improve _ constants in item signature handling

compiler/rustc_middle/src/ty/context.rs

@@ -1210,11 +1210,25 @@ impl<'tcx> TyCtxt<'tcx> {
         self.mk_ty(Error(DelaySpanBugEmitted(())))
     }
 
-    /// Like `err` but for constants.
+    /// Like [TyCtxt::ty_error] but for constants.
     #[track_caller]
     pub fn const_error(self, ty: Ty<'tcx>) -> &'tcx Const<'tcx> {
-        self.sess
-            .delay_span_bug(DUMMY_SP, "ty::ConstKind::Error constructed but no error reported.");
+        self.const_error_with_message(
+            ty,
+            DUMMY_SP,
+            "ty::ConstKind::Error constructed but no error reported",
+        )
+    }
+
+    /// Like [TyCtxt::ty_error_with_message] but for constants.
+    #[track_caller]
+    pub fn const_error_with_message<S: Into<MultiSpan>>(
+        self,
+        ty: Ty<'tcx>,
+        span: S,
+        msg: &str,
+    ) -> &'tcx Const<'tcx> {
+        self.sess.delay_span_bug(span, msg);
         self.mk_const(ty::Const { val: ty::ConstKind::Error(DelaySpanBugEmitted(())), ty })
     }
 

compiler/rustc_privacy/src/lib.rs

@@ -1148,19 +1148,11 @@ impl<'tcx> Visitor<'tcx> for TypePrivacyVisitor<'tcx> {
                 if self.visit(ty).is_break() {
                     return;
                 }
+            } else {
+                // We don't do anything for const infers here.
             }
         } else {
-            let local_id = self.tcx.hir().local_def_id(inf.hir_id);
-            if let Some(did) = self.tcx.opt_const_param_of(local_id) {
-                if self.visit_def_id(did, "inferred", &"").is_break() {
-                    return;
-                }
-            }
-
-            // FIXME see above note for same issue.
-            if self.visit(rustc_typeck::hir_ty_to_ty(self.tcx, &inf.to_ty())).is_break() {
-                return;
-            }
+            bug!("visit_infer without typeck_results");
         }
         intravisit::walk_inf(self, inf);
     }

compiler/rustc_typeck/src/astconv/mod.rs

@@ -6,7 +6,7 @@ mod errors;
 mod generics;
 
 use crate::bounds::Bounds;
-use crate::collect::PlaceholderHirTyCollector;
+use crate::collect::HirPlaceholderCollector;
 use crate::errors::{
     AmbiguousLifetimeBound, MultipleRelaxedDefaultBounds, TraitObjectDeclaredWithNoTraits,
     TypeofReservedKeywordUsed, ValueOfAssociatedStructAlreadySpecified,
@@ -2478,7 +2478,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         debug!(?bound_vars);
 
         // We proactively collect all the inferred type params to emit a single error per fn def.
-        let mut visitor = PlaceholderHirTyCollector::default();
+        let mut visitor = HirPlaceholderCollector::default();
         for ty in decl.inputs {
             visitor.visit_ty(ty);
         }

compiler/rustc_typeck/src/collect.rs

@@ -112,9 +112,9 @@ pub struct ItemCtxt<'tcx> {
 ///////////////////////////////////////////////////////////////////////////
 
 #[derive(Default)]
-crate struct PlaceholderHirTyCollector(crate Vec<Span>);
+crate struct HirPlaceholderCollector(crate Vec<Span>);
 
-impl<'v> Visitor<'v> for PlaceholderHirTyCollector {
+impl<'v> Visitor<'v> for HirPlaceholderCollector {
     fn visit_ty(&mut self, t: &'v hir::Ty<'v>) {
         if let hir::TyKind::Infer = t.kind {
             self.0.push(t.span);
@@ -131,6 +131,12 @@ impl<'v> Visitor<'v> for PlaceholderHirTyCollector {
             _ => {}
         }
     }
+    fn visit_array_length(&mut self, length: &'v hir::ArrayLen) {
+        if let &hir::ArrayLen::Infer(_, span) = length {
+            self.0.push(span);
+        }
+        intravisit::walk_array_len(self, length)
+    }
 }
 
 struct CollectItemTypesVisitor<'tcx> {
@@ -175,7 +181,7 @@ crate fn placeholder_type_error<'tcx>(
         sugg.push((span, format!(", {}", type_name)));
     }
 
-    let mut err = bad_placeholder(tcx, "type", placeholder_types, kind);
+    let mut err = bad_placeholder(tcx, placeholder_types, kind);
 
     // Suggest, but only if it is not a function in const or static
     if suggest {
@@ -233,7 +239,7 @@ fn reject_placeholder_type_signatures_in_item<'tcx>(
         _ => return,
     };
 
