Remove Session.trait_methods_not_found

Instead, avoid registering the problematic well-formed obligation
to begin with. This removes global untracked mutable state,
and avoids potential issues with incremental compilation.

compiler/rustc_infer/src/traits/error_reporting/mod.rs

@@ -104,11 +104,5 @@ pub fn report_object_safety_error(
          to be resolvable dynamically; for more information visit \
          <https://doc.rust-lang.org/reference/items/traits.html#object-safety>",
     );
-
-    if tcx.sess.trait_methods_not_found.borrow().iter().any(|full_span| full_span.contains(span)) {
-        // Avoid emitting error caused by non-existing method (#58734)
-        err.cancel();
-    }
-
     err
 }

compiler/rustc_session/src/session.rs

@@ -189,9 +189,6 @@ pub struct Session {
     /// Cap lint level specified by a driver specifically.
     pub driver_lint_caps: FxHashMap<lint::LintId, lint::Level>,
 
-    /// `Span`s of trait methods that weren't found to avoid emitting object safety errors
-    pub trait_methods_not_found: Lock<FxHashSet<Span>>,
-
     /// Mapping from ident span to path span for paths that don't exist as written, but that
     /// exist under `std`. For example, wrote `str::from_utf8` instead of `std::str::from_utf8`.
     pub confused_type_with_std_module: Lock<FxHashMap<Span, Span>>,
@@ -1353,7 +1350,6 @@ pub fn build_session(
         print_fuel,
         jobserver: jobserver::client(),
         driver_lint_caps,
-        trait_methods_not_found: Lock::new(Default::default()),
         confused_type_with_std_module: Lock::new(Default::default()),
         ctfe_backtrace,
         miri_unleashed_features: Lock::new(Default::default()),

compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs

@@ -858,13 +858,25 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     path.segments,
                 );
             }
-            QPath::TypeRelative(ref qself, ref segment) => (self.to_ty(qself), qself, segment),
+            QPath::TypeRelative(ref qself, ref segment) => {
+                // Don't use `self.to_ty`, since this will register a WF obligation.
+                // If we're trying to call a non-existent method on a trait
+                // (e.g. `MyTrait::missing_method`), then resolution will
+                // give us a `QPath::TypeRelative` with a trait object as
+                // `qself`. In that case, we want to avoid registering a WF obligation
+                // for `dyn MyTrait`, since we don't actually need the trait
+                // to be object-safe.
+                // We manually call `register_wf_obligation` in the success path
+                // below.
+                (<dyn AstConv<'_>>::ast_ty_to_ty(self, qself), qself, segment)
+            }
             QPath::LangItem(..) => {
                 bug!("`resolve_ty_and_res_fully_qualified_call` called on `LangItem`")
             }
         };
         if let Some(&cached_result) = self.typeck_results.borrow().type_dependent_defs().get(hir_id)
         {
+            self.register_wf_obligation(ty.into(), qself.span, traits::WellFormed(None));
             // Return directly on cache hit. This is useful to avoid doubly reporting
             // errors with default match binding modes. See #44614.
             let def = cached_result.map_or(Res::Err, |(kind, def_id)| Res::Def(kind, def_id));
@@ -878,6 +890,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     method::MethodError::PrivateMatch(kind, def_id, _) => Ok((kind, def_id)),
                     _ => Err(ErrorReported),
                 };
+
+                // If we have a path like `MyTrait::missing_method`, then don't register
+                // a WF obligation for `dyn MyTrait` when method lookup fails. Otherwise,
+                // register a WF obligation so that we can detect any additional
+                // errors in the self type.
+                if !(matches!(error, method::MethodError::NoMatch(_)) && ty.is_trait()) {
+                    self.register_wf_obligation(ty.into(), qself.span, traits::WellFormed(None));
+                }
                 if item_name.name != kw::Empty {
                     if let Some(mut e) = self.report_method_error(
                         span,
@@ -895,6 +915,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         if result.is_ok() {
             self.maybe_lint_bare_trait(qpath, hir_id);
+            self.register_wf_obligation(ty.into(), qself.span, traits::WellFormed(None));
         }
 
         // Write back the new resolution.

compiler/rustc_typeck/src/check/method/suggest.rs

@@ -70,15 +70,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     pub fn report_method_error(
         &self,
-        span: Span,
+        mut span: Span,
         rcvr_ty: Ty<'tcx>,
         item_name: Ident,
         source: SelfSource<'tcx>,
         error: MethodError<'tcx>,
         args: Option<&'tcx [hir::Expr<'tcx>]>,
     ) -> Option<DiagnosticBuilder<'_>> {
-        let orig_span = span;
-        let mut span = span;
         // Avoid suggestions when we don't know what's going on.
         if rcvr_ty.references_error() {
             return None;
@@ -545,7 +543,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     } else {
                         err.span_label(span, format!("{item_kind} cannot be called on `{ty_str}` due to unsatisfied trait bounds"));
                     }
-                    self.tcx.sess.trait_methods_not_found.borrow_mut().insert(orig_span);
                 };
 
                 // If the method name is the name of a field with a function or closure type,