misc coercion cleanups and handle safety correctly

tests/ui/coercion/hr_alias_normalization_leaking_vars.rs

@@ -0,0 +1,36 @@
+// We have two function parameters with types:
+// - `&?0`
+// - `Box<for<'a> fn(<?0 as Trait<'a>>::Item)>`
+//
+// As the alias in the second parameter has a `?0` it is an ambig
+// alias, and as it references bound vars it can't be normalized to
+// an infer var.
+//
+// When checking function arguments we try to coerce both:
+// - `&()` to `&?0`
+// - `FnDef(f)` to `Box<for<'a> fn(<?0 as Trait<'a>>::Item)>`
+//
+// The first coercion infers `?0=()`. Previously when handling
+// the second coercion we wound *re-normalize* the alias, which
+// now that `?0` has been inferred allowed us to determine this
+// alias is not wellformed and normalize it to some infer var `?1`.
+//
+// We would then see that `FnDef(f)` can't be coerced to `Box<fn(?1)>`
+// and return a `TypeError` referencing this new variable `?1`. This
+// then caused ICEs as diagnostics would encounter inferences variables
+// from the result of normalization inside of the probe used be coercion.
+
+
+trait LendingIterator {
+    type Item<'q>;
+    fn for_each(&self, _f: Box<fn(Self::Item<'_>)>) {}
+}
+
+fn f(_: ()) {}
+
+fn main() {
+    LendingIterator::for_each(&(), f);
+    //~^ ERROR: the trait bound `(): LendingIterator` is not satisfied
+    //~| ERROR: the trait bound `(): LendingIterator` is not satisfied
+    //~| ERROR: mismatched types
+}

tests/ui/coercion/hr_alias_normalization_leaking_vars.stderr

@@ -0,0 +1,46 @@
+error[E0277]: the trait bound `(): LendingIterator` is not satisfied
+  --> $DIR/hr_alias_normalization_leaking_vars.rs:32:31
+   |
+LL |     LendingIterator::for_each(&(), f);
+   |     ------------------------- ^^^ the trait `LendingIterator` is not implemented for `()`
+   |     |
+   |     required by a bound introduced by this call
+   |
+help: this trait has no implementations, consider adding one
+  --> $DIR/hr_alias_normalization_leaking_vars.rs:24:1
+   |
+LL | trait LendingIterator {
+   | ^^^^^^^^^^^^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/hr_alias_normalization_leaking_vars.rs:32:36
+   |
+LL |     LendingIterator::for_each(&(), f);
+   |     -------------------------      ^ expected `Box<fn(...)>`, found fn item
+   |     |
+   |     arguments to this function are incorrect
+   |
+   = note: expected struct `Box<for<'a> fn(<() as LendingIterator>::Item<'a>)>`
+             found fn item `fn(()) {f}`
+note: method defined here
+  --> $DIR/hr_alias_normalization_leaking_vars.rs:26:8
+   |
+LL |     fn for_each(&self, _f: Box<fn(Self::Item<'_>)>) {}
+   |        ^^^^^^^^        ---------------------------
+
+error[E0277]: the trait bound `(): LendingIterator` is not satisfied
+  --> $DIR/hr_alias_normalization_leaking_vars.rs:32:36
+   |
+LL |     LendingIterator::for_each(&(), f);
+   |                                    ^ the trait `LendingIterator` is not implemented for `()`
+   |
+help: this trait has no implementations, consider adding one
+  --> $DIR/hr_alias_normalization_leaking_vars.rs:24:1
+   |
+LL | trait LendingIterator {
+   | ^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0277, E0308.
+For more information about an error, try `rustc --explain E0277`.

tests/ui/coercion/issue-88097.rs

@@ -3,6 +3,9 @@
 // behavior has been fixed.
 
 //@ check-pass
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
 
 fn peculiar() -> impl Fn(u8) -> u8 {
     return |x| x + 1

tests/ui/coercion/leak_check_fndef_lub.rs

@@ -0,0 +1,32 @@
+//@ check-pass
+
+fn foo<T>() {}
+
+fn fndef_lub_leak_check() {
+    macro_rules! lub {
+        ($lhs:expr, $rhs:expr) => {
+            if true { $lhs } else { $rhs }
+        };
+    }
+
+    // These don't currently lub but could in theory one day.
+    // If that happens this test should be adjusted to use
+    // fn ptrs that can't be lub'd.
+    let lhs = foo::<for<'a> fn(&'static (), &'a ())>;
+    let rhs = foo::<for<'a> fn(&'a (), &'static ())>;
+
+    // If we leak check then we know we should coerce these
+    // to `fn()`, if we don't leak check we may try to keep
+    // them as `FnDef`s which would result in a borrowck
+    // error.
+    let lubbed = lub!(lhs, rhs);
+
+    // assert that we coerced lhs/rhs to a fn ptr
+    is_fnptr(lubbed);
+}
+
+trait FnPtr {}
+impl FnPtr for fn() {}
+fn is_fnptr<T: FnPtr>(_: T) {}
+
+fn main() {}

