Add an `ErrorGuaranteed` to `ast::TyKind::Err`.

This makes it more like `hir::TyKind::Err`, and avoids a
`span_delayed_bug` call in `LoweringContext::lower_ty_direct`.

It also requires adding `ast::TyKind::Dummy`, now that
`ast::TyKind::Err` can't be used for that purpose in the absence of an
error emission.

There are a couple of cases that aren't as neat as I would have liked,
marked with `FIXME` comments.

14 files changed, 75 insertions, 36 deletions

diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs
index 098e2606a3b..08fc8e3c6cc 100644
--- a/compiler/rustc_ast/src/ast.rs
+++ b/compiler/rustc_ast/src/ast.rs
@@ -2136,10 +2136,12 @@ pub enum TyKind {
     ImplicitSelf,
     /// A macro in the type position.
     MacCall(P<MacCall>),
-    /// Placeholder for a kind that has failed to be defined.
-    Err,
     /// Placeholder for a `va_list`.
     CVarArgs,
+    /// Sometimes we need a dummy value when no error has occurred.
+    Dummy,
+    /// Placeholder for a kind that has failed to be defined.
+    Err(ErrorGuaranteed),
 }
 
 impl TyKind {
diff --git a/compiler/rustc_ast/src/mut_visit.rs b/compiler/rustc_ast/src/mut_visit.rs
index d482ada170e..c42c4199973 100644
--- a/compiler/rustc_ast/src/mut_visit.rs
+++ b/compiler/rustc_ast/src/mut_visit.rs
@@ -481,7 +481,12 @@ pub fn noop_visit_ty<T: MutVisitor>(ty: &mut P<Ty>, vis: &mut T) {
     let Ty { id, kind, span, tokens } = ty.deref_mut();
     vis.visit_id(id);
     match kind {
-        TyKind::Infer | TyKind::ImplicitSelf | TyKind::Err | TyKind::Never | TyKind::CVarArgs => {}
+        TyKind::Infer
+        | TyKind::ImplicitSelf
+        | TyKind::Err(_)
+        | TyKind::Dummy
+        | TyKind::Never
+        | TyKind::CVarArgs => {}
         TyKind::Slice(ty) => vis.visit_ty(ty),
         TyKind::Ptr(mt) => vis.visit_mt(mt),
         TyKind::Ref(lt, mt) => {
@@ -1649,7 +1654,7 @@ impl DummyAstNode for Ty {
     fn dummy() -> Self {
         Ty {
             id: DUMMY_NODE_ID,
-            kind: TyKind::Err,
+            kind: TyKind::Dummy,
             span: Default::default(),
             tokens: Default::default(),
         }
diff --git a/compiler/rustc_ast/src/visit.rs b/compiler/rustc_ast/src/visit.rs
index 4aaaa0ba424..83f6746bdeb 100644
--- a/compiler/rustc_ast/src/visit.rs
+++ b/compiler/rustc_ast/src/visit.rs
@@ -447,7 +447,7 @@ pub fn walk_ty<'a, V: Visitor<'a>>(visitor: &mut V, typ: &'a Ty) {
             walk_list!(visitor, visit_param_bound, bounds, BoundKind::Impl);
         }
         TyKind::Typeof(expression) => visitor.visit_anon_const(expression),
-        TyKind::Infer | TyKind::ImplicitSelf | TyKind::Err => {}
+        TyKind::Infer | TyKind::ImplicitSelf | TyKind::Dummy | TyKind::Err(_) => {}
         TyKind::MacCall(mac) => visitor.visit_mac_call(mac),
         TyKind::Never | TyKind::CVarArgs => {}
         TyKind::AnonStruct(_, ref fields) | TyKind::AnonUnion(_, ref fields) => {
diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index 6b5fc014240..c1b7af94b02 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -1285,7 +1285,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn lower_ty_direct(&mut self, t: &Ty, itctx: ImplTraitContext) -> hir::Ty<'hir> {
         let kind = match &t.kind {
             TyKind::Infer => hir::TyKind::Infer,
-            TyKind::Err => hir::TyKind::Err(self.dcx().has_errors().unwrap()),
+            TyKind::Err(guar) => hir::TyKind::Err(*guar),
             // Lower the anonymous structs or unions in a nested lowering context.
             //
             // ```
@@ -1503,6 +1503,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 );
                 hir::TyKind::Err(guar)
             }
+            TyKind::Dummy => panic!("`TyKind::Dummy` should never be lowered"),
         };
 
