Rollup merge of rust-lang#135044 - compiler-errors:better-infer-sugge…

…stions-in-const, r=oli-obk

Improve infer (`_`) suggestions in `const`s and `static`s

Fixes rust-lang#135010.

This PR does a few things to (imo) greatly improve the error message when users write something like `static FOO: [i32; _] = [1, 2, 3]`.

Firstly, it adapts the recovery code for when we encounter `_` in a const/static to work a bit more like `fn foo() -> _`, and removes the somewhat redundant query `diagnostic_only_typeck`.

Secondly, it changes the lowering for `[T; _]` to always lower under the `feature(generic_arg_infer)` logic to `ConstArgKind::Infer`. We still issue the feature error, so it's not doing anything *observable* on the good path, but it does mean that we no longer erroneously interpret `[T; _]`'s array length as a `_` **wildcard expression** (à la destructuring assignment, like `(_, y) = expr`).

Lastly it makes the suggestions verbose and fixes (well, suppresses) a bug with stashing and suggestions.

r? oli-obk

compiler/rustc_ast_lowering/src/lib.rs

@@ -2031,20 +2031,17 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn lower_array_length_to_const_arg(&mut self, c: &AnonConst) -> &'hir hir::ConstArg<'hir> {
         match c.value.kind {
             ExprKind::Underscore => {
-                if self.tcx.features().generic_arg_infer() {
-                    let ct_kind = hir::ConstArgKind::Infer(self.lower_span(c.value.span));
-                    self.arena
-                        .alloc(hir::ConstArg { hir_id: self.lower_node_id(c.id), kind: ct_kind })
-                } else {
+                if !self.tcx.features().generic_arg_infer() {
                     feature_err(
                         &self.tcx.sess,
                         sym::generic_arg_infer,
                         c.value.span,
                         fluent_generated::ast_lowering_underscore_array_length_unstable,
                     )
                     .stash(c.value.span, StashKey::UnderscoreForArrayLengths);
-                    self.lower_anon_const_to_const_arg(c)
                 }
+                let ct_kind = hir::ConstArgKind::Infer(self.lower_span(c.value.span));
+                self.arena.alloc(hir::ConstArg { hir_id: self.lower_node_id(c.id), kind: ct_kind })
             }
             _ => self.lower_anon_const_to_const_arg(c),
         }

compiler/rustc_hir/src/hir.rs

@@ -3387,7 +3387,7 @@ impl<'hir> FnRetTy<'hir> {
         }
     }
 
-    pub fn get_infer_ret_ty(&self) -> Option<&'hir Ty<'hir>> {
+    pub fn is_suggestable_infer_ty(&self) -> Option<&'hir Ty<'hir>> {
         if let Self::Return(ty) = self {
             if ty.is_suggestable_infer_ty() {
                 return Some(*ty);

compiler/rustc_hir_analysis/src/collect.rs

@@ -131,19 +131,25 @@ pub struct ItemCtxt<'tcx> {
 ///////////////////////////////////////////////////////////////////////////
 
 #[derive(Default)]
-pub(crate) struct HirPlaceholderCollector(pub(crate) Vec<Span>);
+pub(crate) struct HirPlaceholderCollector {
+    pub spans: Vec<Span>,
+    // If any of the spans points to a const infer var, then suppress any messages
+    // that may try to turn that const infer into a type parameter.
+    pub may_contain_const_infer: bool,
+}
 
 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);
+            self.spans.push(t.span);
         }
         intravisit::walk_ty(self, t)
     }
     fn visit_generic_arg(&mut self, generic_arg: &'v hir::GenericArg<'v>) {
         match generic_arg {
             hir::GenericArg::Infer(inf) => {
-                self.0.push(inf.span);
+                self.spans.push(inf.span);
+                self.may_contain_const_infer = true;
                 intravisit::walk_inf(self, inf);
             }
             hir::GenericArg::Type(t) => self.visit_ty(t),
@@ -152,7 +158,8 @@ impl<'v> Visitor<'v> for HirPlaceholderCollector {
     }
     fn visit_const_arg(&mut self, const_arg: &'v hir::ConstArg<'v>) {
         if let hir::ConstArgKind::Infer(span) = const_arg.kind {
-            self.0.push(span);
+            self.may_contain_const_infer = true;
+            self.spans.push(span);
         }
         intravisit::walk_const_arg(self, const_arg)
     }
@@ -277,8 +284,8 @@ fn reject_placeholder_type_signatures_in_item<'tcx>(
     placeholder_type_error(
         icx.lowerer(),
         Some(generics),
-        visitor.0,
-        suggest,
+        visitor.spans,
+        suggest && !visitor.may_contain_const_infer,
         None,
         item.kind.descr(),
     );
@@ -607,16 +614,16 @@ impl<'tcx> HirTyLowerer<'tcx> for ItemCtxt<'tcx> {
             hir::FnRetTy::DefaultReturn(..) => tcx.types.unit,
         };
 
