Rollup merge of #146037 - aapoalas:reborrow-lang-experiment, r=tmandry

Introduce CoerceShared lang item and trait, and basic Reborrow tests

Part of rust-lang/rust#145612: This introduces the `CoerceShared` trait which is the `Reborrow` equivalent of a `&mut T` -> `&T` coercion. The trait has a `Target` GAT which makes this (currently) unique in the `core/src/marker.rs`; I'm not sure if this can be considered problematic. Maybe this is not the way such things should be done at the marker trait level? Or maybe it is fine.

Improtantly, this PR introduces a battery of basic `Reborrow` and `CoerceShared` tests. These test the very basics of the feature; custom marker types intended to have exclusive semantics (`Custom<'a>(PhantomData<&'a mut ()>)`), custom exclusive reference wrappers, and standard library exclusive reference wrappers (`Pin<&mut T>` and `Option<&mut T>`). None of these of course work since the implementation for `Reborrow` and `CoerceShared` is entirely missing, but this is the first step towards making these work.

Future PRs will introduce more tests, such as "recursive" reborrowing (ie. reborrowing structs that contain multiple reborrowable fields) and checks around the lifetime semantics of reborrowing ie. that a reborrow produces a new type with the same lifetime as the original.

21 files changed, 254 insertions, 11 deletions

diff --git a/compiler/rustc_hir/src/lang_items.rs b/compiler/rustc_hir/src/lang_items.rs
index 2e099a97b65..311cf8f995c 100644
--- a/compiler/rustc_hir/src/lang_items.rs
+++ b/compiler/rustc_hir/src/lang_items.rs
@@ -440,6 +440,7 @@ language_item_table! {
 
     // Reborrowing related lang-items
     Reborrow,                sym::reborrow,            reborrow,                   Target::Trait,          GenericRequirement::Exact(0);
+    CoerceShared,            sym::coerce_shared,       coerce_shared,              Target::Trait,          GenericRequirement::Exact(0);
 }
 
 /// The requirement imposed on the generics of a lang item
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index cd422da0c1c..63b0de377c9 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -679,6 +679,7 @@ symbols! {
         cmpxchg16b_target_feature,
         cmse_nonsecure_entry,
         coerce_pointee_validated,
+        coerce_shared,
         coerce_unsized,
         cold,
         cold_path,
diff --git a/library/core/src/marker.rs b/library/core/src/marker.rs
index 1c100312a9a..4b767d8d622 100644
--- a/library/core/src/marker.rs
+++ b/library/core/src/marker.rs
@@ -1341,11 +1341,3 @@ pub macro CoercePointee($item:item) {
 pub trait CoercePointeeValidated {
     /* compiler built-in */
 }
-
-/// Allows value to be reborrowed as exclusive, creating a copy of the value
-/// that disables the source for reads and writes for the lifetime of the copy.
-#[lang = "reborrow"]
-#[unstable(feature = "reborrow", issue = "145612")]
-pub trait Reborrow {
-    // Empty.
-}
diff --git a/library/core/src/ops/mod.rs b/library/core/src/ops/mod.rs
index 87dd873fdb5..ab1ad407ee2 100644
--- a/library/core/src/ops/mod.rs
+++ b/library/core/src/ops/mod.rs
@@ -149,6 +149,7 @@ mod function;
 mod index;
 mod index_range;
 mod range;
+mod reborrow;
 mod try_trait;
 mod unsize;
 