-    let mut visitor = PlaceholderHirTyCollector::default();
+    let mut visitor = HirPlaceholderCollector::default();
     visitor.visit_item(item);
 
     placeholder_type_error(
@@ -311,7 +317,6 @@ impl<'tcx> Visitor<'tcx> for CollectItemTypesVisitor<'tcx> {
 
 fn bad_placeholder<'tcx>(
     tcx: TyCtxt<'tcx>,
-    placeholder_kind: &'static str,
     mut spans: Vec<Span>,
     kind: &'static str,
 ) -> rustc_errors::DiagnosticBuilder<'tcx> {
@@ -322,8 +327,7 @@ fn bad_placeholder<'tcx>(
         tcx.sess,
         spans.clone(),
         E0121,
-        "the {} placeholder `_` is not allowed within types on item signatures for {}",
-        placeholder_kind,
+        "the placeholder `_` is not allowed within types on item signatures for {}",
         kind
     );
     for span in spans {
@@ -381,7 +385,7 @@ impl<'tcx> AstConv<'tcx> for ItemCtxt<'tcx> {
     }
 
     fn ty_infer(&self, _: Option<&ty::GenericParamDef>, span: Span) -> Ty<'tcx> {
-        self.tcx().ty_error_with_message(span, "bad_placeholder_type")
+        self.tcx().ty_error_with_message(span, "bad placeholder type")
     }
 
     fn ct_infer(
@@ -390,13 +394,11 @@ impl<'tcx> AstConv<'tcx> for ItemCtxt<'tcx> {
         _: Option<&ty::GenericParamDef>,
         span: Span,
     ) -> &'tcx Const<'tcx> {
-        bad_placeholder(self.tcx(), "const", vec![span], "generic").emit();
-        // Typeck doesn't expect erased regions to be returned from `type_of`.
         let ty = self.tcx.fold_regions(ty, &mut false, |r, _| match r {
             ty::ReErased => self.tcx.lifetimes.re_static,
             _ => r,
         });
-        self.tcx().const_error(ty)
+        self.tcx().const_error_with_message(ty, span, "bad placeholder constant")
     }
 
     fn projected_ty_from_poly_trait_ref(
@@ -739,7 +741,7 @@ fn convert_item(tcx: TyCtxt<'_>, item_id: hir::ItemId) {
                 match item.kind {
                     hir::ForeignItemKind::Fn(..) => tcx.ensure().fn_sig(item.def_id),
                     hir::ForeignItemKind::Static(..) => {
-                        let mut visitor = PlaceholderHirTyCollector::default();
+                        let mut visitor = HirPlaceholderCollector::default();
                         visitor.visit_foreign_item(item);
                         placeholder_type_error(
                             tcx,
@@ -822,7 +824,7 @@ fn convert_item(tcx: TyCtxt<'_>, item_id: hir::ItemId) {
                 hir::ItemKind::Const(ty, ..) | hir::ItemKind::Static(ty, ..) => {
                     // (#75889): Account for `const C: dyn Fn() -> _ = "";`
                     if let hir::TyKind::TraitObject(..) = ty.kind {
-                        let mut visitor = PlaceholderHirTyCollector::default();
+                        let mut visitor = HirPlaceholderCollector::default();
                         visitor.visit_item(it);
                         placeholder_type_error(
                             tcx,
@@ -858,7 +860,7 @@ fn convert_trait_item(tcx: TyCtxt<'_>, trait_item_id: hir::TraitItemId) {
         hir::TraitItemKind::Const(..) => {
             tcx.ensure().type_of(trait_item_id.def_id);
             // Account for `const C: _;`.
-            let mut visitor = PlaceholderHirTyCollector::default();
+            let mut visitor = HirPlaceholderCollector::default();
             visitor.visit_trait_item(trait_item);
             placeholder_type_error(tcx, None, &[], visitor.0, false, None, "constant");
         }
@@ -867,7 +869,7 @@ fn convert_trait_item(tcx: TyCtxt<'_>, trait_item_id: hir::TraitItemId) {
             tcx.ensure().item_bounds(trait_item_id.def_id);
             tcx.ensure().type_of(trait_item_id.def_id);
             // Account for `type T = _;`.
-            let mut visitor = PlaceholderHirTyCollector::default();
+            let mut visitor = HirPlaceholderCollector::default();
             visitor.visit_trait_item(trait_item);
             placeholder_type_error(tcx, None, &[], visitor.0, false, None, "associated type");
         }
@@ -876,7 +878,7 @@ fn convert_trait_item(tcx: TyCtxt<'_>, trait_item_id: hir::TraitItemId) {
             tcx.ensure().item_bounds(trait_item_id.def_id);
             // #74612: Visit and try to find bad placeholders
             // even if there is no concrete type.
-            let mut visitor = PlaceholderHirTyCollector::default();
+            let mut visitor = HirPlaceholderCollector::default();
             visitor.visit_trait_item(trait_item);
 
             placeholder_type_error(tcx, None, &[], visitor.0, false, None, "associated type");
@@ -898,7 +900,7 @@ fn convert_impl_item(tcx: TyCtxt<'_>, impl_item_id: hir::ImplItemId) {
         }
         hir::ImplItemKind::TyAlias(_) => {
             // Account for `type T = _;`
-            let mut visitor = PlaceholderHirTyCollector::default();
+            let mut visitor = HirPlaceholderCollector::default();
             visitor.visit_impl_item(impl_item);
 