         hir::Ty { kind, span: self.lower_span(t.span), hir_id: self.lower_node_id(t.id) }
diff --git a/compiler/rustc_ast_passes/src/ast_validation.rs b/compiler/rustc_ast_passes/src/ast_validation.rs
index ebc5c17a4c4..fa0f5326196 100644
--- a/compiler/rustc_ast_passes/src/ast_validation.rs
+++ b/compiler/rustc_ast_passes/src/ast_validation.rs
@@ -881,7 +881,8 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                         &item.vis,
                         errors::VisibilityNotPermittedNote::TraitImpl,
                     );
-                    if let TyKind::Err = self_ty.kind {
+                    // njn: use Dummy here
+                    if let TyKind::Err(_) = self_ty.kind {
                         this.dcx().emit_err(errors::ObsoleteAuto { span: item.span });
                     }
                     if let (&Unsafe::Yes(span), &ImplPolarity::Negative(sp)) = (unsafety, polarity)
diff --git a/compiler/rustc_ast_pretty/src/pprust/state.rs b/compiler/rustc_ast_pretty/src/pprust/state.rs
index cda746894e8..480506272d2 100644
--- a/compiler/rustc_ast_pretty/src/pprust/state.rs
+++ b/compiler/rustc_ast_pretty/src/pprust/state.rs
@@ -1048,11 +1048,16 @@ impl<'a> State<'a> {
             ast::TyKind::Infer => {
                 self.word("_");
             }
-            ast::TyKind::Err => {
+            ast::TyKind::Err(_) => {
                 self.popen();
                 self.word("/*ERROR*/");
                 self.pclose();
             }
+            ast::TyKind::Dummy => {
+                self.popen();
+                self.word("/*DUMMY*/");
+                self.pclose();
+            }
             ast::TyKind::ImplicitSelf => {
                 self.word("Self");
             }
diff --git a/compiler/rustc_expand/src/base.rs b/compiler/rustc_expand/src/base.rs
index cfeb31fc4c8..20cfc72f6d3 100644
--- a/compiler/rustc_expand/src/base.rs
+++ b/compiler/rustc_expand/src/base.rs
@@ -567,10 +567,13 @@ impl DummyResult {
     }
 
     /// A plain dummy type.
-    pub fn raw_ty(sp: Span, is_error: bool) -> P<ast::Ty> {
+    pub fn raw_ty(sp: Span) -> P<ast::Ty> {
+        // FIXME(nnethercote): you might expect `ast::TyKind::Dummy` to be used here, but some
+        // values produced here end up being lowered to HIR, which `ast::TyKind::Dummy` does not
+        // support, so we use an empty tuple instead.
         P(ast::Ty {
             id: ast::DUMMY_NODE_ID,
-            kind: if is_error { ast::TyKind::Err } else { ast::TyKind::Tup(ThinVec::new()) },
+            kind: ast::TyKind::Tup(ThinVec::new()),
             span: sp,
             tokens: None,
         })
@@ -611,7 +614,7 @@ impl MacResult for DummyResult {
     }
 
     fn make_ty(self: Box<DummyResult>) -> Option<P<ast::Ty>> {
-        Some(DummyResult::raw_ty(self.span, self.is_error))
+        Some(DummyResult::raw_ty(self.span))
     }
 
     fn make_arms(self: Box<DummyResult>) -> Option<SmallVec<[ast::Arm; 1]>> {
diff --git a/compiler/rustc_parse/src/parser/diagnostics.rs b/compiler/rustc_parse/src/parser/diagnostics.rs
index 445d5b2ce79..69518287f38 100644
--- a/compiler/rustc_parse/src/parser/diagnostics.rs
+++ b/compiler/rustc_parse/src/parser/diagnostics.rs
@@ -46,14 +46,14 @@ use std::ops::{Deref, DerefMut};
 use thin_vec::{thin_vec, ThinVec};
 
