Auto merge of rust-lang#109788 - compiler-errors:trait-item-from-non-trait, r=petrochenkov

More descriptive error when qself path doesnt have a trait on the RHS of `as`

`<Ty as Enum>::Assoc` should report that `Enum` is a trait. Main question is whether to eagerly report the error, or raise it with `return Err(..)` -- i'll note that in an inline comment though.

cc `@GuillaumeGomez` who said this came up at a Paris Rust meetup.

r? `@petrochenkov`

Author: bors

compiler/rustc_resolve/src/late.rs

@@ -122,6 +122,12 @@ pub(crate) enum ConstantItemKind {
     Static,
 }
 
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+enum RecordPartialRes {
+    Yes,
+    No,
+}
+
 /// The rib kind restricts certain accesses,
 /// e.g. to a `Res::Local` of an outer item.
 #[derive(Copy, Clone, Debug)]
@@ -2682,6 +2688,7 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
                 &path,
                 PathSource::Trait(AliasPossibility::No),
                 Finalize::new(trait_ref.ref_id, trait_ref.path.span),
+                RecordPartialRes::Yes,
             );
             self.diagnostic_metadata.currently_processing_impl_trait = None;
             if let Some(def_id) = res.expect_full_res().opt_def_id() {
@@ -3420,6 +3427,7 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
             &Segment::from_path(path),
             source,
             Finalize::new(id, path.span),
+            RecordPartialRes::Yes,
         );
     }
 
@@ -3430,6 +3438,7 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
         path: &[Segment],
         source: PathSource<'ast>,
         finalize: Finalize,
+        record_partial_res: RecordPartialRes,
     ) -> PartialRes {
         let ns = source.namespace();
 
@@ -3636,7 +3645,7 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
             _ => report_errors(self, None),
         };
 
-        if !matches!(source, PathSource::TraitItem(..)) {
+        if record_partial_res == RecordPartialRes::Yes {
             // Avoid recording definition of `A::B` in `<T as A>::B::C`.
             self.r.record_partial_res(node_id, partial_res);
             self.resolve_elided_lifetimes_in_path(node_id, partial_res, path, source, path_span);
@@ -3740,7 +3749,25 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
                 )));
             }
 
-            // Make sure `A::B` in `<T as A::B>::C` is a trait item.
+            let num_privacy_errors = self.r.privacy_errors.len();
+            // Make sure that `A` in `<T as A>::B::C` is a trait.
+            let trait_res = self.smart_resolve_path_fragment(
+                &None,
+                &path[..qself.position],
+                PathSource::Trait(AliasPossibility::No),
+                Finalize::new(finalize.node_id, qself.path_span),
+                RecordPartialRes::No,
+            );
+
+            if trait_res.expect_full_res() == Res::Err {
+                return Ok(Some(trait_res));
+            }
+
+            // Truncate additional privacy errors reported above,
+            // because they'll be recomputed below.
+            self.r.privacy_errors.truncate(num_privacy_errors);
+
+            // Make sure `A::B` in `<T as A>::B::C` is a trait item.
             //
             // Currently, `path` names the full item (`A::B::C`, in
             // our example). so we extract the prefix of that that is
@@ -3753,6 +3780,7 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
                 &path[..=qself.position],
                 PathSource::TraitItem(ns),
                 Finalize::with_root_span(finalize.node_id, finalize.path_span, qself.path_span),
+                RecordPartialRes::No,
             );
 
             // The remaining segments (the `C` in our example) will

tests/ui/parser/dyn-trait-compatibility.rs

@@ -9,6 +9,6 @@ type A2 = dyn<dyn, dyn>;
 type A3 = dyn<<dyn as dyn>::dyn>;
 //~^ ERROR cannot find type `dyn` in this scope
 //~| ERROR cannot find type `dyn` in this scope
-//~| ERROR use of undeclared crate or module `dyn`
+//~| ERROR cannot find trait `dyn` in this scope
 
 fn main() {}

tests/ui/parser/dyn-trait-compatibility.stderr

@@ -4,12 +4,6 @@ error[E0433]: failed to resolve: use of undeclared crate or module `dyn`
 LL | type A1 = dyn::dyn;
    |           ^^^ use of undeclared crate or module `dyn`
 
