Improve error message for impl Self

compiler/rustc_resolve/src/late.rs

@@ -394,6 +394,9 @@ struct DiagnosticMetadata<'ast> {
     /// Used to detect possible `if let` written without `let` and to provide structured suggestion.
     in_if_condition: Option<&'ast Expr>,
 
+    /// When processing a impl self type and encountering a `Self`, suggest not using it.
+    currently_processing_impl_self_ty: bool,
+
     /// If we are currently in a trait object definition. Used to point at the bounds when
     /// encountering a struct or enum.
     current_trait_object: Option<&'ast [ast::GenericBound]>,
@@ -1318,7 +1321,12 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
                             visit::walk_trait_ref(this, trait_ref);
                         }
                         // Resolve the self type.
+                        let prev = replace(
+                            &mut this.diagnostic_metadata.currently_processing_impl_self_ty,
+                            true,
+                        );
                         this.visit_ty(self_type);
+                        this.diagnostic_metadata.currently_processing_impl_self_ty = prev;
                         // Resolve the generic parameters.
                         this.visit_generics(generics);
                         // Resolve the items within the impl.
@@ -2038,6 +2046,16 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
             Ok(Some(partial_res)) if partial_res.unresolved_segments() == 0 => {
                 if source.is_expected(partial_res.base_res()) || partial_res.base_res() == Res::Err
                 {
+                    if self.diagnostic_metadata.currently_processing_impl_self_ty {
+                        if let Res::SelfTy { .. } = partial_res.base_res() {
+                            if path.len() == 1 && path[0].ident.name == kw::SelfUpper {
+                                self.r.session.span_err(
+                                    span,
+                                    "`Self` is only available in impls, traits, and type definitions",
+                                );
+                            }
+                        }
+                    }
                     partial_res
                 } else {
                     report_errors(self, Some(partial_res.base_res()))

src/test/ui/resolve/issue-23305.rs

@@ -4,5 +4,6 @@ pub trait ToNbt<T> {
 
 impl dyn ToNbt<Self> {}
 //~^ ERROR cycle detected
+//~| ERROR `Self` is only available in impls, traits, and type definitions
 
 fn main() {}

src/test/ui/resolve/issue-23305.stderr

@@ -1,3 +1,9 @@
+error: `Self` is only available in impls, traits, and type definitions
+  --> $DIR/issue-23305.rs:5:16
+   |
+LL | impl dyn ToNbt<Self> {}
+   |                ^^^^
+
 error[E0391]: cycle detected when computing type of `<impl at $DIR/issue-23305.rs:5:1: 5:24>`
   --> $DIR/issue-23305.rs:5:16
    |
@@ -11,6 +17,6 @@ note: cycle used when collecting item types in top-level module
 LL | pub trait ToNbt<T> {
    | ^^^^^^^^^^^^^^^^^^
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0391`.

src/test/ui/resolve/resolve-self-in-impl.rs

@@ -12,9 +12,13 @@ impl Tr for S where S<Self>: Copy {} // OK
 impl Tr for S where Self::A: Copy {} // OK
 
 impl Tr for Self {} //~ ERROR cycle detected
+//~^ ERROR `Self` is only available in impls, traits, and type definitions
 impl Tr for S<Self> {} //~ ERROR cycle detected
+//~^ ERROR `Self` is only available in impls, traits, and type definitions
 impl Self {} //~ ERROR cycle detected
+//~^ ERROR `Self` is only available in impls, traits, and type definitions
 impl S<Self> {} //~ ERROR cycle detected
+//~^ ERROR `Self` is only available in impls, traits, and type definitions
 impl Tr<Self::A> for S {} //~ ERROR cycle detected
 
 fn main() {}

src/test/ui/resolve/resolve-self-in-impl.stderr

@@ -1,3 +1,27 @@
+error: `Self` is only available in impls, traits, and type definitions
+  --> $DIR/resolve-self-in-impl.rs:14:13
+   |
+LL | impl Tr for Self {}
+   |             ^^^^
+
+error: `Self` is only available in impls, traits, and type definitions
+  --> $DIR/resolve-self-in-impl.rs:16:15
+   |
+LL | impl Tr for S<Self> {}
+   |               ^^^^
+
+error: `Self` is only available in impls, traits, and type definitions
+  --> $DIR/resolve-self-in-impl.rs:18:6
+   |
+LL | impl Self {}
+   |      ^^^^
+
+error: `Self` is only available in impls, traits, and type definitions
+  --> $DIR/resolve-self-in-impl.rs:20:8
+   |
+LL | impl S<Self> {}
+   |        ^^^^
+
 error[E0391]: cycle detected when computing type of `<impl at $DIR/resolve-self-in-impl.rs:14:1: 14:20>`
   --> $DIR/resolve-self-in-impl.rs:14:13
    |
@@ -17,13 +41,13 @@ LL | |
 LL | | fn main() {}
    | |____________^
 