 /// Creates a placeholder argument.
-pub(super) fn dummy_arg(ident: Ident) -> Param {
+pub(super) fn dummy_arg(ident: Ident, guar: ErrorGuaranteed) -> Param {
     let pat = P(Pat {
         id: ast::DUMMY_NODE_ID,
         kind: PatKind::Ident(BindingAnnotation::NONE, ident, None),
         span: ident.span,
         tokens: None,
     });
-    let ty = Ty { kind: TyKind::Err, span: ident.span, id: ast::DUMMY_NODE_ID, tokens: None };
+    let ty = Ty { kind: TyKind::Err(guar), span: ident.span, id: ast::DUMMY_NODE_ID, tokens: None };
     Param {
         attrs: AttrVec::default(),
         id: ast::DUMMY_NODE_ID,
@@ -1540,14 +1540,14 @@ impl<'a> Parser<'a> {
     pub(super) fn maybe_recover_from_question_mark(&mut self, ty: P<Ty>) -> P<Ty> {
         if self.token == token::Question {
             self.bump();
-            self.dcx().emit_err(QuestionMarkInType {
+            let guar = self.dcx().emit_err(QuestionMarkInType {
                 span: self.prev_token.span,
                 sugg: QuestionMarkInTypeSugg {
                     left: ty.span.shrink_to_lo(),
                     right: self.prev_token.span,
                 },
             });
-            self.mk_ty(ty.span.to(self.prev_token.span), TyKind::Err)
+            self.mk_ty(ty.span.to(self.prev_token.span), TyKind::Err(guar))
         } else {
             ty
         }
@@ -2304,8 +2304,8 @@ impl<'a> Parser<'a> {
 
     pub(super) fn recover_bad_self_param(&mut self, mut param: Param) -> PResult<'a, Param> {
         let span = param.pat.span;
-        param.ty.kind = TyKind::Err;
-        self.dcx().emit_err(SelfParamNotFirst { span });
+        let guar = self.dcx().emit_err(SelfParamNotFirst { span });
+        param.ty.kind = TyKind::Err(guar);
         Ok(param)
     }
 
