Rollup merge of #138528 - dianne:implicit-deref-patterns, r=Nadrieril

deref patterns: implement implicit deref patterns

This implements implicit deref patterns (per https://hackmd.io/4qDDMcvyQ-GDB089IPcHGg#Implicit-deref-patterns) and adds tests and an unstable book chapter.

Best reviewed commit-by-commit. Overall there's a lot of additions, but a lot of that is tests, documentation, and simple(?) refactoring.

Tracking issue: #87121

r? ``@Nadrieril``

21 files changed, 437 insertions, 6 deletions

diff --git a/tests/ui/pattern/deref-patterns/bindings.rs b/tests/ui/pattern/deref-patterns/bindings.rs
index 5881e4166a4..c14d57f3f24 100644
--- a/tests/ui/pattern/deref-patterns/bindings.rs
+++ b/tests/ui/pattern/deref-patterns/bindings.rs
@@ -1,7 +1,9 @@
+//@ revisions: explicit implicit
 //@ run-pass
 #![feature(deref_patterns)]
 #![allow(incomplete_features)]
 
+#[cfg(explicit)]
 fn simple_vec(vec: Vec<u32>) -> u32 {
     match vec {
         deref!([]) => 100,
@@ -13,6 +15,19 @@ fn simple_vec(vec: Vec<u32>) -> u32 {
     }
 }
 
+#[cfg(implicit)]
+fn simple_vec(vec: Vec<u32>) -> u32 {
+    match vec {
+        [] => 100,
+        [x] if x == 4 => x + 4,
+        [x] => x,
+        [1, x] => x + 200,
+        deref!(ref slice) => slice.iter().sum(),
+        _ => 2000,
+    }
+}
+
+#[cfg(explicit)]
 fn nested_vec(vecvec: Vec<Vec<u32>>) -> u32 {
     match vecvec {
         deref!([]) => 0,
@@ -24,6 +39,19 @@ fn nested_vec(vecvec: Vec<Vec<u32>>) -> u32 {
     }
 }
 
+#[cfg(implicit)]
+fn nested_vec(vecvec: Vec<Vec<u32>>) -> u32 {
+    match vecvec {
+        [] => 0,
+        [[x]] => x,
+        [[0, x] | [1, x]] => x,
+        [ref x] => x.iter().sum(),
+        [[], [1, x, y]] => y - x,
+        _ => 2000,
+    }
+}
+
+#[cfg(explicit)]
 fn ref_mut(val: u32) -> u32 {
     let mut b = Box::new(0u32);
     match &mut b {
@@ -37,6 +65,21 @@ fn ref_mut(val: u32) -> u32 {
     *x
 }
 
+#[cfg(implicit)]
+fn ref_mut(val: u32) -> u32 {
+    let mut b = Box::new((0u32,));
+    match &mut b {
+        (_x,) if false => unreachable!(),
+        (x,) => {
+            *x = val;
+        }
+        _ => unreachable!(),
+    }
+    let (x,) = &b else { unreachable!() };
+    *x
+}
+
+#[cfg(explicit)]
 #[rustfmt::skip]
 fn or_and_guard(tuple: (u32, u32)) -> u32 {
     let mut sum = 0;
@@ -48,6 +91,18 @@ fn or_and_guard(tuple: (u32, u32)) -> u32 {
     sum
 }
 
+#[cfg(implicit)]
+#[rustfmt::skip]
+fn or_and_guard(tuple: (u32, u32)) -> u32 {
+    let mut sum = 0;
+    let b = Box::new(tuple);
+    match b {
+        (x, _) | (_, x) if { sum += x; false } => {},
+        _ => {},
+    }
+    sum
+}
+
 fn main() {
     assert_eq!(simple_vec(vec![1]), 1);
     assert_eq!(simple_vec(vec![1, 2]), 202);
diff --git a/tests/ui/pattern/deref-patterns/branch.rs b/tests/ui/pattern/deref-patterns/branch.rs
index 1bac1006d9d..9d72b35fd2f 100644
--- a/tests/ui/pattern/deref-patterns/branch.rs
+++ b/tests/ui/pattern/deref-patterns/branch.rs
@@ -1,8 +1,10 @@
+//@ revisions: explicit implicit
 //@ run-pass
 // Test the execution of deref patterns.
 #![feature(deref_patterns)]
 #![allow(incomplete_features)]
 
+#[cfg(explicit)]
 fn branch(vec: Vec<u32>) -> u32 {
     match vec {
         deref!([]) => 0,
@@ -12,6 +14,17 @@ fn branch(vec: Vec<u32>) -> u32 {
     }
 }
 
