Don't suggest unnameable generic arguments

compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs

@@ -1,4 +1,4 @@
-use crate::infer::type_variable::TypeVariableOriginKind;
+use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use crate::infer::InferCtxt;
 use rustc_errors::{pluralize, struct_span_err, Applicability, DiagnosticBuilder, ErrorGuaranteed};
 use rustc_hir as hir;
@@ -8,12 +8,12 @@ use rustc_hir::def_id::DefId;
 use rustc_hir::intravisit::{self, Visitor};
 use rustc_hir::{Body, Closure, Expr, ExprKind, FnRetTy, HirId, Local, LocalSource};
 use rustc_middle::hir::nested_filter;
-use rustc_middle::infer::unify_key::ConstVariableOriginKind;
+use rustc_middle::infer::unify_key::{ConstVariableOrigin, ConstVariableOriginKind};
 use rustc_middle::ty::adjustment::{Adjust, Adjustment, AutoBorrow, AutoBorrowMutability};
 use rustc_middle::ty::print::{FmtPrinter, PrettyPrinter, Print, Printer};
 use rustc_middle::ty::subst::{GenericArg, GenericArgKind, Subst, SubstsRef};
 use rustc_middle::ty::{self, DefIdTree, InferConst};
-use rustc_middle::ty::{Ty, TyCtxt, TypeckResults};
+use rustc_middle::ty::{IsSuggestable, Ty, TyCtxt, TypeckResults};
 use rustc_span::symbol::{kw, Ident};
 use rustc_span::{BytePos, Span};
 use std::borrow::Cow;
@@ -407,11 +407,40 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
 
                 err.span_label(span, cannot_infer_msg);
 
-                let printer = fmt_printer(self, Namespace::TypeNS);
-                let args = printer.comma_sep(generic_args.iter().copied()).unwrap().into_buffer();
+                let args = fmt_printer(self, Namespace::TypeNS)
+                    .comma_sep(generic_args.iter().copied().map(|arg| {
+                        if arg.is_suggestable(self.tcx, true) {
+                            return arg;
+                        }
+
+                        match arg.unpack() {
+                            GenericArgKind::Lifetime(_) => bug!("unexpected lifetime"),
+                            GenericArgKind::Type(_) => self
+                                .next_ty_var(TypeVariableOrigin {
+                                    span: rustc_span::DUMMY_SP,
+                                    kind: TypeVariableOriginKind::MiscVariable,
+                                })
+                                .into(),
+                            GenericArgKind::Const(arg) => self
+                                .next_const_var(
+                                    arg.ty(),
+                                    ConstVariableOrigin {
+                                        span: rustc_span::DUMMY_SP,
+                                        kind: ConstVariableOriginKind::MiscVariable,
+                                    },
+                                )
+                                .into(),
+                        }
+                    }))
+                    .unwrap()
+                    .into_buffer();
+
                 err.span_suggestion_verbose(
                     insert_span,
-                    &format!("consider specifying the generic argument{}", pluralize!(args.len()),),
+                    &format!(
+                        "consider specifying the generic argument{}",
+                        pluralize!(generic_args.len()),
+                    ),
                     format!("::<{}>", args),
                     Applicability::HasPlaceholders,
                 );

compiler/rustc_middle/src/ty/diagnostics.rs

@@ -3,7 +3,7 @@
 use std::ops::ControlFlow;
 
 use crate::ty::{
-    visit::TypeVisitable, Const, ConstKind, DefIdTree, ExistentialPredicate, InferTy,
+    visit::TypeVisitable, Const, ConstKind, DefIdTree, ExistentialPredicate, InferConst, InferTy,
     PolyTraitPredicate, Ty, TyCtxt, TypeSuperVisitable, TypeVisitor,
 };
 
