Refer to const params as "const params" and not "type params"

Author: estebank

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

@@ -10,7 +10,7 @@ use rustc_middle::hir::map::Map;
 use rustc_middle::infer::unify_key::ConstVariableOriginKind;
 use rustc_middle::ty::print::Print;
 use rustc_middle::ty::subst::{GenericArg, GenericArgKind};
-use rustc_middle::ty::{self, DefIdTree, InferConst, Ty, TyCtxt, TypeFoldable, TypeFolder};
+use rustc_middle::ty::{self, Const, DefIdTree, InferConst, Ty, TyCtxt, TypeFoldable, TypeFolder};
 use rustc_span::symbol::kw;
 use rustc_span::Span;
 use std::borrow::Cow;
@@ -305,6 +305,15 @@ pub enum UnderspecifiedArgKind {
     Const { is_parameter: bool },
 }
 
+impl UnderspecifiedArgKind {
+    fn descr(&self) -> &'static str {
+        match self {
+            Self::Type { .. } => "type",
+            Self::Const { .. } => "const",
+        }
+    }
+}
+
 impl InferenceDiagnosticsData {
     /// Generate a label for a generic argument which can't be inferred. When not
     /// much is known about the argument, `use_diag` may be used to describe the
@@ -587,6 +596,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             }
         }
 
+        let param_type = arg_data.kind.descr();
         let suffix = match local_visitor.found_node_ty {
             Some(ty) if ty.is_closure() => {
                 let substs =
@@ -625,15 +635,15 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             }
             Some(ty) if is_named_and_not_impl_trait(ty) && arg_data.name == "_" => {
                 let ty = ty_to_string(ty);
-                format!("the explicit type `{}`, with the type parameters specified", ty)
+                format!("the explicit type `{}`, with the {} parameters specified", ty, param_type)
             }
             Some(ty) if is_named_and_not_impl_trait(ty) && ty.to_string() != arg_data.name => {
                 let ty = ResolvedTypeParamEraser::new(self.tcx).fold_ty(ty);
                 let ty = ErrTypeParamEraser(self.tcx).fold_ty(ty);
                 let ty = ty_to_string(ty);
                 format!(
-                    "the explicit type `{}`, where the type parameter `{}` is specified",
-                    ty, arg_data.name,
+                    "the explicit type `{}`, where the {} parameter `{}` is specified",
+                    ty, param_type, arg_data.name,
                 )
             }
             _ => "a type".to_string(),
@@ -910,7 +920,12 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     }
 }
 