+#[cfg(implicit)]
+fn branch(vec: Vec<u32>) -> u32 {
+    match vec {
+        [] => 0,
+        [1, _, 3] => 1,
+        [2, ..] => 2,
+        _ => 1000,
+    }
+}
+
+#[cfg(explicit)]
 fn nested(vec: Vec<Vec<u32>>) -> u32 {
     match vec {
         deref!([deref!([]), ..]) => 1,
@@ -20,6 +33,15 @@ fn nested(vec: Vec<Vec<u32>>) -> u32 {
     }
 }
 
+#[cfg(implicit)]
+fn nested(vec: Vec<Vec<u32>>) -> u32 {
+    match vec {
+        [[], ..] => 1,
+        [[0, ..], [1, ..]] => 2,
+        _ => 1000,
+    }
+}
+
 fn main() {
     assert!(matches!(Vec::<u32>::new(), deref!([])));
     assert!(matches!(vec![1], deref!([1])));
diff --git a/tests/ui/pattern/deref-patterns/cant_move_out_of_pattern.rs b/tests/ui/pattern/deref-patterns/cant_move_out_of_pattern.rs
index 84b5ec09dc7..791776be5ac 100644
--- a/tests/ui/pattern/deref-patterns/cant_move_out_of_pattern.rs
+++ b/tests/ui/pattern/deref-patterns/cant_move_out_of_pattern.rs
@@ -21,4 +21,22 @@ fn cant_move_out_rc(rc: Rc<Struct>) -> Struct {
     }
 }
 
+struct Container(Struct);
+
+fn cant_move_out_box_implicit(b: Box<Container>) -> Struct {
+    match b {
+        //~^ ERROR: cannot move out of a shared reference
+        Container(x) => x,
+        _ => unreachable!(),
+    }
+}
+
+fn cant_move_out_rc_implicit(rc: Rc<Container>) -> Struct {
+    match rc {
+        //~^ ERROR: cannot move out of a shared reference
+        Container(x) => x,
+        _ => unreachable!(),
+    }
+}
+
 fn main() {}
diff --git a/tests/ui/pattern/deref-patterns/cant_move_out_of_pattern.stderr b/tests/ui/pattern/deref-patterns/cant_move_out_of_pattern.stderr
index 2cf435b1179..1887800fc38 100644
--- a/tests/ui/pattern/deref-patterns/cant_move_out_of_pattern.stderr
+++ b/tests/ui/pattern/deref-patterns/cant_move_out_of_pattern.stderr
@@ -32,6 +32,40 @@ help: consider borrowing the pattern binding
 LL |         deref!(ref x) => x,
    |                +++
 
-error: aborting due to 2 previous errors
+error[E0507]: cannot move out of a shared reference
+  --> $DIR/cant_move_out_of_pattern.rs:27:11
+   |
+LL |     match b {
+   |           ^
+LL |
+LL |         Container(x) => x,
+   |                   -
+   |                   |
+   |                   data moved here
+   |                   move occurs because `x` has type `Struct`, which does not implement the `Copy` trait
+   |
+help: consider borrowing the pattern binding
+   |
+LL |         Container(ref x) => x,
+   |                   +++
+
+error[E0507]: cannot move out of a shared reference
+  --> $DIR/cant_move_out_of_pattern.rs:35:11
+   |
+LL |     match rc {
+   |           ^^
+LL |
+LL |         Container(x) => x,
+   |                   -
+   |                   |
+   |                   data moved here
+   |                   move occurs because `x` has type `Struct`, which does not implement the `Copy` trait
+   |
+help: consider borrowing the pattern binding
+   |
+LL |         Container(ref x) => x,
+   |                   +++
+
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0507`.
diff --git a/tests/ui/pattern/deref-patterns/closure_capture.rs b/tests/ui/pattern/deref-patterns/closure_capture.rs
index fc0ddedac2b..08586b6c7ab 100644
--- a/tests/ui/pattern/deref-patterns/closure_capture.rs
+++ b/tests/ui/pattern/deref-patterns/closure_capture.rs
@@ -11,6 +11,15 @@ fn main() {
     assert_eq!(b.len(), 3);
     f();
 