-        if !(visitor.0.is_empty() && infer_replacements.is_empty()) {
+        if !(visitor.spans.is_empty() && infer_replacements.is_empty()) {
             // We check for the presence of
             // `ident_span` to not emit an error twice when we have `fn foo(_: fn() -> _)`.
 
             let mut diag = crate::collect::placeholder_type_error_diag(
                 self,
                 generics,
-                visitor.0,
+                visitor.spans,
                 infer_replacements.iter().map(|(s, _)| *s).collect(),
-                true,
+                !visitor.may_contain_const_infer,
                 hir_ty,
                 "function",
             );
@@ -712,7 +719,7 @@ fn lower_item(tcx: TyCtxt<'_>, item_id: hir::ItemId) {
                         placeholder_type_error(
                             icx.lowerer(),
                             None,
-                            visitor.0,
+                            visitor.spans,
                             false,
                             None,
                             "static variable",
@@ -780,7 +787,7 @@ fn lower_item(tcx: TyCtxt<'_>, item_id: hir::ItemId) {
                 placeholder_type_error(
                     icx.lowerer(),
                     None,
-                    visitor.0,
+                    visitor.spans,
                     false,
                     None,
                     it.kind.descr(),
@@ -822,7 +829,7 @@ fn lower_trait_item(tcx: TyCtxt<'_>, trait_item_id: hir::TraitItemId) {
                 placeholder_type_error(
                     icx.lowerer(),
                     None,
-                    visitor.0,
+                    visitor.spans,
                     false,
                     None,
                     "associated constant",
@@ -837,7 +844,14 @@ fn lower_trait_item(tcx: TyCtxt<'_>, trait_item_id: hir::TraitItemId) {
             // Account for `type T = _;`.
             let mut visitor = HirPlaceholderCollector::default();
             visitor.visit_trait_item(trait_item);
-            placeholder_type_error(icx.lowerer(), None, visitor.0, false, None, "associated type");
+            placeholder_type_error(
+                icx.lowerer(),
+                None,
+                visitor.spans,
+                false,
+                None,
+                "associated type",
+            );
         }
 
         hir::TraitItemKind::Type(_, None) => {
@@ -848,7 +862,14 @@ fn lower_trait_item(tcx: TyCtxt<'_>, trait_item_id: hir::TraitItemId) {
             let mut visitor = HirPlaceholderCollector::default();
             visitor.visit_trait_item(trait_item);
 
-            placeholder_type_error(icx.lowerer(), None, visitor.0, false, None, "associated type");
+            placeholder_type_error(
+                icx.lowerer(),
+                None,
+                visitor.spans,
+                false,
+                None,
+                "associated type",
+            );
         }
     };
 
@@ -872,7 +893,14 @@ fn lower_impl_item(tcx: TyCtxt<'_>, impl_item_id: hir::ImplItemId) {
             let mut visitor = HirPlaceholderCollector::default();
             visitor.visit_impl_item(impl_item);
 
-            placeholder_type_error(icx.lowerer(), None, visitor.0, false, None, "associated type");
+            placeholder_type_error(
+                icx.lowerer(),
+                None,
+                visitor.spans,
+                false,
+                None,
+                "associated type",
+            );
         }
         hir::ImplItemKind::Const(ty, _) => {
             // Account for `const T: _ = ..;`
@@ -882,7 +910,7 @@ fn lower_impl_item(tcx: TyCtxt<'_>, impl_item_id: hir::ImplItemId) {
                 placeholder_type_error(
                     icx.lowerer(),
                     None,
-                    visitor.0,
+                    visitor.spans,
                     false,
                     None,
                     "associated constant",
@@ -1371,7 +1399,7 @@ fn lower_fn_sig_recovering_infer_ret_ty<'tcx>(
     generics: &'tcx hir::Generics<'tcx>,
     def_id: LocalDefId,
 ) -> ty::PolyFnSig<'tcx> {
-    if let Some(infer_ret_ty) = sig.decl.output.get_infer_ret_ty() {
+    if let Some(infer_ret_ty) = sig.decl.output.is_suggestable_infer_ty() {
         return recover_infer_ret_ty(icx, infer_ret_ty, generics, def_id);
     }
 
@@ -1422,7 +1450,7 @@ fn recover_infer_ret_ty<'tcx>(
     let mut visitor = HirPlaceholderCollector::default();
     visitor.visit_ty(infer_ret_ty);
 
-    let mut diag = bad_placeholder(icx.lowerer(), visitor.0, "return type");
+    let mut diag = bad_placeholder(icx.lowerer(), visitor.spans, "return type");
     let ret_ty = fn_sig.output();
 
     // Don't leak types into signatures unless they're nameable!