tests/ui/coercion/leak_check_fndef_lub_deadcode_breakage.rs

@@ -0,0 +1,29 @@
+fn foo<T>() {}
+
+fn fndef_lub_leak_check() {
+    macro_rules! lub {
+        ($lhs:expr, $rhs:expr) => {
+            if true { $lhs } else { $rhs }
+        };
+    }
+
+    // These don't currently lub but could in theory one day.
+    // If that happens this test should be adjusted to use
+    // fn ptrs that can't be lub'd.
+    let lhs = foo::<for<'a> fn(&'static (), &'a ())>;
+    let rhs = foo::<for<'a> fn(&'a (), &'static ())>;
+
+    loop {}
+
+    // If we leak check then we know we should coerce these
+    // to `fn()`, if we don't leak check we may try to keep
+    // them as `FnDef`s which would cause this code to compile
+    // as borrowck won't emit errors for deadcode.
+    let lubbed = lub!(lhs, rhs);
+
+    // assert that `lubbed` is a ZST/`FnDef`
+    unsafe { std::mem::transmute::<_, ()>(lubbed) }
+    //~^ ERROR: cannot transmute between types of different sizes
+}
+
+fn main() {}

tests/ui/coercion/leak_check_fndef_lub_deadcode_breakage.stderr

@@ -0,0 +1,12 @@
+error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
+  --> $DIR/leak_check_fndef_lub_deadcode_breakage.rs:25:14
+   |
+LL |     unsafe { std::mem::transmute::<_, ()>(lubbed) }
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: source type: `fn()` (64 bits)
+   = note: target type: `()` (0 bits)
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0512`.

tests/ui/coercion/lub_closures_before_fnptr_coercion.rs

@@ -0,0 +1,70 @@
+//@ check-pass
+//@ compile-flags: -Znext-solver
+
+#![feature(type_alias_impl_trait)]
+
+// Test that when lubbing two equal closure tys with different
+// structural identities (i.e. `PartialEq::eq` on `ty::Ty` would be false)
+// we don't coerce-lub to a fnptr.
+//
+// Most of this test is involved jank to be able to leak the hidden type
+// of an opaque with a hidden type of `Closure<C1, C2>`. This then allows
+// us to substitute `C1` and `C2` for arbitrary types in the parent scope.
+//
+// See: <https://github.com/lcnr/random-rust-snippets/issues/13>
+
+struct WaddupGamers<T, U>(Option<T>, U);
+impl<T: Leak<Unpin = U>, U> Unpin for WaddupGamers<T, U> {}
+unsafe impl<T: Leak<Send = U>, U> Send for WaddupGamers<T, U> {}
+pub trait Leak {
+    type Unpin;
+    type Send;
+}
+impl<T> Leak for (T,) {
+    type Unpin = T;
+    type Send = T;
+}
+fn define<C1, C2>() -> impl Sized {
+    WaddupGamers(None::<C1>, || ())
+}
+
+fn require_unpin<T: Unpin>(_: T) {}
+fn require_send<T: Send>(_: T) {}
+fn mk<T>() -> T { todo!() }
+
+type NameMe<T> = impl Sized;
+type NameMe2<T> = impl Sized;
+
+#[define_opaque(NameMe, NameMe2)]
+fn leak<T>()
+where
+    T: Leak<Unpin = NameMe<T>, Send = NameMe2<T>>,
+{
+    require_unpin(define::<T, for<'a> fn(&'a ())>());
+    require_send(define::<T, for<'a> fn(&'a ())>());
+
+    // This is the actual logic for lubbing two closures
+    // with syntactically different `ty::Ty`s:
+
+    // lhs: Closure<C1=T, C2=for<'a1> fn(&'a1 ())>
+    let lhs = mk::<NameMe<T>>();
+    // lhs: Closure<C1=T, C2=for<'a2> fn(&'a2 ())>
+    let rhs = mk::<NameMe2<T>>();
+
+    macro_rules! lub {
+        ($lhs:expr, $rhs:expr) => {
+            if true { $lhs } else { $rhs }
+        };
+    }
+
+    // Lubbed to either:
+    // - `Closure<C1=T, C2=for<'a> fn(&'a ())>`
+    // - `fn(&())`
+    let lubbed = lub!(lhs, rhs);
+
+    // Use transmute to assert the size of `lubbed` is (), i.e.
+    // that it is a ZST closure type not a fnptr.
+    unsafe { std::mem::transmute::<_, ()>(lubbed) };
+}
+
+fn main() {}

tests/ui/coercion/structural_identity_dependent_reborrows.rs

@@ -0,0 +1,24 @@
+//@ edition: 2024
+
+// We avoid emitting reborrow coercions if it seems like it would
+// not result in a different lifetime on the borrow. This can effect
+// capture analysis resulting in borrow checking errors.
+
+fn foo<'a>(b: &'a ()) -> impl Fn() {
+    || {
+        expected::<&()>(b);
+    }
+}
+
+// No reborrow of `b` is emitted which means our closure captures
+// `b` by ref resulting in an upvar of `&&'a ()`
+fn bar<'a>(b: &'a ()) -> impl Fn() {
+    || {
+        //~^ ERROR: closure may outlive the current function
+        expected::<&'a ()>(b);
+    }
+}
+
+fn expected<T>(_: T) {}
+
+fn main() {}

tests/ui/coercion/structural_identity_dependent_reborrows.stderr

@@ -0,0 +1,25 @@
+error[E0373]: closure may outlive the current function, but it borrows `b`, which is owned by the current function
+  --> $DIR/structural_identity_dependent_reborrows.rs:16:5
+   |
+LL |     || {
+   |     ^^ may outlive borrowed value `b`
+LL |
+LL |         expected::<&'a ()>(b);
+   |                            - `b` is borrowed here
+   |
+note: closure is returned here
+  --> $DIR/structural_identity_dependent_reborrows.rs:16:5
+   |
+LL | /     || {
+LL | |
+LL | |         expected::<&'a ()>(b);
+LL | |     }
+   | |_____^
+help: to force the closure to take ownership of `b` (and any other referenced variables), use the `move` keyword
+   |
+LL |     move || {
+   |     ++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0373`.