@@ -82,15 +82,18 @@ pub trait IsSuggestable<'tcx> {
     /// meaningful rendered suggestions when pretty-printed. We leave some
     /// nonsense, such as region vars, since those render as `'_` and are
     /// usually okay to reinterpret as elided lifetimes.
-    fn is_suggestable(self, tcx: TyCtxt<'tcx>) -> bool;
+    ///
+    /// Only if `infer_suggestable` is true, we consider type and const
+    /// inference variables to be suggestable.
+    fn is_suggestable(self, tcx: TyCtxt<'tcx>, infer_suggestable: bool) -> bool;
 }
 
 impl<'tcx, T> IsSuggestable<'tcx> for T
 where
     T: TypeVisitable<'tcx>,
 {
-    fn is_suggestable(self, tcx: TyCtxt<'tcx>) -> bool {
-        self.visit_with(&mut IsSuggestableVisitor { tcx }).is_continue()
+    fn is_suggestable(self, tcx: TyCtxt<'tcx>, infer_suggestable: bool) -> bool {
+        self.visit_with(&mut IsSuggestableVisitor { tcx, infer_suggestable }).is_continue()
     }
 }
 
@@ -100,7 +103,7 @@ pub fn suggest_arbitrary_trait_bound<'tcx>(
     err: &mut Diagnostic,
     trait_pred: PolyTraitPredicate<'tcx>,
 ) -> bool {
-    if !trait_pred.is_suggestable(tcx) {
+    if !trait_pred.is_suggestable(tcx, false) {
         return false;
     }
 
@@ -419,13 +422,16 @@ impl<'v> hir::intravisit::Visitor<'v> for StaticLifetimeVisitor<'v> {
 
 pub struct IsSuggestableVisitor<'tcx> {
     tcx: TyCtxt<'tcx>,
+    infer_suggestable: bool,
 }
 
 impl<'tcx> TypeVisitor<'tcx> for IsSuggestableVisitor<'tcx> {
     type BreakTy = ();
 
     fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
         match t.kind() {
+            Infer(InferTy::TyVar(_)) if self.infer_suggestable => {}
+
             FnDef(..)
             | Closure(..)
             | Infer(..)
@@ -479,6 +485,8 @@ impl<'tcx> TypeVisitor<'tcx> for IsSuggestableVisitor<'tcx> {
 
     fn visit_const(&mut self, c: Const<'tcx>) -> ControlFlow<Self::BreakTy> {
         match c.kind() {
+            ConstKind::Infer(InferConst::Var(_)) if self.infer_suggestable => {}
+
             ConstKind::Infer(..)
             | ConstKind::Bound(..)
             | ConstKind::Placeholder(..)

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -378,7 +378,7 @@ fn suggest_restriction<'tcx>(
             replace_ty: ty::ParamTy::new(generics.count() as u32, Symbol::intern(&type_param_name))
                 .to_ty(tcx),
         });
-        if !trait_pred.is_suggestable(tcx) {
+        if !trait_pred.is_suggestable(tcx, false) {
             return;
         }
         // We know we have an `impl Trait` that doesn't satisfy a required projection.
@@ -417,7 +417,7 @@ fn suggest_restriction<'tcx>(
             Applicability::MaybeIncorrect,
         );
     } else {
-        if !trait_pred.is_suggestable(tcx) {
+        if !trait_pred.is_suggestable(tcx, false) {
             return;
         }
         // Trivial case: `T` needs an extra bound: `T: Bound`.
@@ -586,7 +586,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                     // else in the predicate.
                     if !trait_pred.skip_binder().trait_ref.substs[1..]
                         .iter()
-                        .all(|g| g.is_suggestable(self.tcx))
+                        .all(|g| g.is_suggestable(self.tcx, false))
                     {
                         return;
                     }

compiler/rustc_typeck/src/astconv/generics.rs

@@ -86,7 +86,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                         let param_type = tcx.infer_ctxt().enter(|infcx| {
                             infcx.resolve_numeric_literals_with_default(tcx.type_of(param.def_id))
                         });
-                        if param_type.is_suggestable(tcx) {
+                        if param_type.is_suggestable(tcx, false) {
                             err.span_suggestion(
                                 tcx.def_span(src_def_id),
                                 "consider changing this type parameter to be a `const` generic",

compiler/rustc_typeck/src/astconv/mod.rs

@@ -2676,7 +2676,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                     span,
                     ty,
                     opt_sugg: Some((span, Applicability::MachineApplicable))
-                        .filter(|_| ty.is_suggestable(tcx)),
+                        .filter(|_| ty.is_suggestable(tcx, false)),
                 });
 