@@ -2437,7 +2437,7 @@ impl<'a> Parser<'a> {
     pub(super) fn deduplicate_recovered_params_names(&self, fn_inputs: &mut ThinVec<Param>) {
         let mut seen_inputs = FxHashSet::default();
         for input in fn_inputs.iter_mut() {
-            let opt_ident = if let (PatKind::Ident(_, ident, _), TyKind::Err) =
+            let opt_ident = if let (PatKind::Ident(_, ident, _), TyKind::Err(_)) =
                 (&input.pat.kind, &input.ty.kind)
             {
                 Some(*ident)
@@ -2644,8 +2644,10 @@ impl<'a> Parser<'a> {
                         "::",
                         Applicability::MaybeIncorrect,
                     );
-                    err.emit();
-                    return Ok(GenericArg::Type(self.mk_ty(start.to(expr.span), TyKind::Err)));
+                    let guar = err.emit();
+                    return Ok(GenericArg::Type(
+                        self.mk_ty(start.to(expr.span), TyKind::Err(guar)),
+                    ));
                 } else if token::Comma == self.token.kind || self.token.kind.should_end_const_arg()
                 {
                     // Avoid the following output by checking that we consumed a full const arg:
diff --git a/compiler/rustc_parse/src/parser/item.rs b/compiler/rustc_parse/src/parser/item.rs
index 8050b34956c..a0605b8a44c 100644
--- a/compiler/rustc_parse/src/parser/item.rs
+++ b/compiler/rustc_parse/src/parser/item.rs
@@ -591,7 +591,23 @@ impl<'a> Parser<'a> {
         let ty_second = if self.token == token::DotDot {
             // We need to report this error after `cfg` expansion for compatibility reasons
             self.bump(); // `..`, do not add it to expected tokens
-            Some(self.mk_ty(self.prev_token.span, TyKind::Err))
+
+            // FIXME(nnethercote): AST validation later detects this
+            // `TyKind::Err` and emits an errors. So why the unchecked
+            // ErrorGuaranteed?
+            // - A `span_delayed_bug` doesn't work here, because rustfmt can
+            //   hit this path but then not hit the follow-up path in the AST
+            //   validator that issues the error, which results in ICEs.
+            // - `TyKind::Dummy` doesn't work, because it ends up reaching HIR
+            //   lowering, which results in ICEs. Changing `TyKind::Dummy` to
+            //   `TyKind::Err` during AST validation might fix that, but that's
+            //   not possible because AST validation doesn't allow mutability.
+            //
+            // #121072 will hopefully remove all this special handling of the
+            // obsolete `impl Trait for ..` and then this can go away.
+            #[allow(deprecated)]
+            let guar = rustc_errors::ErrorGuaranteed::unchecked_error_guaranteed();
+            Some(self.mk_ty(self.prev_token.span, TyKind::Err(guar)))
         } else if has_for || self.token.can_begin_type() {
             Some(self.parse_ty()?)
         } else {
@@ -2628,13 +2644,13 @@ impl<'a> Parser<'a> {
             p.recover_diff_marker();
             let snapshot = p.create_snapshot_for_diagnostic();
             let param = p.parse_param_general(req_name, first_param).or_else(|e| {
-                e.emit();
+                let guar = e.emit();
                 let lo = p.prev_token.span;
                 p.restore_snapshot(snapshot);
                 // Skip every token until next possible arg or end.
                 p.eat_to_tokens(&[&token::Comma, &token::CloseDelim(Delimiter::Parenthesis)]);
                 // Create a placeholder argument for proper arg count (issue #34264).
-                Ok(dummy_arg(Ident::new(kw::Empty, lo.to(p.prev_token.span))))
+                Ok(dummy_arg(Ident::new(kw::Empty, lo.to(p.prev_token.span)), guar))
             });
             // ...now that we've parsed the first argument, `self` is no longer allowed.
             first_param = false;
@@ -2671,8 +2687,8 @@ impl<'a> Parser<'a> {
                     return if let Some(ident) =
                         this.parameter_without_type(&mut err, pat, is_name_required, first_param)
                     {
-                        err.emit();
-                        Ok((dummy_arg(ident), TrailingToken::None))
+                        let guar = err.emit();
+                        Ok((dummy_arg(ident, guar), TrailingToken::None))
                     } else {
                         Err(err)
                     };
diff --git a/compiler/rustc_parse/src/parser/path.rs b/compiler/rustc_parse/src/parser/path.rs
index e7cad74b4dd..681039999a6 100644
--- a/compiler/rustc_parse/src/parser/path.rs
+++ b/compiler/rustc_parse/src/parser/path.rs
@@ -678,8 +678,9 @@ impl<'a> Parser<'a> {
                 c.into()
             }
             Some(GenericArg::Lifetime(lt)) => {
-                self.dcx().emit_err(errors::AssocLifetime { span, lifetime: lt.ident.span });
-                self.mk_ty(span, ast::TyKind::Err).into()
+                let guar =
+                    self.dcx().emit_err(errors::AssocLifetime { span, lifetime: lt.ident.span });
+                self.mk_ty(span, ast::TyKind::Err(guar)).into()
             }
             None => {
                 let after_eq = eq.shrink_to_hi();
@@ -779,7 +780,7 @@ impl<'a> Parser<'a> {
                     // type to determine if error recovery has occurred and if the input is not a
                     // syntactically valid type after all.
                     if let ast::TyKind::Slice(inner_ty) | ast::TyKind::Array(inner_ty, _) = &ty.kind
-                        && let ast::TyKind::Err = inner_ty.kind
+                        && let ast::TyKind::Err(_) = inner_ty.kind
                         && let Some(snapshot) = snapshot
                         && let Some(expr) =
                             self.recover_unbraced_const_arg_that_can_begin_ty(snapshot)
diff --git a/compiler/rustc_parse/src/parser/ty.rs b/compiler/rustc_parse/src/parser/ty.rs
index 157fb9e505a..f79f2a813b2 100644
--- a/compiler/rustc_parse/src/parser/ty.rs
+++ b/compiler/rustc_parse/src/parser/ty.rs
@@ -346,8 +346,10 @@ impl<'a> Parser<'a> {
                 AllowCVariadic::No => {
                     // FIXME(Centril): Should we just allow `...` syntactically
                     // anywhere in a type and use semantic restrictions instead?
-                    self.dcx().emit_err(NestedCVariadicType { span: lo.to(self.prev_token.span) });
-                    TyKind::Err
+                    let guar = self
+                        .dcx()
+                        .emit_err(NestedCVariadicType { span: lo.to(self.prev_token.span) });
+                    TyKind::Err(guar)
                 }
             }
         } else {
@@ -493,8 +495,8 @@ impl<'a> Parser<'a> {
             {
                 // Recover from `[LIT; EXPR]` and `[LIT]`
                 self.bump();
-                err.emit();
-                self.mk_ty(self.prev_token.span, TyKind::Err)
+                let guar = err.emit();
+                self.mk_ty(self.prev_token.span, TyKind::Err(guar))
             }
             Err(err) => return Err(err),
         };
diff --git a/compiler/rustc_passes/src/hir_stats.rs b/compiler/rustc_passes/src/hir_stats.rs
index d02e86dd456..96429bb7788 100644
--- a/compiler/rustc_passes/src/hir_stats.rs
+++ b/compiler/rustc_passes/src/hir_stats.rs
@@ -616,8 +616,9 @@ impl<'v> ast_visit::Visitor<'v> for StatCollector<'v> {
                 Infer,
                 ImplicitSelf,
                 MacCall,
-                Err,
-                CVarArgs
+                CVarArgs,
+                Dummy,
+                Err
             ]
         );
 
