Auto merge of rust-lang#131840 - compiler-errors:impossible-maybe, r=…

…lcnr

Dont consider predicates that may hold as impossible in `is_impossible_associated_item`

Use infer vars to account for ambiguities when considering if methods are impossible to instantiate for a given self type. Also while we're at it, let's use the new trait solver instead of `evaluate` since this is used in rustdoc.

r? lcnr
Fixes rust-lang#131839

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -562,11 +562,20 @@ fn is_impossible_associated_item(
 
  let generics = tcx.generics_of(trait_item_def_id);
  let predicates = tcx.predicates_of(trait_item_def_id);
+
+ // Be conservative in cases where we have `W<T: ?Sized>` and a method like `Self: Sized`,
+ // since that method *may* have some substitutions where the predicates hold.
+ //
+ // This replicates the logic we use in coherence.
+ let infcx =
+ tcx.infer_ctxt().ignoring_regions().with_next_trait_solver(true).intercrate(true).build();
+ let param_env = ty::ParamEnv::empty();
+ let fresh_args = infcx.fresh_args_for_item(tcx.def_span(impl_def_id), impl_def_id);
+
  let impl_trait_ref = tcx
  .impl_trait_ref(impl_def_id)
  .expect("expected impl to correspond to trait")
- .instantiate_identity();
- let param_env = tcx.param_env(impl_def_id);
+ .instantiate(tcx, fresh_args);
 
  let mut visitor = ReferencesOnlyParentGenerics { tcx, generics, trait_item_def_id };
  let predicates_for_trait = predicates.predicates.iter().filter_map(|(pred, span)| {
@@ -580,16 +589,9 @@ fn is_impossible_associated_item(
  })
  });
 
- let infcx = tcx.infer_ctxt().ignoring_regions().build();
- for obligation in predicates_for_trait {
- // Ignore overflow error, to be conservative.
- if let Ok(result) = infcx.evaluate_obligation(&obligation)
- && !result.may_apply()
- {
- return true;
- }
- }
- false
+ let ocx = ObligationCtxt::new(&infcx);
+ ocx.register_obligations(predicates_for_trait);
+ !ocx.select_where_possible().is_empty()
 }
 
 pub fn provide(providers: &mut Providers) {

tests/rustdoc/impossible-default.rs

@@ -18,3 +18,11 @@ pub trait Foo {
 pub struct Bar([u8]);
 
 impl Foo for Bar {}
+
+//@ has foo/struct.Generic.html '//*[@id="method.needs_sized"]//h4[@class="code-header"]' \
+// "fn needs_sized"
+//@ has foo/struct.Generic.html '//*[@id="method.no_needs_sized"]//h4[@class="code-header"]' \
+// "fn no_needs_sized"
+pub struct Generic<T: ?Sized>(T);
+
+impl<T: ?Sized> Foo for Generic<T> {}