Rollup merge of #108830 - compiler-errors:new-solver-fast-reject-faster, r=lcnr

Treat projections with infer as placeholder during fast reject in new solver

r? ``@lcnr``

Kind of a shame that we need to change all of the call sites for `for_each_relevant_impl`, etc. to pass an extra parameter. I guess I could have the "default" fn which calls a configurable fn?

3 files changed, 67 insertions, 0 deletions

diff --git a/tests/ui/traits/new-solver/nested-obligations-with-bound-vars-gat.rs b/tests/ui/traits/new-solver/nested-obligations-with-bound-vars-gat.rs
new file mode 100644
index 00000000000..92bad959095
--- /dev/null
+++ b/tests/ui/traits/new-solver/nested-obligations-with-bound-vars-gat.rs
@@ -0,0 +1,43 @@
+// check-pass
+// compile-flags: -Ztrait-solver=next
+// Issue 96230
+
+use std::fmt::Debug;
+
+trait Classic {
+    type Assoc;
+}
+
+trait Gat {
+    type Assoc<'a>;
+}
+
+struct Foo;
+
+impl Classic for Foo {
+    type Assoc = ();
+}
+
+impl Gat for Foo {
+    type Assoc<'i> = ();
+}
+
+fn classic_debug<T: Classic>(_: T)
+where
+    T::Assoc: Debug,
+{
+}
+
+fn gat_debug<T: Gat>(_: T)
+where
+    for<'a> T::Assoc<'a>: Debug,
+{
+}
+
+fn main() {
+    classic_debug::<Foo>(Foo); // fine
+    classic_debug(Foo); // fine
+
+    gat_debug::<Foo>(Foo); // fine
+    gat_debug(Foo); // boom
+}
diff --git a/tests/ui/traits/new-solver/runaway-impl-candidate-selection.rs b/tests/ui/traits/new-solver/runaway-impl-candidate-selection.rs
new file mode 100644
index 00000000000..1dca86d3630
--- /dev/null
+++ b/tests/ui/traits/new-solver/runaway-impl-candidate-selection.rs
@@ -0,0 +1,15 @@
+// compile-flags: -Ztrait-solver=next
+
+// In the new solver, we are trying to select `<?0 as Iterator>::Item: Debug`,
+// which, naively can be unified with every impl of `Debug` if we're not careful.
+// This test makes sure that we treat projections with inference var substs as
+// placeholders during fast reject.
+
+fn iter<T: Iterator>() -> <T as Iterator>::Item {
+    todo!()
+}
+
+fn main() {
+    println!("{:?}", iter::<_>());
+    //~^ ERROR type annotations needed
+}
diff --git a/tests/ui/traits/new-solver/runaway-impl-candidate-selection.stderr b/tests/ui/traits/new-solver/runaway-impl-candidate-selection.stderr
new file mode 100644
index 00000000000..47004821ad7
--- /dev/null
+++ b/tests/ui/traits/new-solver/runaway-impl-candidate-selection.stderr
@@ -0,0 +1,9 @@
+error[E0282]: type annotations needed
+  --> $DIR/runaway-impl-candidate-selection.rs:13:22
+   |
+LL |     println!("{:?}", iter::<_>());
+   |                      ^^^^^^^^^ cannot infer type of the type parameter `T` declared on the function `iter`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.