+    let v = vec![1, 2, 3];
+    let f = || {
+        // this should count as a borrow of `v` as a whole
+        let [.., x] = v else { unreachable!() };
+        assert_eq!(x, 3);
+    };
+    assert_eq!(v, [1, 2, 3]);
+    f();
+
     let mut b = Box::new("aaa".to_string());
     let mut f = || {
         let deref!(ref mut s) = b else { unreachable!() };
@@ -18,4 +27,22 @@ fn main() {
     };
     f();
     assert_eq!(b.len(), 5);
+
+    let mut v = vec![1, 2, 3];
+    let mut f = || {
+        // this should count as a mutable borrow of `v` as a whole
+        let [.., ref mut x] = v else { unreachable!() };
+        *x = 4;
+    };
+    f();
+    assert_eq!(v, [1, 2, 4]);
+
+    let mut v = vec![1, 2, 3];
+    let mut f = || {
+        // here, `[.., x]` is adjusted by both an overloaded deref and a builtin deref
+        let [.., x] = &mut v else { unreachable!() };
+        *x = 4;
+    };
+    f();
+    assert_eq!(v, [1, 2, 4]);
 }
diff --git a/tests/ui/pattern/deref-patterns/fake_borrows.rs b/tests/ui/pattern/deref-patterns/fake_borrows.rs
index 35fa9cbf7d8..bf614d7d66f 100644
--- a/tests/ui/pattern/deref-patterns/fake_borrows.rs
+++ b/tests/ui/pattern/deref-patterns/fake_borrows.rs
@@ -11,4 +11,11 @@ fn main() {
         deref!(false) => {}
         _ => {},
     }
+    match b {
+        true => {}
+        _ if { *b = true; false } => {}
+        //~^ ERROR cannot assign `*b` in match guard
+        false => {}
+        _ => {},
+    }
 }
diff --git a/tests/ui/pattern/deref-patterns/fake_borrows.stderr b/tests/ui/pattern/deref-patterns/fake_borrows.stderr
index 6a591e6416c..8c060236d0d 100644
--- a/tests/ui/pattern/deref-patterns/fake_borrows.stderr
+++ b/tests/ui/pattern/deref-patterns/fake_borrows.stderr
@@ -7,6 +7,15 @@ LL |         deref!(true) => {}
 LL |         _ if { *b = true; false } => {}
    |                ^^^^^^^^^ cannot assign
 