-/// Turn resolved type params into `[type error]` to signal we don't want to display them.
+/// Turn *resolved* type params into `[type error]` to signal we don't want to display them. After
+/// performing that replacement, we'll turn all remaining infer type params to use their name from
+/// their definition, and replace all the `[type error]`s back to being infer so they display in
+/// the output as `_`. If we didn't go through `[type error]`, we would either show all type params
+/// by their name *or* `_`, neither of which is desireable: we want to show all types that we could
+/// infer as `_` to reduce verbosity and avoid telling the user about unnecessary type annotations.
 struct ResolvedTypeParamEraser<'tcx> {
     tcx: TyCtxt<'tcx>,
     level: usize,
@@ -920,7 +935,18 @@ impl<'tcx> ResolvedTypeParamEraser<'tcx> {
     fn new(tcx: TyCtxt<'tcx>) -> Self {
         ResolvedTypeParamEraser { tcx, level: 0 }
     }
+
+    /// Replace not yet inferred const params with their def name.
+    fn replace_infers(&self, c: &'tcx Const<'tcx>, index: u32, name: Symbol) -> &'tcx Const<'tcx> {
+        match c.val {
+            ty::ConstKind::Infer(..) => {
+                self.tcx().mk_const_param(index, name, c.ty)
+            }
+            _ => c,
+        }
+    }
 }
+
 impl<'tcx> TypeFolder<'tcx> for ResolvedTypeParamEraser<'tcx> {
     fn tcx<'a>(&'a self) -> TyCtxt<'tcx> {
         self.tcx
@@ -940,29 +966,22 @@ impl<'tcx> TypeFolder<'tcx> for ResolvedTypeParamEraser<'tcx> {
                     .map(|(subst, param)| match &(subst.unpack(), &param.kind) {
                         (_, ty::GenericParamDefKind::Type { has_default: true, .. }) => subst,
                         (crate::infer::GenericArgKind::Const(c), _) => {
-                            match c.val {
-                                ty::ConstKind::Infer(..) => {
-                                    // Replace not yet inferred const params with their def name.
-                                    self.tcx().mk_const_param(param.index, param.name, c.ty).into()
-                                }
-                                _ => subst,
-                            }
+                            self.replace_infers(c, param.index, param.name).into()
                         }
                         _ => subst.super_fold_with(self),
                     })
                     .collect();
-                if self.level == 1
-                    || substs.iter().any(|subst| match subst.unpack() {
-                        ty::subst::GenericArgKind::Type(t) => match t.kind() {
-                            ty::Error(_) => false,
-                            _ => true,
-                        },
-                        // Account for `const` params here, otherwise `doesnt_infer.rs`
-                        // shows `_` instead of `Foo<{ _: u32 }>`
-                        ty::subst::GenericArgKind::Const(_) => true,
-                        _ => false,
-                    })
-                {
+                let should_keep = |subst: &GenericArg<'_>| match subst.unpack() {
+                    ty::subst::GenericArgKind::Type(t) => match t.kind() {
+                        ty::Error(_) => false,
+                        _ => true,
+                    },
+                    // Account for `const` params here, otherwise `doesnt_infer.rs`
+                    // shows `_` instead of `Foo<{ _: u32 }>`
+                    ty::subst::GenericArgKind::Const(_) => true,
+                    _ => false,
+                };
+                if self.level == 1 || substs.iter().any(should_keep) {
                     let substs = self.tcx().intern_substs(&substs[..]);
                     self.tcx().mk_ty(ty::Adt(def, substs))
                 } else {
@@ -986,18 +1005,9 @@ impl<'tcx> TypeFolder<'tcx> for ResolvedTypeParamEraser<'tcx> {
             | ty::Opaque(..)
             | ty::Projection(_)
             | ty::Never => t.super_fold_with(self),
-            ty::Array(ty, c) => {
-                self.tcx().mk_ty(ty::Array(
-                    self.fold_ty(ty),
-                    match c.val {
-                        ty::ConstKind::Infer(..) => {
-                            // Replace not yet inferred const params with their def name.
-                            self.tcx().mk_const_param(0, Symbol::intern("N"), c.ty).into()
-                        }
-                        _ => c,
-                    },
-                ))
-            }
+            ty::Array(ty, c) => self
+                .tcx()
+                .mk_ty(ty::Array(self.fold_ty(ty), self.replace_infers(c, 0, Symbol::intern("N")))),
             // We don't want to hide type params that haven't been resolved yet.
             // This would be the type that will be written out with the type param
             // name in the output.

src/test/ui/const-generics/defaults/doesnt_infer.stderr

@@ -4,7 +4,7 @@ error[E0282]: type annotations needed for `Foo<{_: u32}>`
 LL |     let foo = Foo::foo();
    |         ---   ^^^^^^^^ cannot infer the value of const parameter `N`
    |         |
-   |         consider giving `foo` the explicit type `Foo<N>`, where the type parameter `N` is specified
+   |         consider giving `foo` the explicit type `Foo<N>`, where the const parameter `N` is specified
 
 error: aborting due to previous error
 

src/test/ui/inference/issue-83606.stderr

@@ -4,7 +4,7 @@ error[E0282]: type annotations needed for `[usize; _]`
 LL |     let _ = foo("foo"); //<- Do not suggest `foo::<N>("foo");`!
    |         -   ^^^ cannot infer the value of const parameter `N` declared on the function `foo`
    |         |
-   |         consider giving this pattern the explicit type `[_; N]`, where the type parameter `N` is specified
+   |         consider giving this pattern the explicit type `[_; N]`, where the const parameter `N` is specified
 
 error: aborting due to previous error