-error[E0391]: cycle detected when computing type of `<impl at $DIR/resolve-self-in-impl.rs:15:1: 15:23>`
-  --> $DIR/resolve-self-in-impl.rs:15:15
+error[E0391]: cycle detected when computing type of `<impl at $DIR/resolve-self-in-impl.rs:16:1: 16:23>`
+  --> $DIR/resolve-self-in-impl.rs:16:15
    |
 LL | impl Tr for S<Self> {}
    |               ^^^^
    |
-   = note: ...which immediately requires computing type of `<impl at $DIR/resolve-self-in-impl.rs:15:1: 15:23>` again
+   = note: ...which immediately requires computing type of `<impl at $DIR/resolve-self-in-impl.rs:16:1: 16:23>` again
 note: cycle used when collecting item types in top-level module
   --> $DIR/resolve-self-in-impl.rs:1:1
    |
@@ -36,13 +60,13 @@ LL | |
 LL | | fn main() {}
    | |____________^
 
-error[E0391]: cycle detected when computing type of `<impl at $DIR/resolve-self-in-impl.rs:16:1: 16:13>`
-  --> $DIR/resolve-self-in-impl.rs:16:6
+error[E0391]: cycle detected when computing type of `<impl at $DIR/resolve-self-in-impl.rs:18:1: 18:13>`
+  --> $DIR/resolve-self-in-impl.rs:18:6
    |
 LL | impl Self {}
    |      ^^^^
    |
-   = note: ...which immediately requires computing type of `<impl at $DIR/resolve-self-in-impl.rs:16:1: 16:13>` again
+   = note: ...which immediately requires computing type of `<impl at $DIR/resolve-self-in-impl.rs:18:1: 18:13>` again
 note: cycle used when collecting item types in top-level module
   --> $DIR/resolve-self-in-impl.rs:1:1
    |
@@ -55,13 +79,13 @@ LL | |
 LL | | fn main() {}
    | |____________^
 
-error[E0391]: cycle detected when computing type of `<impl at $DIR/resolve-self-in-impl.rs:17:1: 17:16>`
-  --> $DIR/resolve-self-in-impl.rs:17:8
+error[E0391]: cycle detected when computing type of `<impl at $DIR/resolve-self-in-impl.rs:20:1: 20:16>`
+  --> $DIR/resolve-self-in-impl.rs:20:8
    |
 LL | impl S<Self> {}
    |        ^^^^
    |
-   = note: ...which immediately requires computing type of `<impl at $DIR/resolve-self-in-impl.rs:17:1: 17:16>` again
+   = note: ...which immediately requires computing type of `<impl at $DIR/resolve-self-in-impl.rs:20:1: 20:16>` again
 note: cycle used when collecting item types in top-level module
   --> $DIR/resolve-self-in-impl.rs:1:1
    |
@@ -74,13 +98,13 @@ LL | |
 LL | | fn main() {}
    | |____________^
 
-error[E0391]: cycle detected when computing trait implemented by `<impl at $DIR/resolve-self-in-impl.rs:18:1: 18:26>`
-  --> $DIR/resolve-self-in-impl.rs:18:1
+error[E0391]: cycle detected when computing trait implemented by `<impl at $DIR/resolve-self-in-impl.rs:22:1: 22:26>`
+  --> $DIR/resolve-self-in-impl.rs:22:1
    |
 LL | impl Tr<Self::A> for S {}
    | ^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: ...which immediately requires computing trait implemented by `<impl at $DIR/resolve-self-in-impl.rs:18:1: 18:26>` again
+   = note: ...which immediately requires computing trait implemented by `<impl at $DIR/resolve-self-in-impl.rs:22:1: 22:26>` again
 note: cycle used when collecting item types in top-level module
   --> $DIR/resolve-self-in-impl.rs:1:1
    |
@@ -93,6 +117,6 @@ LL | |
 LL | | fn main() {}
    | |____________^
 
-error: aborting due to 5 previous errors
+error: aborting due to 9 previous errors
 
 For more information about this error, try `rustc --explain E0391`.