-error: aborting due to 1 previous error
+error[E0510]: cannot assign `*b` in match guard
+  --> $DIR/fake_borrows.rs:16:16
+   |
+LL |     match b {
+   |           - value is immutable in match guard
+LL |         true => {}
+LL |         _ if { *b = true; false } => {}
+   |                ^^^^^^^^^ cannot assign
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0510`.
diff --git a/tests/ui/pattern/deref-patterns/implicit-const-deref.rs b/tests/ui/pattern/deref-patterns/implicit-const-deref.rs
new file mode 100644
index 00000000000..70f89629bc2
--- /dev/null
+++ b/tests/ui/pattern/deref-patterns/implicit-const-deref.rs
@@ -0,0 +1,19 @@
+//! Test that we get an error about structural equality rather than a type error when attempting to
+//! use const patterns of library pointer types. Currently there aren't any smart pointers that can
+//! be used in constant patterns, but we still need to make sure we don't implicitly dereference the
+//! scrutinee and end up with a type error; this would prevent us from reporting that only constants
+//! supporting structural equality can be used as patterns.
+#![feature(deref_patterns)]
+#![allow(incomplete_features)]
+
+const EMPTY: Vec<()> = Vec::new();
+
+fn main() {
+    // FIXME(inline_const_pat): if `inline_const_pat` is reinstated, there should be a case here for
+    // inline const block patterns as well; they're checked differently than named constants.
+    match vec![()] {
+        EMPTY => {}
+        //~^ ERROR: constant of non-structural type `Vec<()>` in a pattern
+        _ => {}
+    }
+}
diff --git a/tests/ui/pattern/deref-patterns/implicit-const-deref.stderr b/tests/ui/pattern/deref-patterns/implicit-const-deref.stderr
new file mode 100644
index 00000000000..21d09ec44c4
--- /dev/null
+++ b/tests/ui/pattern/deref-patterns/implicit-const-deref.stderr
@@ -0,0 +1,16 @@
+error: constant of non-structural type `Vec<()>` in a pattern
+  --> $DIR/implicit-const-deref.rs:15:9
+   |
+LL | const EMPTY: Vec<()> = Vec::new();
+   | -------------------- constant defined here
+...
+LL |         EMPTY => {}
+   |         ^^^^^ constant of non-structural type
+  --> $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
+   |
+   = note: `Vec<()>` must be annotated with `#[derive(PartialEq)]` to be usable in patterns
+   |
+   = note: see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details
+
+error: aborting due to 1 previous error
+
diff --git a/tests/ui/pattern/deref-patterns/implicit-cow-deref.rs b/tests/ui/pattern/deref-patterns/implicit-cow-deref.rs
new file mode 100644
index 00000000000..a9b8de86010
--- /dev/null
+++ b/tests/ui/pattern/deref-patterns/implicit-cow-deref.rs
@@ -0,0 +1,45 @@
+//@ run-pass
+//! Test that implicit deref patterns interact as expected with `Cow` constructor patterns.
+#![feature(deref_patterns)]
+#![allow(incomplete_features)]
+
+use std::borrow::Cow;
+
+fn main() {
+    let cow: Cow<'static, [u8]> = Cow::Borrowed(&[1, 2, 3]);
+
+    match cow {
+        [..] => {}
+        _ => unreachable!(),
+    }
+
+    match cow {
+        Cow::Borrowed(_) => {}
+        Cow::Owned(_) => unreachable!(),
+    }
+
+    match Box::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).
+    match cow_of_cow {
+        Cow::Borrowed(_) => unreachable!(),
+        Cow::Owned(_) => {}
+    }
+
+    match Box::new(&cow_of_cow) {
+        Cow::Borrowed { 0: _ } => unreachable!(),
+        Cow::Owned { 0: _ } => {}
+        _ => unreachable!(),
+    }
+}
diff --git a/tests/ui/pattern/deref-patterns/needs-gate.rs b/tests/ui/pattern/deref-patterns/needs-gate.rs
new file mode 100644
index 00000000000..2d5ec45217f
--- /dev/null
+++ b/tests/ui/pattern/deref-patterns/needs-gate.rs
@@ -0,0 +1,15 @@
+// gate-test-deref_patterns
+
+fn main() {
+    match Box::new(0) {
+        deref!(0) => {}
+        //~^ ERROR: use of unstable library feature `deref_patterns`: placeholder syntax for deref patterns
+        _ => {}
+    }
+
+    match Box::new(0) {
+        0 => {}
+        //~^ ERROR: mismatched types
+        _ => {}
+    }
+}
diff --git a/tests/ui/pattern/deref-patterns/needs-gate.stderr b/tests/ui/pattern/deref-patterns/needs-gate.stderr
new file mode 100644
index 00000000000..8687b5dc977
--- /dev/null
+++ b/tests/ui/pattern/deref-patterns/needs-gate.stderr
@@ -0,0 +1,29 @@
+error[E0658]: use of unstable library feature `deref_patterns`: placeholder syntax for deref patterns
+  --> $DIR/needs-gate.rs:5:9
+   |
+LL |         deref!(0) => {}
+   |         ^^^^^
+   |
+   = note: see issue #87121 <https://github.com/rust-lang/rust/issues/87121> for more information
+   = help: add `#![feature(deref_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[E0308]: mismatched types
+  --> $DIR/needs-gate.rs:11:9
+   |
+LL |     match Box::new(0) {
+   |           ----------- this expression has type `Box<{integer}>`
+LL |         0 => {}
+   |         ^ expected `Box<{integer}>`, found integer
+   |
+   = note: expected struct `Box<{integer}>`
+                found type `{integer}`
+help: consider dereferencing to access the inner value using the Deref trait
+   |
+LL |     match *Box::new(0) {
+   |           +
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0308, E0658.
+For more information about an error, try `rustc --explain E0308`.
diff --git a/tests/ui/pattern/deref-patterns/recursion-limit.rs b/tests/ui/pattern/deref-patterns/recursion-limit.rs
new file mode 100644
index 00000000000..c5fe520f6f1
--- /dev/null
+++ b/tests/ui/pattern/deref-patterns/recursion-limit.rs
@@ -0,0 +1,23 @@
+//! Test that implicit deref patterns respect the recursion limit
+#![feature(deref_patterns)]
+#![allow(incomplete_features)]
+#![recursion_limit = "8"]
+
+use std::ops::Deref;
+
+struct Cyclic;
+impl Deref for Cyclic {
+    type Target = Cyclic;
+    fn deref(&self) -> &Cyclic {
+        &Cyclic
+    }
+}
+
+fn main() {
+    match &Box::new(Cyclic) {
+        () => {}
+        //~^ ERROR: reached the recursion limit while auto-dereferencing `Cyclic`
+        //~| ERROR: the trait bound `Cyclic: DerefPure` is not satisfied
+        _ => {}
+    }
+}
diff --git a/tests/ui/pattern/deref-patterns/recursion-limit.stderr b/tests/ui/pattern/deref-patterns/recursion-limit.stderr
new file mode 100644
index 00000000000..9a83d1eb5a4
--- /dev/null
+++ b/tests/ui/pattern/deref-patterns/recursion-limit.stderr
@@ -0,0 +1,18 @@
+error[E0055]: reached the recursion limit while auto-dereferencing `Cyclic`
+  --> $DIR/recursion-limit.rs:18:9
+   |
+LL |         () => {}
+   |         ^^ deref recursion limit reached
+   |
+   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "16"]` attribute to your crate (`recursion_limit`)
+
+error[E0277]: the trait bound `Cyclic: DerefPure` is not satisfied
+  --> $DIR/recursion-limit.rs:18:9
+   |
+LL |         () => {}
+   |         ^^ the trait `DerefPure` is not implemented for `Cyclic`
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0055, E0277.
+For more information about an error, try `rustc --explain E0055`.
diff --git a/tests/ui/pattern/deref-patterns/ref-mut.rs b/tests/ui/pattern/deref-patterns/ref-mut.rs
index 1918008a761..43738671346 100644
--- a/tests/ui/pattern/deref-patterns/ref-mut.rs
+++ b/tests/ui/pattern/deref-patterns/ref-mut.rs
@@ -8,10 +8,19 @@ fn main() {
         deref!(x) => {}
         _ => {}
     }
+    match &mut vec![1] {
+        [x] => {}
+        _ => {}
+    }
 
