Merging upstream version 1.76.0+dfsg1.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

4 files changed, 136 insertions, 0 deletions

diff --git a/tests/ui/traits/next-solver/cycles/coinduction/fixpoint-exponential-growth.rs b/tests/ui/traits/next-solver/cycles/coinduction/fixpoint-exponential-growth.rs
new file mode 100644
index 000000000..947b52da7
--- /dev/null
+++ b/tests/ui/traits/next-solver/cycles/coinduction/fixpoint-exponential-growth.rs
@@ -0,0 +1,35 @@
+// compile-flags: -Znext-solver
+
+// Proving `W<?0>: Trait` instantiates `?0` with `(W<?1>, W<?2>)` and then
+// proves `W<?1>: Trait` and `W<?2>: Trait`, resulting in a coinductive cycle.
+//
+// Proving coinductive cycles runs until we reach a fixpoint. However, after
+// computing `try_evaluate_added_goals` in the second fixpoint iteration, the
+// self type already has a depth equal to the number of steps. This results
+// in enormous constraints, causing the canonicalizer to hang without ever
+// reaching the recursion limit. We currently avoid that by erasing the constraints
+// from overflow.
+//
+// This previously caused a hang in the trait solver, see
+// https://github.com/rust-lang/trait-system-refactor-initiative/issues/13.
+
+#![feature(rustc_attrs)]
+
+#[rustc_coinductive]
+trait Trait {}
+
+struct W<T>(T);
+
+impl<T, U> Trait for W<(W<T>, W<U>)>
+where
+    W<T>: Trait,
+    W<U>: Trait,
+{
+}
+
+fn impls<T: Trait>() {}
+
+fn main() {
+    impls::<W<_>>();
+    //~^ ERROR overflow evaluating the requirement
+}
diff --git a/tests/ui/traits/next-solver/cycles/coinduction/fixpoint-exponential-growth.stderr b/tests/ui/traits/next-solver/cycles/coinduction/fixpoint-exponential-growth.stderr
new file mode 100644
index 000000000..150100f2c
--- /dev/null
+++ b/tests/ui/traits/next-solver/cycles/coinduction/fixpoint-exponential-growth.stderr
@@ -0,0 +1,16 @@
+error[E0275]: overflow evaluating the requirement `W<_>: Trait`
+  --> $DIR/fixpoint-exponential-growth.rs:33:13
+   |
+LL |     impls::<W<_>>();
+   |             ^^^^
+   |
+   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`fixpoint_exponential_growth`)
+note: required by a bound in `impls`
+  --> $DIR/fixpoint-exponential-growth.rs:30:13
+   |
+LL | fn impls<T: Trait>() {}
+   |             ^^^^^ required by this bound in `impls`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0275`.
diff --git a/tests/ui/traits/next-solver/cycles/coinduction/incompleteness-unstable-result.rs b/tests/ui/traits/next-solver/cycles/coinduction/incompleteness-unstable-result.rs
new file mode 100644
index 000000000..a3c07b987
--- /dev/null
+++ b/tests/ui/traits/next-solver/cycles/coinduction/incompleteness-unstable-result.rs
@@ -0,0 +1,69 @@
+// compile-flags: -Znext-solver
+#![feature(rustc_attrs)]
+
+// This test is incredibly subtle. At its core the goal is to get a coinductive cycle,
+// which, depending on its root goal, either holds or errors. We achieve this by getting
+// incomplete inference via a `ParamEnv` candidate in the `A<T>` impl and required
+// inference from an `Impl` candidate in the `B<T>` impl.
+//
+// To make global cache accesses stronger than the guidance from the where-bounds, we add
+// another coinductive cycle from `A<T>: Trait<U, V, D>` to `A<T>: Trait<U, D, V>` and only
+// constrain `D` directly. This means that any candidates which rely on `V` only make
+// progress in the second iteration, allowing a cache access in the first iteration to take
+// precedence.
+//
+// tl;dr: our caching of coinductive cycles was broken and this is a regression
+// test for that.
+
+#[rustc_coinductive]
+trait Trait<T: ?Sized, V: ?Sized, D: ?Sized> {}
+struct A<T: ?Sized>(*const T);
+struct B<T: ?Sized>(*const T);
+
+trait IncompleteGuidance<T: ?Sized, V: ?Sized> {}
+impl<T: ?Sized, U: ?Sized + 'static> IncompleteGuidance<U, u8> for T {}
+impl<T: ?Sized, U: ?Sized + 'static> IncompleteGuidance<U, i8> for T {}
+impl<T: ?Sized, U: ?Sized + 'static> IncompleteGuidance<U, i16> for T {}
+
+trait ImplGuidance<T: ?Sized, V: ?Sized> {}
+impl<T: ?Sized> ImplGuidance<u32, u8> for T {}
+impl<T: ?Sized> ImplGuidance<i32, i8> for T {}
+
+impl<T: ?Sized, U: ?Sized, V: ?Sized, D: ?Sized> Trait<U, V, D> for A<T>
+where
+    T: IncompleteGuidance<U, V>,
+    A<T>: Trait<U, D, V>,
+    B<T>: Trait<U, V, D>,
+    (): ToU8<D>,
+{
+}
+
+trait ToU8<T: ?Sized> {}
+impl ToU8<u8> for () {}
+
+impl<T: ?Sized, U: ?Sized, V: ?Sized, D: ?Sized> Trait<U, V, D> for B<T>
+where
+    T: ImplGuidance<U, V>,
+    A<T>: Trait<U, V, D>,
+{
+}
+
+fn impls_trait<T: ?Sized + Trait<U, V, D>, U: ?Sized, V: ?Sized, D: ?Sized>() {}
+
+fn with_bound<X>()
+where
+    X: IncompleteGuidance<i32, u8>,
+    X: IncompleteGuidance<u32, i8>,
+    X: IncompleteGuidance<u32, i16>,
+{
+    impls_trait::<B<X>, _, _, _>(); // entering the cycle from `B` works
+
+    // entering the cycle from `A` fails, but would work if we were to use the cache
+    // result of `B<X>`.
+    impls_trait::<A<X>, _, _, _>();
+    //~^ ERROR the trait bound `A<X>: Trait<_, _, _>` is not satisfied
+}
+
+fn main() {
+    with_bound::<u32>();
+}
diff --git a/tests/ui/traits/next-solver/cycles/coinduction/incompleteness-unstable-result.stderr b/tests/ui/traits/next-solver/cycles/coinduction/incompleteness-unstable-result.stderr
new file mode 100644
index 000000000..d49321917
--- /dev/null
+++ b/tests/ui/traits/next-solver/cycles/coinduction/incompleteness-unstable-result.stderr
@@ -0,0 +1,16 @@
+error[E0277]: the trait bound `A<X>: Trait<_, _, _>` is not satisfied
+  --> $DIR/incompleteness-unstable-result.rs:63:19
+   |
+LL |     impls_trait::<A<X>, _, _, _>();
+   |                   ^^^^ the trait `Trait<_, _, _>` is not implemented for `A<X>`
+   |
+   = help: the trait `Trait<U, V, D>` is implemented for `A<T>`
+note: required by a bound in `impls_trait`
+  --> $DIR/incompleteness-unstable-result.rs:51:28
+   |
+LL | fn impls_trait<T: ?Sized + Trait<U, V, D>, U: ?Sized, V: ?Sized, D: ?Sized>() {}
+   |                            ^^^^^^^^^^^^^^ required by this bound in `impls_trait`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.