Emit method not found error for object safe traits without dyn

Given `Trait::foo()` in edition 2021 where `Trait` is object safe and `foo` isn't resolved, emit E0782 instead of marking the expression as `{type error}`, as `foo` not being part of `Trait` is more useful information.

If `Trait` *isn't* object safe, then the resolve error isn't as useful because E0782 will have more actionable feedback.

Author: estebank

compiler/rustc_hir_analysis/src/hir_ty_lowering/lint.rs

@@ -81,14 +81,32 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             }
 
             if !object_safe && self_ty.span.can_be_used_for_suggestions() {
-                // suggest_impl_trait_on_bare_trait(tcx, &mut diag, self_ty);
                 let parent = tcx.parent_hir_node(self_ty.hir_id);
                 suggest_path_on_bare_trait(tcx, &mut diag, parent);
             }
 
             // Check if the impl trait that we are considering is an impl of a local trait.
             self.maybe_suggest_blanket_trait_impl(self_ty, &mut diag);
             self.maybe_suggest_assoc_ty_bound(self_ty, &mut diag);
+
+            if object_safe {
+                let parents = self.tcx().hir().parent_iter(self_ty.hir_id);
+                for (_, parent) in parents {
+                    let hir::Node::Expr(expr) = parent else {
+                        break;
+                    };
+                    if let hir::ExprKind::Path(hir::QPath::TypeRelative(_, segment)) = expr.kind
+                        && let Res::Err = segment.res
+                    {
+                        // If the trait is object safe *and* there's a path segment that couldn't be
+                        // resolved, we know that we will have a resolve error later. If there's an
+                        // unresolved segment *and* the trait is not object safe, then no other
+                        // error would have been emitted, so we always emit an error in that case.
+                        diag.emit();
+                        return None;
+                    }
+                }
+            }
             diag.stash(self_ty.span, StashKey::TraitMissingMethod)
         } else {
             tcx.node_span_lint(BARE_TRAIT_OBJECTS, self_ty.hir_id, self_ty.span, |lint| {

tests/ui/resolve/issue-111312.rs

@@ -7,6 +7,7 @@ trait Has {
 trait HasNot {}
 
 fn main() {
-    <dyn HasNot>::has();
+    HasNot::has();
     //~^ ERROR no function or associated item named `has` found for trait `HasNot`
+    //~| ERROR trait objects must include the `dyn` keyword
 }

tests/ui/resolve/issue-111312.stderr

@@ -1,15 +1,27 @@
-error[E0599]: no function or associated item named `has` found for trait `HasNot`
-  --> $DIR/issue-111312.rs:10:19
+error[E0782]: trait objects must include the `dyn` keyword
+  --> $DIR/issue-111312.rs:10:5
+   |
+LL |     HasNot::has();
+   |     ^^^^^^
+   |
+help: add `dyn` keyword before this trait
    |
 LL |     <dyn HasNot>::has();
-   |                   ^^^ function or associated item not found in `HasNot`
+   |     ++++       +
+
+error[E0599]: no function or associated item named `has` found for trait `HasNot`
+  --> $DIR/issue-111312.rs:10:13
+   |
+LL |     HasNot::has();
+   |             ^^^ function or associated item not found in `HasNot`
    |
 note: `Has` defines an item `has`
   --> $DIR/issue-111312.rs:3:1
    |
 LL | trait Has {
    | ^^^^^^^^^
 
-error: aborting due to 1 previous error
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0599`.
+Some errors have detailed explanations: E0599, E0782.
+For more information about an error, try `rustc --explain E0599`.

tests/ui/resolve/issue-111727.rs

@@ -2,5 +2,6 @@
 
 fn main() {
     std::any::Any::create();
-    //~^ ERROR trait objects must include the `dyn` keyword
+    //~^ ERROR no function or associated item named `create` found for trait `Any`
+    //~| ERROR trait objects must include the `dyn` keyword
 }

tests/ui/resolve/issue-111727.stderr

@@ -9,6 +9,13 @@ help: add `dyn` keyword before this trait
 LL |     <dyn std::any::Any>::create();
    |     ++++              +
 
-error: aborting due to 1 previous error
+error[E0599]: no function or associated item named `create` found for trait `Any`
+  --> $DIR/issue-111727.rs:4:20
+   |
+LL |     std::any::Any::create();
+   |                    ^^^^^^ function or associated item not found in `Any`
+
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0782`.
+Some errors have detailed explanations: E0599, E0782.
+For more information about an error, try `rustc --explain E0599`.