-error[E0433]: failed to resolve: use of undeclared crate or module `dyn`
-  --> $DIR/dyn-trait-compatibility.rs:9:23
-   |
-LL | type A3 = dyn<<dyn as dyn>::dyn>;
-   |                       ^^^ use of undeclared crate or module `dyn`
-
 error[E0412]: cannot find type `dyn` in this scope
   --> $DIR/dyn-trait-compatibility.rs:1:11
    |
@@ -40,6 +34,12 @@ error[E0412]: cannot find type `dyn` in this scope
 LL | type A3 = dyn<<dyn as dyn>::dyn>;
    |           ^^^ not found in this scope
 
+error[E0405]: cannot find trait `dyn` in this scope
+  --> $DIR/dyn-trait-compatibility.rs:9:23
+   |
+LL | type A3 = dyn<<dyn as dyn>::dyn>;
+   |                       ^^^ not found in this scope
+
 error[E0412]: cannot find type `dyn` in this scope
   --> $DIR/dyn-trait-compatibility.rs:9:16
    |
@@ -48,5 +48,5 @@ LL | type A3 = dyn<<dyn as dyn>::dyn>;
 
 error: aborting due to 8 previous errors
 
-Some errors have detailed explanations: E0412, E0433.
-For more information about an error, try `rustc --explain E0412`.
+Some errors have detailed explanations: E0405, E0412, E0433.
+For more information about an error, try `rustc --explain E0405`.

tests/ui/rust-2018/edition-lint-fully-qualified-paths.fixed

@@ -18,9 +18,11 @@ mod foo {
 fn main() {
     let _: <foo::Baz as crate::foo::Foo>::Bar = ();
     //~^ ERROR absolute paths must start with
-    //~| this is accepted in the current edition
+    //~| WARN this is accepted in the current edition
+    //~| ERROR absolute paths must start with
+    //~| WARN this is accepted in the current edition
 
     let _: <crate::foo::Baz as foo::Foo>::Bar = ();
     //~^ ERROR absolute paths must start with
-    //~| this is accepted in the current edition
+    //~| WARN this is accepted in the current edition
 }

tests/ui/rust-2018/edition-lint-fully-qualified-paths.rs

@@ -18,9 +18,11 @@ mod foo {
 fn main() {
     let _: <foo::Baz as ::foo::Foo>::Bar = ();
     //~^ ERROR absolute paths must start with
-    //~| this is accepted in the current edition
+    //~| WARN this is accepted in the current edition
+    //~| ERROR absolute paths must start with
+    //~| WARN this is accepted in the current edition
 
     let _: <::foo::Baz as foo::Foo>::Bar = ();
     //~^ ERROR absolute paths must start with
-    //~| this is accepted in the current edition
+    //~| WARN this is accepted in the current edition
 }

tests/ui/rust-2018/edition-lint-fully-qualified-paths.stderr

@@ -13,13 +13,22 @@ LL | #![deny(absolute_paths_not_starting_with_crate)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: absolute paths must start with `self`, `super`, `crate`, or an external crate name in the 2018 edition
-  --> $DIR/edition-lint-fully-qualified-paths.rs:23:13
+  --> $DIR/edition-lint-fully-qualified-paths.rs:19:25
+   |
+LL |     let _: <foo::Baz as ::foo::Foo>::Bar = ();
+   |                         ^^^^^^^^^^ help: use `crate`: `crate::foo::Foo`
+   |
+   = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2018!
+   = note: for more information, see issue #53130 <https://github.com/rust-lang/rust/issues/53130>
+
+error: absolute paths must start with `self`, `super`, `crate`, or an external crate name in the 2018 edition
+  --> $DIR/edition-lint-fully-qualified-paths.rs:25:13
    |
 LL |     let _: <::foo::Baz as foo::Foo>::Bar = ();
    |             ^^^^^^^^^^ help: use `crate`: `crate::foo::Baz`
    |
    = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2018!
    = note: for more information, see issue #53130 <https://github.com/rust-lang/rust/issues/53130>
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 

