Emit a nicer error on impl Self {

compiler/rustc_error_messages/locales/en-US/hir_analysis.ftl

@@ -146,3 +146,7 @@ hir_analysis_const_impl_for_non_const_trait =
 
 hir_analysis_const_bound_for_non_const_trait =
     ~const can only be applied to `#[const_trait]` traits
+
+hir_analysis_self_in_impl_self =
+    `Self` is not valid in the self type of an impl block
+    .note = replace `Self` with a different type

compiler/rustc_hir/src/hir.rs

@@ -2418,6 +2418,30 @@ impl<'hir> Ty<'hir> {
         }
         final_ty
     }
+
+    pub fn find_self_aliases(&self) -> Vec<Span> {
+        use crate::intravisit::Visitor;
+        struct MyVisitor(Vec<Span>);
+        impl<'v> Visitor<'v> for MyVisitor {
+            fn visit_ty(&mut self, t: &'v Ty<'v>) {
+                if matches!(
+                    &t.kind,
+                    TyKind::Path(QPath::Resolved(
+                        _,
+                        Path { res: crate::def::Res::SelfTyAlias { .. }, .. },
+                    ))
+                ) {
+                    self.0.push(t.span);
+                    return;
+                }
+                crate::intravisit::walk_ty(self, t);
+            }
+        }
+
+        let mut my_visitor = MyVisitor(vec![]);
+        my_visitor.visit_ty(self);
+        my_visitor.0
+    }
 }
 
 /// Not represented directly in the AST; referred to by name through a `ty_path`.

compiler/rustc_hir_analysis/src/collect/type_of.rs

@@ -319,7 +319,15 @@ pub(super) fn type_of(tcx: TyCtxt<'_>, def_id: DefId) -> Ty<'_> {
                     }
                 }
                 ItemKind::TyAlias(self_ty, _) => icx.to_ty(self_ty),
-                ItemKind::Impl(hir::Impl { self_ty, .. }) => icx.to_ty(*self_ty),
+                ItemKind::Impl(hir::Impl { self_ty, .. }) => {
+                    match self_ty.find_self_aliases() {
+                        spans if spans.len() > 0 => {
+                            tcx.sess.emit_err(crate::errors::SelfInImplSelf { span: spans.into(), note: (), });
+                            tcx.ty_error()
+                        },
+                        _ => icx.to_ty(*self_ty),
+                    }
+                },
                 ItemKind::Fn(..) => {
                     let substs = InternalSubsts::identity_for_item(tcx, def_id.to_def_id());
                     tcx.mk_fn_def(def_id.to_def_id(), substs)

compiler/rustc_hir_analysis/src/errors.rs

@@ -1,7 +1,7 @@
 //! Errors emitted by `rustc_hir_analysis`.
 
-use rustc_errors::IntoDiagnostic;
 use rustc_errors::{error_code, Applicability, DiagnosticBuilder, ErrorGuaranteed, Handler};
+use rustc_errors::{IntoDiagnostic, MultiSpan};
 use rustc_macros::{Diagnostic, LintDiagnostic};
 use rustc_middle::ty::Ty;
 use rustc_span::{symbol::Ident, Span, Symbol};
@@ -270,3 +270,12 @@ pub struct ConstBoundForNonConstTrait {
     #[primary_span]
     pub span: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(hir_analysis_self_in_impl_self)]
+pub struct SelfInImplSelf {
+    #[primary_span]
+    pub span: MultiSpan,
+    #[note]
+    pub note: (),
+}

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

@@ -3,6 +3,6 @@ pub trait ToNbt<T> {
 }
 
 impl dyn ToNbt<Self> {}
-//~^ ERROR cycle detected
+//~^ ERROR `Self` is not valid in the self type of an impl block
 
 fn main() {}

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

@@ -1,22 +1,10 @@
-error[E0391]: cycle detected when computing type of `<impl at $DIR/issue-23305.rs:5:1: 5:21>`
+error: `Self` is not valid in the self type of an impl block
   --> $DIR/issue-23305.rs:5:16
    |
 LL | impl dyn ToNbt<Self> {}
    |                ^^^^
    |
-   = note: ...which immediately requires computing type of `<impl at $DIR/issue-23305.rs:5:1: 5:21>` again
-note: cycle used when collecting item types in top-level module
-  --> $DIR/issue-23305.rs:1:1
-   |
-LL | / pub trait ToNbt<T> {
-LL | |     fn new(val: T) -> Self;
-LL | | }
-LL | |
-...  |
-LL | |
-LL | | fn main() {}
-   | |____________^
+   = note: replace `Self` with a different type
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0391`.