     match &mut Rc::new(1) {
         deref!(x) => {}
         //~^ ERROR the trait bound `Rc<{integer}>: DerefMut` is not satisfied
         _ => {}
     }
+    match &mut Rc::new((1,)) {
+        (x,) => {}
+        //~^ ERROR the trait bound `Rc<({integer},)>: DerefMut` is not satisfied
+        _ => {}
+    }
 }
diff --git a/tests/ui/pattern/deref-patterns/ref-mut.stderr b/tests/ui/pattern/deref-patterns/ref-mut.stderr
index 41f1c3061ce..24a35b418e9 100644
--- a/tests/ui/pattern/deref-patterns/ref-mut.stderr
+++ b/tests/ui/pattern/deref-patterns/ref-mut.stderr
@@ -8,13 +8,19 @@ LL | #![feature(deref_patterns)]
    = note: `#[warn(incomplete_features)]` on by default
 
 error[E0277]: the trait bound `Rc<{integer}>: DerefMut` is not satisfied
-  --> $DIR/ref-mut.rs:13:9
+  --> $DIR/ref-mut.rs:17:9
    |
 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
+error[E0277]: the trait bound `Rc<({integer},)>: DerefMut` is not satisfied
+  --> $DIR/ref-mut.rs:22:9
+   |
+LL |         (x,) => {}
+   |         ^^^^ the trait `DerefMut` is not implemented for `Rc<({integer},)>`
+
+error: aborting due to 2 previous errors; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/pattern/deref-patterns/typeck.rs b/tests/ui/pattern/deref-patterns/typeck.rs
index f23f7042cd8..3a7ce9d1deb 100644
--- a/tests/ui/pattern/deref-patterns/typeck.rs
+++ b/tests/ui/pattern/deref-patterns/typeck.rs
@@ -10,26 +10,32 @@ fn main() {
     let vec: Vec<u32> = Vec::new();
     match vec {
         deref!([..]) => {}
+        [..] => {}
         _ => {}
     }
     match Box::new(true) {
         deref!(true) => {}
+        true => {}
         _ => {}
     }
     match &Box::new(true) {
         deref!(true) => {}
+        true => {}
         _ => {}
     }
     match &Rc::new(0) {
         deref!(1..) => {}
+        1.. => {}
         _ => {}
     }
     let _: &Struct = match &Rc::new(Struct) {
         deref!(x) => x,
+        Struct => &Struct,
         _ => unreachable!(),
     };
     let _: &[Struct] = match &Rc::new(vec![Struct]) {
         deref!(deref!(x)) => x,
+        [Struct] => &[Struct],
         _ => unreachable!(),
     };
 }
diff --git a/tests/ui/pattern/deref-patterns/typeck_fail.rs b/tests/ui/pattern/deref-patterns/typeck_fail.rs
index 040118449ec..4b9ad7d25f0 100644
--- a/tests/ui/pattern/deref-patterns/typeck_fail.rs
+++ b/tests/ui/pattern/deref-patterns/typeck_fail.rs
@@ -7,11 +7,22 @@ fn main() {
     match "foo".to_string() {
         deref!("foo") => {}
         //~^ ERROR: mismatched types
+        "foo" => {}
+        //~^ ERROR: mismatched types
         _ => {}
     }
     match &"foo".to_string() {
         deref!("foo") => {}
         //~^ ERROR: mismatched types
+        "foo" => {}
+        //~^ ERROR: mismatched types
+        _ => {}
+    }
+
+    // Make sure we don't try implicitly dereferncing any ADT.
+    match Some(0) {
+        Ok(0) => {}
+        //~^ ERROR: mismatched types
         _ => {}
     }
 }
diff --git a/tests/ui/pattern/deref-patterns/typeck_fail.stderr b/tests/ui/pattern/deref-patterns/typeck_fail.stderr
index 1c14802745a..3e2f3561882 100644
--- a/tests/ui/pattern/deref-patterns/typeck_fail.stderr
+++ b/tests/ui/pattern/deref-patterns/typeck_fail.stderr
@@ -7,13 +7,45 @@ LL |         deref!("foo") => {}
    |                ^^^^^ expected `str`, found `&str`
 