@@ -189,6 +190,8 @@ pub use self::range::{Bound, RangeBounds, RangeInclusive, RangeToInclusive};
 pub use self::range::{OneSidedRange, OneSidedRangeBound};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::range::{Range, RangeFrom, RangeFull, RangeTo};
+#[unstable(feature = "reborrow", issue = "145612")]
+pub use self::reborrow::{CoerceShared, Reborrow};
 #[unstable(feature = "try_trait_v2_residual", issue = "91285")]
 pub use self::try_trait::Residual;
 #[unstable(feature = "try_trait_v2_yeet", issue = "96374")]
diff --git a/library/core/src/ops/reborrow.rs b/library/core/src/ops/reborrow.rs
new file mode 100644
index 00000000000..f83f4233a4d
--- /dev/null
+++ b/library/core/src/ops/reborrow.rs
@@ -0,0 +1,16 @@
+/// Allows value to be reborrowed as exclusive, creating a copy of the value
+/// that disables the source for reads and writes for the lifetime of the copy.
+#[lang = "reborrow"]
+#[unstable(feature = "reborrow", issue = "145612")]
+pub trait Reborrow {
+    // Empty.
+}
+
+/// Allows reborrowable value to be reborrowed as shared, creating a copy
+/// that disables the source for writes for the lifetime of the copy.
+#[lang = "coerce_shared"]
+#[unstable(feature = "reborrow", issue = "145612")]
+pub trait CoerceShared: Reborrow {
+    /// The type of this value when reborrowed as shared.
+    type Target: Copy;
+}
diff --git a/tests/ui/feature-gates/feature-gate-reborrow-coerce-shared.rs b/tests/ui/feature-gates/feature-gate-reborrow-coerce-shared.rs
new file mode 100644
index 00000000000..c8ca4537089
--- /dev/null
+++ b/tests/ui/feature-gates/feature-gate-reborrow-coerce-shared.rs
@@ -0,0 +1,3 @@
+use std::ops::CoerceShared; //~ ERROR  use of unstable library feature `reborrow`
+
+fn main() {}
diff --git a/tests/ui/feature-gates/feature-gate-reborrow-coerce-shared.stderr b/tests/ui/feature-gates/feature-gate-reborrow-coerce-shared.stderr
new file mode 100644
index 00000000000..dbbbcdf2fd5
--- /dev/null
+++ b/tests/ui/feature-gates/feature-gate-reborrow-coerce-shared.stderr
@@ -0,0 +1,13 @@
+error[E0658]: use of unstable library feature `reborrow`
+  --> $DIR/feature-gate-reborrow-coerce-shared.rs:1:5
+   |
+LL | use std::ops::CoerceShared;
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #145612 <https://github.com/rust-lang/rust/issues/145612> for more information
+   = help: add `#![feature(reborrow)]` 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`.
diff --git a/tests/ui/feature-gates/feature-gate-reborrow.rs b/tests/ui/feature-gates/feature-gate-reborrow.rs
index f016f6c6bfa..96eecfb28a1 100644
--- a/tests/ui/feature-gates/feature-gate-reborrow.rs
+++ b/tests/ui/feature-gates/feature-gate-reborrow.rs
@@ -1,3 +1,3 @@
-use std::marker::Reborrow; //~ ERROR  use of unstable library feature `reborrow`
+use std::ops::Reborrow; //~ ERROR  use of unstable library feature `reborrow`
 