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

@@ -11,10 +11,11 @@ impl Tr for S where Self: Copy {} // OK
 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
-impl Tr for S<Self> {} //~ ERROR cycle detected
-impl Self {} //~ ERROR cycle detected
-impl S<Self> {} //~ ERROR cycle detected
+impl Tr for Self {} //~ ERROR `Self` is not valid in the self type of an impl block
+impl Tr for S<Self> {} //~ ERROR `Self` is not valid in the self type of an impl block
+impl Self {} //~ ERROR `Self` is not valid in the self type of an impl block
+impl S<Self> {} //~ ERROR `Self` is not valid in the self type of an impl block
+impl (Self, Self) {} //~ ERROR `Self` is not valid in the self type of an impl block
 impl Tr<Self::A> for S {} //~ ERROR cycle detected
 
 fn main() {}

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

@@ -1,86 +1,50 @@
-error[E0391]: cycle detected when computing type of `<impl at $DIR/resolve-self-in-impl.rs:14:1: 14:17>`
+error: `Self` is not valid in the self type of an impl block
   --> $DIR/resolve-self-in-impl.rs:14:13
    |
 LL | impl Tr for Self {}
    |             ^^^^
    |
-   = note: ...which immediately requires computing type of `<impl at $DIR/resolve-self-in-impl.rs:14:1: 14:17>` again
-note: cycle used when collecting item types in top-level module
-  --> $DIR/resolve-self-in-impl.rs:1:1
-   |
-LL | / #![feature(associated_type_defaults)]
-LL | |
-LL | | struct S<T = u8>(T);
-LL | | trait Tr<T = u8> {
-...  |
-LL | |
-LL | | fn main() {}
-   | |____________^
+   = note: replace `Self` with a different type
 
-error[E0391]: cycle detected when computing type of `<impl at $DIR/resolve-self-in-impl.rs:15:1: 15:20>`
+error: `Self` is not valid in the self type of an impl block
   --> $DIR/resolve-self-in-impl.rs:15:15
    |
 LL | impl Tr for S<Self> {}
    |               ^^^^
    |
-   = note: ...which immediately requires computing type of `<impl at $DIR/resolve-self-in-impl.rs:15:1: 15:20>` again
-note: cycle used when collecting item types in top-level module
-  --> $DIR/resolve-self-in-impl.rs:1:1
-   |
-LL | / #![feature(associated_type_defaults)]
-LL | |
-LL | | struct S<T = u8>(T);
-LL | | trait Tr<T = u8> {
-...  |
-LL | |
-LL | | fn main() {}
-   | |____________^
+   = note: replace `Self` with a different type
 
-error[E0391]: cycle detected when computing type of `<impl at $DIR/resolve-self-in-impl.rs:16:1: 16:10>`
+error: `Self` is not valid in the self type of an impl block
   --> $DIR/resolve-self-in-impl.rs:16:6
    |
 LL | impl Self {}
    |      ^^^^
    |
-   = note: ...which immediately requires computing type of `<impl at $DIR/resolve-self-in-impl.rs:16:1: 16:10>` again
-note: cycle used when collecting item types in top-level module
-  --> $DIR/resolve-self-in-impl.rs:1:1
-   |
-LL | / #![feature(associated_type_defaults)]
-LL | |
-LL | | struct S<T = u8>(T);
-LL | | trait Tr<T = u8> {
-...  |
-LL | |
-LL | | fn main() {}
-   | |____________^
+   = note: replace `Self` with a different type
 
-error[E0391]: cycle detected when computing type of `<impl at $DIR/resolve-self-in-impl.rs:17:1: 17:13>`
+error: `Self` is not valid in the self type of an impl block
   --> $DIR/resolve-self-in-impl.rs:17:8
    |
 LL | impl S<Self> {}
    |        ^^^^
    |
-   = note: ...which immediately requires computing type of `<impl at $DIR/resolve-self-in-impl.rs:17:1: 17:13>` again
-note: cycle used when collecting item types in top-level module
-  --> $DIR/resolve-self-in-impl.rs:1:1
+   = note: replace `Self` with a different type
+
+error: `Self` is not valid in the self type of an impl block
+  --> $DIR/resolve-self-in-impl.rs:18:7
    |
-LL | / #![feature(associated_type_defaults)]
-LL | |
-LL | | struct S<T = u8>(T);
-LL | | trait Tr<T = u8> {
-...  |
-LL | |
-LL | | fn main() {}
-   | |____________^
+LL | impl (Self, Self) {}
+   |       ^^^^  ^^^^
+   |
+   = note: replace `Self` with a different type
 
-error[E0391]: cycle detected when computing trait implemented by `<impl at $DIR/resolve-self-in-impl.rs:18:1: 18:23>`
-  --> $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:19:1: 19:23>`
+  --> $DIR/resolve-self-in-impl.rs:19:1
    |
 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: 18:23>` again
+   = note: ...which immediately requires computing trait implemented by `<impl at $DIR/resolve-self-in-impl.rs:19:1: 19:23>` again
 note: cycle used when collecting item types in top-level module
   --> $DIR/resolve-self-in-impl.rs:1:1
    |
@@ -93,6 +57,6 @@ LL | |
 LL | | fn main() {}
    | |____________^
 
-error: aborting due to 5 previous errors
+error: aborting due to 6 previous errors
 
 For more information about this error, try `rustc --explain E0391`.