                 ty

compiler/rustc_typeck/src/check/fn_ctxt/checks.rs

@@ -1070,7 +1070,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     let (_, expected_ty) = formal_and_expected_inputs[expected_idx];
                     if expected_ty.is_unit() {
                         "()".to_string()
-                    } else if expected_ty.is_suggestable(tcx) {
+                    } else if expected_ty.is_suggestable(tcx, false) {
                         format!("/* {} */", expected_ty)
                     } else {
                         "/* value */".to_string()

compiler/rustc_typeck/src/check/fn_ctxt/suggestions.rs

@@ -507,7 +507,12 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             self.resolve_numeric_literals_with_default(self.resolve_vars_if_possible(found));
         // Only suggest changing the return type for methods that
         // haven't set a return type at all (and aren't `fn main()` or an impl).
-        match (&fn_decl.output, found.is_suggestable(self.tcx), can_suggest, expected.is_unit()) {
+        match (
+            &fn_decl.output,
+            found.is_suggestable(self.tcx, false),
+            can_suggest,
+            expected.is_unit(),
+        ) {
             (&hir::FnRetTy::DefaultReturn(span), true, true, true) => {
                 err.subdiagnostic(AddReturnTypeSuggestion::Add { span, found });
                 true

compiler/rustc_typeck/src/collect.rs

@@ -1929,7 +1929,7 @@ fn infer_return_ty_for_fn_sig<'tcx>(
             visitor.visit_ty(ty);
             let mut diag = bad_placeholder(tcx, visitor.0, "return type");
             let ret_ty = fn_sig.skip_binder().output();
-            if ret_ty.is_suggestable(tcx) {
+            if ret_ty.is_suggestable(tcx, false) {
                 diag.span_suggestion(
                     ty.span,
                     "replace with the correct return type",
@@ -1938,7 +1938,12 @@ fn infer_return_ty_for_fn_sig<'tcx>(
                 );
             } else if matches!(ret_ty.kind(), ty::FnDef(..)) {
                 let fn_sig = ret_ty.fn_sig(tcx);
-                if fn_sig.skip_binder().inputs_and_output.iter().all(|t| t.is_suggestable(tcx)) {
+                if fn_sig
+                    .skip_binder()
+                    .inputs_and_output
+                    .iter()
+                    .all(|t| t.is_suggestable(tcx, false))
+                {
                     diag.span_suggestion(
                         ty.span,
                         "replace with the correct return type",

src/test/ui/closures/issue-99565.rs

@@ -0,0 +1,7 @@
+#![crate_type = "lib"]
+
+fn foo<T, U>(_: U) {}
+
+fn bar() {
+    foo(|| {}); //~ ERROR type annotations needed
+}

src/test/ui/closures/issue-99565.stderr

@@ -0,0 +1,14 @@
+error[E0282]: type annotations needed
+  --> $DIR/issue-99565.rs:6:5
+   |
+LL |     foo(|| {});
+   |     ^^^ cannot infer type of the type parameter `T` declared on the function `foo`
+   |
+help: consider specifying the generic arguments
+   |
+LL |     foo::<T, _>(|| {});
+   |        ++++++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.

src/test/ui/issues/issue-23041.stderr

@@ -4,7 +4,7 @@ error[E0282]: type annotations needed
 LL |     b.downcast_ref::<fn(_)->_>();
    |       ^^^^^^^^^^^^ cannot infer type of the type parameter `T` declared on the associated function `downcast_ref`
    |
-help: consider specifying the generic arguments
+help: consider specifying the generic argument
    |
 LL |     b.downcast_ref::<fn(_) -> _>();
    |                   ~~~~~~~~~~~~~~

src/test/ui/issues/issue-24013.stderr

@@ -4,7 +4,7 @@ error[E0282]: type annotations needed
 LL |     unsafe {swap::<&mut _>(transmute(&a), transmute(&b))};
    |             ^^^^^^^^^^^^^^ cannot infer type of the type parameter `T` declared on the function `swap`
    |
-help: consider specifying the generic arguments
+help: consider specifying the generic argument
    |
 LL |     unsafe {swap::<&mut _>(transmute(&a), transmute(&b))};
    |                 ~~~~~~~~~~