tests/ui/traits/associated_type_bound/assoc_type_bound_with_struct.rs

@@ -18,6 +18,6 @@ fn qux<'a, T: Bar>(_: &'a T) where <&'a T as Bar>::Baz: String { //~ ERROR expec
 
 fn issue_95327() where <u8 as Unresolved>::Assoc: String {}
 //~^ ERROR expected trait, found struct
-//~| ERROR use of undeclared type `Unresolved`
+//~| ERROR cannot find trait `Unresolved` in this scope
 
 fn main() {}

tests/ui/traits/associated_type_bound/assoc_type_bound_with_struct.stderr

@@ -1,9 +1,3 @@
-error[E0433]: failed to resolve: use of undeclared type `Unresolved`
-  --> $DIR/assoc_type_bound_with_struct.rs:19:31
-   |
-LL | fn issue_95327() where <u8 as Unresolved>::Assoc: String {}
-   |                               ^^^^^^^^^^ use of undeclared type `Unresolved`
-
 error[E0404]: expected trait, found struct `String`
   --> $DIR/assoc_type_bound_with_struct.rs:5:46
    |
@@ -76,6 +70,12 @@ help: a trait with a similar name exists
 LL | fn qux<'a, T: Bar>(_: &'a T) where <&'a T as Bar>::Baz: ToString {
    |                                                         ~~~~~~~~
 
+error[E0405]: cannot find trait `Unresolved` in this scope
+  --> $DIR/assoc_type_bound_with_struct.rs:19:31
+   |
+LL | fn issue_95327() where <u8 as Unresolved>::Assoc: String {}
+   |                               ^^^^^^^^^^ not found in this scope
+
 error[E0404]: expected trait, found struct `String`
   --> $DIR/assoc_type_bound_with_struct.rs:19:51
    |
@@ -87,5 +87,5 @@ LL | fn issue_95327() where <u8 as Unresolved>::Assoc: String {}
 
 error: aborting due to 6 previous errors
 
-Some errors have detailed explanations: E0404, E0433.
+Some errors have detailed explanations: E0404, E0405.
 For more information about an error, try `rustc --explain E0404`.

tests/ui/ufcs/ufcs-partially-resolved.rs

@@ -17,37 +17,37 @@ type A = u32;
 
 fn main() {
     let _: <u8 as Tr>::N; //~ ERROR cannot find associated type `N` in trait `Tr`
-    let _: <u8 as E>::N; //~ ERROR cannot find associated type `N` in enum `E`
-    let _: <u8 as A>::N; //~ ERROR cannot find associated type `N` in `A`
+    let _: <u8 as E>::N; //~ ERROR expected trait, found enum `E`
+    let _: <u8 as A>::N; //~ ERROR expected trait, found type alias `A`
     <u8 as Tr>::N; //~ ERROR cannot find method or associated constant `N` in trait `Tr`
-    <u8 as E>::N; //~ ERROR cannot find method or associated constant `N` in enum `E`
-    <u8 as A>::N; //~ ERROR cannot find method or associated constant `N` in `A`
+    <u8 as E>::N; //~ ERROR expected trait, found enum `E`
+    <u8 as A>::N; //~ ERROR expected trait, found type alias `A`
     let _: <u8 as Tr>::Y; // OK
-    let _: <u8 as E>::Y; //~ ERROR expected associated type, found variant `E::Y`
+    let _: <u8 as E>::Y; //~ ERROR expected trait, found enum `E`
     <u8 as Tr>::Y; // OK
-    <u8 as E>::Y; //~ ERROR expected method or associated constant, found unit variant `E::Y`
+    <u8 as E>::Y; //~ ERROR expected trait, found enum `E`
 