 fn main() {}
diff --git a/tests/ui/feature-gates/feature-gate-reborrow.stderr b/tests/ui/feature-gates/feature-gate-reborrow.stderr
index 5e3033f3bf1..1224909f564 100644
--- a/tests/ui/feature-gates/feature-gate-reborrow.stderr
+++ b/tests/ui/feature-gates/feature-gate-reborrow.stderr
@@ -1,8 +1,8 @@
 error[E0658]: use of unstable library feature `reborrow`
   --> $DIR/feature-gate-reborrow.rs:1:5
    |
-LL | use std::marker::Reborrow;
-   |     ^^^^^^^^^^^^^^^^^^^^^
+LL | use std::ops::Reborrow;
+   |     ^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #145612 <https://github.com/rust-lang/rust/issues/145612> for more information
    = help: add `#![feature(reborrow)]` to the crate attributes to enable
diff --git a/tests/ui/reborrow/custom_mut.rs b/tests/ui/reborrow/custom_mut.rs
new file mode 100644
index 00000000000..1e7c4693238
--- /dev/null
+++ b/tests/ui/reborrow/custom_mut.rs
@@ -0,0 +1,13 @@
+#![feature(reborrow)]
+use std::ops::Reborrow;
+
+struct CustomMut<'a, T>(&'a mut T);
+impl<'a, T> Reborrow for CustomMut<'a, T> {}
+
+fn method(a: CustomMut<'_, ()>) {}
+
+fn main() {
+    let a = CustomMut(&mut ());
+    let _ = method(a);
+    let _ = method(a); //~ERROR use of moved value: `a`
+}
diff --git a/tests/ui/reborrow/custom_mut.stderr b/tests/ui/reborrow/custom_mut.stderr
new file mode 100644
index 00000000000..3b3f47b62d6
--- /dev/null
+++ b/tests/ui/reborrow/custom_mut.stderr
@@ -0,0 +1,29 @@
+error[E0382]: use of moved value: `a`
+  --> $DIR/custom_mut.rs:12:20
+   |
+LL |     let a = CustomMut(&mut ());
+   |         - move occurs because `a` has type `CustomMut<'_, ()>`, which does not implement the `Copy` trait
+LL |     let _ = method(a);
+   |                    - value moved here
+LL |     let _ = method(a);
+   |                    ^ value used here after move
+   |
+note: consider changing this parameter type in function `method` to borrow instead if owning the value isn't necessary
+  --> $DIR/custom_mut.rs:7:14
+   |
+LL | fn method(a: CustomMut<'_, ()>) {}
+   |    ------    ^^^^^^^^^^^^^^^^^ this parameter takes ownership of the value
+   |    |
+   |    in this function
+note: if `CustomMut<'_, ()>` implemented `Clone`, you could clone the value
+  --> $DIR/custom_mut.rs:4:1
+   |
+LL | struct CustomMut<'a, T>(&'a mut T);
+   | ^^^^^^^^^^^^^^^^^^^^^^^ consider implementing `Clone` for this type
+...
+LL |     let _ = method(a);
+   |                    - you could clone this value
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0382`.
diff --git a/tests/ui/reborrow/custom_mut_coerce_shared.rs b/tests/ui/reborrow/custom_mut_coerce_shared.rs
new file mode 100644
index 00000000000..e2d25835c09
--- /dev/null
+++ b/tests/ui/reborrow/custom_mut_coerce_shared.rs
@@ -0,0 +1,28 @@
+#![feature(reborrow)]
+use std::ops::{CoerceShared, Reborrow};
+
+struct CustomMut<'a, T>(&'a mut T);
+impl<'a, T> Reborrow for CustomMut<'a, T> {}
+impl<'a, T> CoerceShared for CustomMut<'a, T> {
+    type Target = CustomRef<'a, T>;
+}
+
+struct CustomRef<'a, T>(&'a T);
+
+impl<'a, T> Clone for CustomRef<'a, T> {
+    fn clone(&self) -> Self {
+        Self(self.0)
+    }
+}
+impl<'a, T> Copy for CustomRef<'a, T> {}
+
+fn method(a: CustomRef<'_, ()>) {}  //~NOTE function defined here
+
+fn main() {
+    let a = CustomMut(&mut ());
+    method(a);
+    //~^ ERROR mismatched types
+    //~| NOTE expected `CustomRef<'_, ()>`, found `CustomMut<'_, ()>`
+    //~| NOTE arguments to this function are incorrect
+    //~| NOTE expected struct `CustomRef<'_, ()>`
+}
diff --git a/tests/ui/reborrow/custom_mut_coerce_shared.stderr b/tests/ui/reborrow/custom_mut_coerce_shared.stderr
new file mode 100644
index 00000000000..508651badc0
--- /dev/null
+++ b/tests/ui/reborrow/custom_mut_coerce_shared.stderr
@@ -0,0 +1,19 @@
+error[E0308]: mismatched types
+  --> $DIR/custom_mut_coerce_shared.rs:23:12
+   |
+LL |     method(a);
+   |     ------ ^ expected `CustomRef<'_, ()>`, found `CustomMut<'_, ()>`
+   |     |
+   |     arguments to this function are incorrect
+   |
+   = note: expected struct `CustomRef<'_, ()>`
+              found struct `CustomMut<'_, ()>`
+note: function defined here
+  --> $DIR/custom_mut_coerce_shared.rs:19:4
+   |
+LL | fn method(a: CustomRef<'_, ()>) {}
+   |    ^^^^^^ --------------------
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.
diff --git a/tests/ui/reborrow/option_mut.rs b/tests/ui/reborrow/option_mut.rs
new file mode 100644
index 00000000000..04d8301772d
--- /dev/null
+++ b/tests/ui/reborrow/option_mut.rs
@@ -0,0 +1,7 @@
+fn method(a: Option<&mut ()>) {}
+
+fn main() {
+    let a = Some(&mut ());
+    let _ = method(a);
+    let _ = method(a); //~ERROR use of moved value: `a`
+}
diff --git a/tests/ui/reborrow/option_mut.stderr b/tests/ui/reborrow/option_mut.stderr
new file mode 100644
index 00000000000..d665e266079
--- /dev/null
+++ b/tests/ui/reborrow/option_mut.stderr
