns/ref-mut.stderr

@@ -0,0 +1,20 @@
+warning: the feature `deref_patterns` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/ref-mut.rs:1:12
+   |
+LL | #![feature(deref_patterns)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: see issue #87121 <https://github.com/rust-lang/rust/issues/87121> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error[E0277]: the trait bound `Rc<{integer}>: DerefMut` is not satisfied
+  --> $DIR/ref-mut.rs:13:9
+   |
+LL |         deref!(x) => {}
+   |         ^^^^^^^^^ the trait `DerefMut` is not implemented for `Rc<{integer}>`
+   |
+   = note: this error originates in the macro `deref` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 1 previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0277`.