 error[E0308]: mismatched types
-  --> $DIR/typeck_fail.rs:13:16
+  --> $DIR/typeck_fail.rs:10:9
+   |
+LL |     match "foo".to_string() {
+   |           ----------------- this expression has type `String`
+...
+LL |         "foo" => {}
+   |         ^^^^^ expected `String`, found `&str`
+
+error[E0308]: mismatched types
+  --> $DIR/typeck_fail.rs:15:16
    |
 LL |     match &"foo".to_string() {
    |           ------------------ this expression has type `&String`
 LL |         deref!("foo") => {}
    |                ^^^^^ expected `str`, found `&str`
 
-error: aborting due to 2 previous errors
+error[E0308]: mismatched types
+  --> $DIR/typeck_fail.rs:17:9
+   |
+LL |     match &"foo".to_string() {
+   |           ------------------ this expression has type `&String`
+...
+LL |         "foo" => {}
+   |         ^^^^^ expected `&String`, found `&str`
+   |
+   = note: expected reference `&String`
+              found reference `&'static str`
+
+error[E0308]: mismatched types
+  --> $DIR/typeck_fail.rs:24:9
+   |
+LL |     match Some(0) {
+   |           ------- this expression has type `Option<{integer}>`
+LL |         Ok(0) => {}
+   |         ^^^^^ expected `Option<{integer}>`, found `Result<_, _>`
+   |
+   = note: expected enum `Option<{integer}>`
+              found enum `Result<_, _>`
+
+error: aborting due to 5 previous errors
 
 For more information about this error, try `rustc --explain E0308`.
diff --git a/tests/ui/pattern/deref-patterns/unsatisfied-bounds.rs b/tests/ui/pattern/deref-patterns/unsatisfied-bounds.rs
new file mode 100644
index 00000000000..00064b2320c
--- /dev/null
+++ b/tests/ui/pattern/deref-patterns/unsatisfied-bounds.rs
@@ -0,0 +1,21 @@
+#![feature(deref_patterns)]
+#![allow(incomplete_features)]
+
+struct MyPointer;
+
+impl std::ops::Deref for MyPointer {
+    type Target = ();
+    fn deref(&self) -> &() {
+        &()
+    }
+}
+
+fn main() {
+    // Test that we get a trait error if a user attempts implicit deref pats on their own impls.
+    // FIXME(deref_patterns): there should be a special diagnostic for missing `DerefPure`.
+    match MyPointer {
+        () => {}
+        //~^ the trait bound `MyPointer: DerefPure` is not satisfied
+        _ => {}
+    }
+}
diff --git a/tests/ui/pattern/deref-patterns/unsatisfied-bounds.stderr b/tests/ui/pattern/deref-patterns/unsatisfied-bounds.stderr
new file mode 100644
index 00000000000..983ce27865c
--- /dev/null
+++ b/tests/ui/pattern/deref-patterns/unsatisfied-bounds.stderr
@@ -0,0 +1,9 @@
+error[E0277]: the trait bound `MyPointer: DerefPure` is not satisfied
+  --> $DIR/unsatisfied-bounds.rs:17:9
+   |
+LL |         () => {}
+   |         ^^ the trait `DerefPure` is not implemented for `MyPointer`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.