     let _: <u8 as Tr>::N::NN; //~ ERROR cannot find associated type `N` in trait `Tr`
-    let _: <u8 as E>::N::NN; //~ ERROR cannot find associated type `N` in enum `E`
-    let _: <u8 as A>::N::NN; //~ ERROR cannot find associated type `N` in `A`
+    let _: <u8 as E>::N::NN; //~ ERROR expected trait, found enum `E`
+    let _: <u8 as A>::N::NN; //~ ERROR expected trait, found type alias `A`
     <u8 as Tr>::N::NN; //~ ERROR cannot find associated type `N` in trait `Tr`
-    <u8 as E>::N::NN; //~ ERROR cannot find associated type `N` in enum `E`
-    <u8 as A>::N::NN; //~ ERROR cannot find associated type `N` in `A`
+    <u8 as E>::N::NN; //~ ERROR expected trait, found enum `E`
+    <u8 as A>::N::NN; //~ ERROR expected trait, found type alias `A`
     let _: <u8 as Tr>::Y::NN; //~ ERROR ambiguous associated type
-    let _: <u8 as E>::Y::NN; //~ ERROR expected associated type, found variant `E::Y`
+    let _: <u8 as E>::Y::NN; //~ ERROR expected trait, found enum `E`
     <u8 as Tr>::Y::NN; //~ ERROR no associated item named `NN` found for type `u16`
-    <u8 as E>::Y::NN; //~ ERROR expected associated type, found variant `E::Y`
+    <u8 as E>::Y::NN; //~ ERROR expected trait, found enum `E`
 
-    let _: <u8 as Tr::N>::NN; //~ ERROR cannot find associated type `NN` in `Tr::N`
-    let _: <u8 as E::N>::NN; //~ ERROR cannot find associated type `NN` in `E::N`
-    let _: <u8 as A::N>::NN; //~ ERROR cannot find associated type `NN` in `A::N`
-    <u8 as Tr::N>::NN; //~ ERROR cannot find method or associated constant `NN` in `Tr::N`
-    <u8 as E::N>::NN; //~ ERROR cannot find method or associated constant `NN` in `E::N`
-    <u8 as A::N>::NN; //~ ERROR cannot find method or associated constant `NN` in `A::N`
-    let _: <u8 as Tr::Y>::NN; //~ ERROR cannot find associated type `NN` in `Tr::Y`
-    let _: <u8 as E::Y>::NN; //~ ERROR failed to resolve: `Y` is a variant, not a module
-    <u8 as Tr::Y>::NN; //~ ERROR cannot find method or associated constant `NN` in `Tr::Y`
-    <u8 as E::Y>::NN; //~ ERROR failed to resolve: `Y` is a variant, not a module
+    let _: <u8 as Tr::N>::NN; //~ ERROR cannot find trait `N` in trait `Tr`
+    let _: <u8 as E::N>::NN; //~ ERROR cannot find trait `N` in enum `E`
+    let _: <u8 as A::N>::NN; //~ ERROR cannot find trait `N` in `A`
+    <u8 as Tr::N>::NN; //~ ERROR cannot find trait `N` in trait `Tr`
+    <u8 as E::N>::NN; //~ ERROR cannot find trait `N` in enum `E`
+    <u8 as A::N>::NN; //~ ERROR cannot find trait `N` in `A`
+    let _: <u8 as Tr::Y>::NN; //~ ERROR expected trait, found associated type `Tr::Y
+    let _: <u8 as E::Y>::NN; //~ ERROR expected trait, found variant `E::Y`
+    <u8 as Tr::Y>::NN; //~ ERROR expected trait, found associated type `Tr::Y`
+    <u8 as E::Y>::NN; //~ ERROR expected trait, found variant `E::Y`
 
     let _: <u8 as Dr>::Z; //~ ERROR expected associated type, found associated function `Dr::Z`
     <u8 as Dr>::X; //~ ERROR expected method or associated constant, found associated type `Dr::X`