Rollup merge of #103609 - BoxyUwU:fix_impl_self_cycle, r=compiler-errors

Emit a nicer error on `impl Self {`

currently it emits a "cycle detected error" but this PR makes it emit a more user friendly error specifically saying that `Self` is disallowed in that position. this is a pretty hacky fix so i dont expect this to be merged (I basically only made this PR because i wanted to see if CI passes)

r? ``@compiler-errors``

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: (),
+}