@@ -0,0 +1,21 @@
+error[E0382]: use of moved value: `a`
+  --> $DIR/option_mut.rs:6:20
+   |
+LL |     let a = Some(&mut ());
+   |         - move occurs because `a` has type `Option<&mut ()>`, which does not implement the `Copy` trait
+LL |     let _ = method(a);
+   |                    - value moved here
+LL |     let _ = method(a);
+   |                    ^ value used here after move
+   |
+note: consider changing this parameter type in function `method` to borrow instead if owning the value isn't necessary
+  --> $DIR/option_mut.rs:1:14
+   |
+LL | fn method(a: Option<&mut ()>) {}
+   |    ------    ^^^^^^^^^^^^^^^ this parameter takes ownership of the value
+   |    |
+   |    in this function
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0382`.
diff --git a/tests/ui/reborrow/option_mut_coerce_shared.rs b/tests/ui/reborrow/option_mut_coerce_shared.rs
new file mode 100644
index 00000000000..95d33ed94dd
--- /dev/null
+++ b/tests/ui/reborrow/option_mut_coerce_shared.rs
@@ -0,0 +1,11 @@
+fn method(a: Option<&()>) {}  //~NOTE function defined here
+
+fn main() {
+    let a = Some(&mut ());
+    method(a);
+    //~^ ERROR mismatched types
+    //~| NOTE arguments to this function are incorrect
+    //~| NOTE types differ in mutability
+    //~| NOTE expected enum `Option<&()>`
+    //~| NOTE    found enum `Option<&mut ()>`
+}
diff --git a/tests/ui/reborrow/option_mut_coerce_shared.stderr b/tests/ui/reborrow/option_mut_coerce_shared.stderr
new file mode 100644
index 00000000000..6ca1a237461
--- /dev/null
+++ b/tests/ui/reborrow/option_mut_coerce_shared.stderr
@@ -0,0 +1,23 @@
+error[E0308]: mismatched types
+  --> $DIR/option_mut_coerce_shared.rs:5:12
+   |
+LL |     method(a);
+   |     ------ ^ types differ in mutability
+   |     |
+   |     arguments to this function are incorrect
+   |
+   = note: expected enum `Option<&()>`
+              found enum `Option<&mut ()>`
+note: function defined here
+  --> $DIR/option_mut_coerce_shared.rs:1:4
+   |
+LL | fn method(a: Option<&()>) {}
+   |    ^^^^^^ --------------
+help: try using `.as_deref()` to convert `Option<&mut ()>` to `Option<&()>`
+   |
+LL |     method(a.as_deref());
+   |             +++++++++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.
diff --git a/tests/ui/reborrow/pin_mut.rs b/tests/ui/reborrow/pin_mut.rs
new file mode 100644
index 00000000000..959cb14f8c9
--- /dev/null
+++ b/tests/ui/reborrow/pin_mut.rs
@@ -0,0 +1,10 @@
+use std::pin::Pin;
+
+fn method(a: Pin<&mut ()>) {}
+
+fn main() {
+    let a = &mut ();
+    let a = Pin::new(a);
+    let _ = method(a);
+    let _ = method(a); //~ERROR use of moved value: `a`
+}
diff --git a/tests/ui/reborrow/pin_mut.stderr b/tests/ui/reborrow/pin_mut.stderr
new file mode 100644
index 00000000000..64e3f603e11
--- /dev/null
+++ b/tests/ui/reborrow/pin_mut.stderr
@@ -0,0 +1,21 @@
+error[E0382]: use of moved value: `a`
+  --> $DIR/pin_mut.rs:9:20
+   |
+LL |     let a = Pin::new(a);
+   |         - move occurs because `a` has type `Pin<&mut ()>`, which does not implement the `Copy` trait
+LL |     let _ = method(a);
+   |                    - value moved here
+LL |     let _ = method(a);
+   |                    ^ value used here after move
+   |
+note: consider changing this parameter type in function `method` to borrow instead if owning the value isn't necessary
+  --> $DIR/pin_mut.rs:3:14
+   |
+LL | fn method(a: Pin<&mut ()>) {}
+   |    ------    ^^^^^^^^^^^^ this parameter takes ownership of the value
+   |    |
+   |    in this function
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0382`.
diff --git a/tests/ui/reborrow/pin_mut_coerce_shared.rs b/tests/ui/reborrow/pin_mut_coerce_shared.rs
new file mode 100644
index 00000000000..06af0b765d0
--- /dev/null
+++ b/tests/ui/reborrow/pin_mut_coerce_shared.rs
@@ -0,0 +1,13 @@
+use std::pin::Pin;
+
+fn method(a: Pin<&()>) {}  //~NOTE function defined here
+
+fn main() {
+    let a = &mut ();
+    let a = Pin::new(a);
+    method(a);
+    //~^ ERROR mismatched types
+    //~| NOTE arguments to this function are incorrect
+    //~| NOTE types differ in mutability
+    //~| NOTE expected struct `Pin<&()>`
+}
diff --git a/tests/ui/reborrow/pin_mut_coerce_shared.stderr b/tests/ui/reborrow/pin_mut_coerce_shared.stderr
new file mode 100644
index 00000000000..74ecf4de4c7
--- /dev/null
+++ b/tests/ui/reborrow/pin_mut_coerce_shared.stderr
@@ -0,0 +1,19 @@
+error[E0308]: mismatched types
+  --> $DIR/pin_mut_coerce_shared.rs:8:12
+   |
+LL |     method(a);
+   |     ------ ^ types differ in mutability
+   |     |
+   |     arguments to this function are incorrect
+   |
+   = note: expected struct `Pin<&()>`
+              found struct `Pin<&mut ()>`
+note: function defined here
+  --> $DIR/pin_mut_coerce_shared.rs:3:4
+   |
+LL | fn method(a: Pin<&()>) {}
+   |    ^^^^^^ -----------
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.