             placeholder_type_error(tcx, None, &[], visitor.0, false, None, "associated type");
@@ -1818,10 +1820,14 @@ fn are_suggestable_generic_args(generic_args: &[hir::GenericArg<'_>]) -> bool {
 /// Whether `ty` is a type with `_` placeholders that can be inferred. Used in diagnostics only to
 /// use inference to provide suggestions for the appropriate type if possible.
 fn is_suggestable_infer_ty(ty: &hir::Ty<'_>) -> bool {
+    debug!(?ty);
     use hir::TyKind::*;
     match &ty.kind {
         Infer => true,
-        Slice(ty) | Array(ty, _) => is_suggestable_infer_ty(ty),
+        Slice(ty) => is_suggestable_infer_ty(ty),
+        Array(ty, length) => {
+            is_suggestable_infer_ty(ty) || matches!(length, hir::ArrayLen::Infer(_, _))
+        }
         Tup(tys) => tys.iter().any(is_suggestable_infer_ty),
         Ptr(mut_ty) | Rptr(_, mut_ty) => is_suggestable_infer_ty(mut_ty.ty),
         OpaqueDef(_, generic_args) => are_suggestable_generic_args(generic_args),
@@ -1873,9 +1879,9 @@ fn fn_sig(tcx: TyCtxt<'_>, def_id: DefId) -> ty::PolyFnSig<'_> {
                     });
                     let fn_sig = ty::Binder::dummy(fn_sig);
 
-                    let mut visitor = PlaceholderHirTyCollector::default();
+                    let mut visitor = HirPlaceholderCollector::default();
                     visitor.visit_ty(ty);
-                    let mut diag = bad_placeholder(tcx, "type", visitor.0, "return type");
+                    let mut diag = bad_placeholder(tcx, visitor.0, "return type");
                     let ret_ty = fn_sig.skip_binder().output();
                     if !ret_ty.references_error() {
                         if !ret_ty.is_closure() {

compiler/rustc_typeck/src/collect/type_of.rs

@@ -20,9 +20,6 @@ use super::{bad_placeholder, is_suggestable_infer_ty};
 /// This should be called using the query `tcx.opt_const_param_of`.
 #[instrument(level = "debug", skip(tcx))]
 pub(super) fn opt_const_param_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Option<DefId> {
-    // FIXME(generic_arg_infer): allow for returning DefIds of inference of
-    // GenericArg::Infer below. This may require a change where GenericArg::Infer has some flag
-    // for const or type.
     use hir::*;
     let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
 
@@ -753,7 +750,7 @@ fn infer_placeholder_type<'a>(
             err.emit();
         }
         None => {
-            let mut diag = bad_placeholder(tcx, "type", vec![span], kind);
+            let mut diag = bad_placeholder(tcx, vec![span], kind);
 
             if !ty.references_error() {
                 let mut mk_nameable = MakeNameable::new(tcx);

src/test/ui/const-generics/generic_arg_infer/in-signature.rs

@@ -0,0 +1,61 @@
+#![crate_type = "rlib"]
+#![feature(generic_arg_infer)]
+
+struct Foo<const N: usize>;
+struct Bar<T, const N: usize>(T);
+
+fn arr_fn() -> [u8; _] {
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types
+    [0; 3]
+}
+
+fn ty_fn() -> Bar<i32, _> {
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types
+    Bar::<i32, 3>(0)
+}
+
+fn ty_fn_mixed() -> Bar<_, _> {
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types
+    Bar::<i32, 3>(0)
+}
+
+const ARR_CT: [u8; _] = [0; 3];
+//~^ ERROR the placeholder `_` is not allowed within types on item signatures for constants
+static ARR_STATIC: [u8; _] = [0; 3];
+//~^ ERROR the placeholder `_` is not allowed within types on item signatures for static variables
+const TY_CT: Bar<i32, _> = Bar::<i32, 3>(0);
+//~^ ERROR the placeholder `_` is not allowed within types on item signatures for constants
+static TY_STATIC: Bar<i32, _> = Bar::<i32, 3>(0);
+//~^ ERROR the placeholder `_` is not allowed within types on item signatures for static variables
+const TY_CT_MIXED: Bar<_, _> = Bar::<i32, 3>(0);
+//~^ ERROR the placeholder `_` is not allowed within types on item signatures for constants
+static TY_STATIC_MIXED: Bar<_, _> = Bar::<i32, 3>(0);
+//~^ ERROR the placeholder `_` is not allowed within types on item signatures for static variables
+trait ArrAssocConst {
+    const ARR: [u8; _];
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for constants
+}
+trait TyAssocConst {
+    const ARR: Bar<i32, _>;
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for constants
+}
+trait TyAssocConstMixed {
+    const ARR: Bar<_, _>;
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for constants
+}
+
+trait AssocTy {
+    type Assoc;
+}
+impl AssocTy for i8 {
+    type Assoc = [u8; _];
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for associated types
+}
+impl AssocTy for i16 {
+    type Assoc = Bar<i32, _>;
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for associated types
+}
+impl AssocTy for i32 {
+    type Assoc = Bar<_, _>;
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for associated types
+}