diff --git a/src/tools/clippy/clippy_utils/src/ast_utils.rs b/src/tools/clippy/clippy_utils/src/ast_utils.rs
index adc35bd82ae..0467a8a6570 100644
--- a/src/tools/clippy/clippy_utils/src/ast_utils.rs
+++ b/src/tools/clippy/clippy_utils/src/ast_utils.rs
@@ -690,7 +690,7 @@ pub fn eq_ty(l: &Ty, r: &Ty) -> bool {
     match (&l.kind, &r.kind) {
         (Paren(l), _) => eq_ty(l, r),
         (_, Paren(r)) => eq_ty(l, r),
-        (Never, Never) | (Infer, Infer) | (ImplicitSelf, ImplicitSelf) | (Err, Err) | (CVarArgs, CVarArgs) => true,
+        (Never, Never) | (Infer, Infer) | (ImplicitSelf, ImplicitSelf) | (Err(_), Err(_)) | (CVarArgs, CVarArgs) => true,
         (Slice(l), Slice(r)) => eq_ty(l, r),
         (Array(le, ls), Array(re, rs)) => eq_ty(le, re) && eq_expr(&ls.value, &rs.value),
         (Ptr(l), Ptr(r)) => l.mutbl == r.mutbl && eq_ty(&l.ty, &r.ty),
diff --git a/src/tools/rustfmt/src/types.rs b/src/tools/rustfmt/src/types.rs
index 4cd8e6a703e..7f220a456a8 100644
--- a/src/tools/rustfmt/src/types.rs
+++ b/src/tools/rustfmt/src/types.rs
@@ -859,7 +859,7 @@ impl Rewrite for ast::Ty {
                 })
             }
             ast::TyKind::CVarArgs => Some("...".to_owned()),
-            ast::TyKind::Err => Some(context.snippet(self.span).to_owned()),
+            ast::TyKind::Dummy | ast::TyKind::Err(_) => Some(context.snippet(self.span).to_owned()),
             ast::TyKind::Typeof(ref anon_const) => rewrite_call(
                 context,
                 "typeof",