Rollup merge of rust-lang#117992 - compiler-errors:sound-but-not-complete, r=lcnr

Don't require intercrate mode for negative coherence

Negative coherence needs to be *sound*, but does not need to be *complete*, since it's looking for the *existence* of a negative goal, not the non-existence of a positive goal.

This removes some trivial and annoying ambiguities when a negative impl has region constraints.

r? lcnr idk if this needs an fcp but if it does, pls kick it off

Author: compiler-errors

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -397,7 +397,11 @@ fn impl_intersection_has_negative_obligation(
 ) -> bool {
     debug!("negative_impl(impl1_def_id={:?}, impl2_def_id={:?})", impl1_def_id, impl2_def_id);
 
-    let ref infcx = tcx.infer_ctxt().intercrate(true).with_next_trait_solver(true).build();
+    // N.B. We don't need to use intercrate mode here because we're trying to prove
+    // the *existence* of a negative goal, not the non-existence of a positive goal.
+    // Without this, we over-eagerly register coherence ambiguity candidates when
+    // impl candidates do exist.
+    let ref infcx = tcx.infer_ctxt().with_next_trait_solver(true).build();
     let universe = infcx.universe();
 
     let impl1_header = fresh_impl_header(infcx, impl1_def_id);

tests/ui/coherence/coherence-negative-outlives-lifetimes.rs

@@ -1,5 +1,5 @@
 // revisions: stock with_negative_coherence
-//[with_negative_coherence] known-bug: unknown
+//[with_negative_coherence] check-pass
 
 #![feature(negative_impls)]
 #![cfg_attr(with_negative_coherence, feature(with_negative_coherence))]

tests/ui/coherence/coherence-negative-outlives-lifetimes.with_negative_coherence.stderr

@@ -1,10 +1,4 @@
-// known-bug: unknown
-
-// This fails because we currently perform negative coherence in coherence mode.
-// This means that when looking for a negative predicate, we also assemble a
-// coherence-unknowable predicate. Since confirming the negative impl has region
-// obligations, we don't prefer the impl over the unknowable predicate
-// unconditionally and instead flounder.
+// check-pass
 
 #![feature(negative_impls)]
 #![feature(rustc_attrs)]

tests/ui/coherence/coherence-overlap-with-regions.rs

@@ -1,5 +1,5 @@
 // revisions: any_lt static_lt
-//[static_lt] known-bug: unknown
+//[static_lt] check-pass
 
 // This fails because we currently perform negative coherence in coherence mode.
 // This means that when looking for a negative predicate, we also assemble a