compiler/rustc_hir_analysis/src/collect/type_of.rs

@@ -4,6 +4,7 @@ use rustc_errors::{Applicability, StashKey, Suggestions};
 use rustc_hir as hir;
 use rustc_hir::HirId;
 use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_hir::intravisit::Visitor;
 use rustc_middle::query::plumbing::CyclePlaceholder;
 use rustc_middle::ty::fold::fold_regions;
 use rustc_middle::ty::print::with_forced_trimmed_paths;
@@ -12,7 +13,7 @@ use rustc_middle::ty::{self, Article, IsSuggestable, Ty, TyCtxt, TypeVisitableEx
 use rustc_middle::{bug, span_bug};
 use rustc_span::{DUMMY_SP, Ident, Span};
 
-use super::{ItemCtxt, bad_placeholder};
+use super::{HirPlaceholderCollector, ItemCtxt, bad_placeholder};
 use crate::errors::TypeofReservedKeywordUsed;
 use crate::hir_ty_lowering::HirTyLowerer;
 
@@ -412,7 +413,7 @@ fn infer_placeholder_type<'tcx>(
     kind: &'static str,
 ) -> Ty<'tcx> {
     let tcx = cx.tcx();
-    let ty = tcx.diagnostic_only_typeck(def_id).node_type(body_id.hir_id);
+    let ty = tcx.typeck(def_id).node_type(body_id.hir_id);
 
     // If this came from a free `const` or `static mut?` item,
     // then the user may have written e.g. `const A = 42;`.
@@ -447,13 +448,37 @@ fn infer_placeholder_type<'tcx>(
             }
         })
         .unwrap_or_else(|| {
-            let mut diag = bad_placeholder(cx, vec![span], kind);
-
-            if !ty.references_error() {
+            let mut visitor = HirPlaceholderCollector::default();
+            let node = tcx.hir_node_by_def_id(def_id);
+            if let Some(ty) = node.ty() {
+                visitor.visit_ty(ty);
+            }
+            // If we have just one span, let's try to steal a const `_` feature error.
+            let try_steal_span = if !tcx.features().generic_arg_infer() && visitor.spans.len() == 1
+            {
+                visitor.spans.first().copied()
+            } else {
+                None
+            };
+            // If we didn't find any infer tys, then just fallback to `span`.
+            if visitor.spans.is_empty() {
+                visitor.spans.push(span);
+            }
+            let mut diag = bad_placeholder(cx, visitor.spans, kind);
+
+            // HACK(#69396): Stashing and stealing diagnostics does not interact
+            // well with macros which may delay more than one diagnostic on the
+            // same span. If this happens, we will fall through to this arm, so
+            // we need to suppress the suggestion since it's invalid. Ideally we
+            // would suppress the duplicated error too, but that's really hard.
+            if span.is_empty() && span.from_expansion() {
+                // An approximately better primary message + no suggestion...
+                diag.primary_message("missing type for item");
+            } else if !ty.references_error() {
                 if let Some(ty) = ty.make_suggestable(tcx, false, None) {
-                    diag.span_suggestion(
+                    diag.span_suggestion_verbose(
                         span,
-                        "replace with the correct type",
+                        "replace this with a fully-specified type",
                         ty,
                         Applicability::MachineApplicable,
                     );
@@ -464,7 +489,16 @@ fn infer_placeholder_type<'tcx>(
                     ));
                 }
             }
-            diag.emit()
+
+            if let Some(try_steal_span) = try_steal_span {
+                cx.dcx().try_steal_replace_and_emit_err(
+                    try_steal_span,
+                    StashKey::UnderscoreForArrayLengths,
+                    diag,
+                )
+            } else {
+                diag.emit()
+            }
         });
     Ty::new_error(tcx, guar)
 }

compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs

@@ -187,7 +187,7 @@ impl TaitConstraintLocator<'_> {
             "foreign items cannot constrain opaque types",
         );
         if let Some(hir_sig) = hir_node.fn_sig()
-            && hir_sig.decl.output.get_infer_ret_ty().is_some()
+            && hir_sig.decl.output.is_suggestable_infer_ty().is_some()
         {
             let guar = self.tcx.dcx().span_delayed_bug(
                 hir_sig.decl.output.span(),

compiler/rustc_hir_typeck/src/expr.rs

@@ -1771,12 +1771,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let parent_node = self.tcx.hir().parent_iter(expr.hir_id).find(|(_, node)| {
             !matches!(node, hir::Node::Expr(hir::Expr { kind: hir::ExprKind::AddrOf(..), .. }))
         });
-        let Some((
-            _,
-            hir::Node::LetStmt(hir::LetStmt { ty: Some(ty), .. })
-            | hir::Node::Item(hir::Item { kind: hir::ItemKind::Const(ty, _, _), .. }),
-        )) = parent_node
-        else {
+        let Some((_, hir::Node::LetStmt(hir::LetStmt { ty: Some(ty), .. }))) = parent_node else {
             return;
         };
         if let hir::TyKind::Array(_, ct) = ty.peel_refs().kind {

compiler/rustc_hir_typeck/src/lib.rs

@@ -87,21 +87,7 @@ fn used_trait_imports(tcx: TyCtxt<'_>, def_id: LocalDefId) -> &UnordSet<LocalDef
 }
 
 fn typeck<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> &'tcx ty::TypeckResults<'tcx> {
-    let fallback = move || tcx.type_of(def_id.to_def_id()).instantiate_identity();
-    typeck_with_fallback(tcx, def_id, fallback, None)
-}
-
-/// Used only to get `TypeckResults` for type inference during error recovery.
-/// Currently only used for type inference of `static`s and `const`s to avoid type cycle errors.
-fn diagnostic_only_typeck<'tcx>(
-    tcx: TyCtxt<'tcx>,
-    def_id: LocalDefId,
-) -> &'tcx ty::TypeckResults<'tcx> {
-    let fallback = move || {
-        let span = tcx.hir().span(tcx.local_def_id_to_hir_id(def_id));
-        Ty::new_error_with_message(tcx, span, "diagnostic only typeck table used")
-    };
-    typeck_with_fallback(tcx, def_id, fallback, None)
+    typeck_with_fallback(tcx, def_id, None)
 }
 
 /// Same as `typeck` but `inspect` is invoked on evaluation of each root obligation.
@@ -113,15 +99,13 @@ pub fn inspect_typeck<'tcx>(
     def_id: LocalDefId,
     inspect: ObligationInspector<'tcx>,
 ) -> &'tcx ty::TypeckResults<'tcx> {
-    let fallback = move || tcx.type_of(def_id.to_def_id()).instantiate_identity();
-    typeck_with_fallback(tcx, def_id, fallback, Some(inspect))
+    typeck_with_fallback(tcx, def_id, Some(inspect))
 }
 
-#[instrument(level = "debug", skip(tcx, fallback, inspector), ret)]
+#[instrument(level = "debug", skip(tcx, inspector), ret)]
 fn typeck_with_fallback<'tcx>(
     tcx: TyCtxt<'tcx>,
     def_id: LocalDefId,
-    fallback: impl Fn() -> Ty<'tcx> + 'tcx,
     inspector: Option<ObligationInspector<'tcx>>,
 ) -> &'tcx ty::TypeckResults<'tcx> {
     // Closures' typeck results come from their outermost function,
@@ -150,7 +134,11 @@ fn typeck_with_fallback<'tcx>(
     let mut fcx = FnCtxt::new(&root_ctxt, param_env, def_id);
 
     if let Some(hir::FnSig { header, decl, .. }) = node.fn_sig() {
-        let fn_sig = if decl.output.get_infer_ret_ty().is_some() {
+        let fn_sig = if decl.output.is_suggestable_infer_ty().is_some() {
+            // In the case that we're recovering `fn() -> W<_>` or some other return
+            // type that has an infer in it, lower the type directly so that it'll
+            // be correctly filled with infer. We'll use this inference to provide
+            // a suggestion later on.
             fcx.lowerer().lower_fn_ty(id, header.safety, header.abi, decl, None, None)
         } else {
             tcx.fn_sig(def_id).instantiate_identity()
@@ -164,8 +152,19 @@ fn typeck_with_fallback<'tcx>(
 
         check_fn(&mut fcx, fn_sig, None, decl, def_id, body, tcx.features().unsized_fn_params());
     } else {
-        let expected_type = infer_type_if_missing(&fcx, node);
-        let expected_type = expected_type.unwrap_or_else(fallback);
+        let expected_type = if let Some(infer_ty) = infer_type_if_missing(&fcx, node) {
+            infer_ty
+        } else if let Some(ty) = node.ty()
+            && ty.is_suggestable_infer_ty()
+        {
+            // In the case that we're recovering `const X: [T; _]` or some other
+            // type that has an infer in it, lower the type directly so that it'll
+            // be correctly filled with infer. We'll use this inference to provide
+            // a suggestion later on.
+            fcx.lowerer().lower_ty(ty)
+        } else {
+            tcx.type_of(def_id).instantiate_identity()
+        };
 
         let expected_type = fcx.normalize(body.value.span, expected_type);
 
@@ -506,5 +505,5 @@ fn fatally_break_rust(tcx: TyCtxt<'_>, span: Span) -> ! {
 
 pub fn provide(providers: &mut Providers) {
     method::provide(providers);
-    *providers = Providers { typeck, diagnostic_only_typeck, used_trait_imports, ..*providers };
+    *providers = Providers { typeck, used_trait_imports, ..*providers };
 }