In "specify type" suggestion, skip type params that are already known

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

@@ -18,6 +18,8 @@ use rustc_middle::ty::{
     TypeFoldable, TypeFolder, TypeSuperFoldable, TypeckResults,
 };
 use rustc_span::{BytePos, DUMMY_SP, FileName, Ident, Span, sym};
+use rustc_type_ir::inherent::*;
+use rustc_type_ir::visit::TypeVisitableExt;
 use tracing::{debug, instrument, warn};
 
 use super::nice_region_error::placeholder_error::Highlighted;
@@ -155,27 +157,92 @@ impl UnderspecifiedArgKind {
     }
 }
 
-struct ClosureEraser<'tcx> {
-    tcx: TyCtxt<'tcx>,
+struct ClosureEraser<'a, 'tcx> {
+    infcx: &'a InferCtxt<'tcx>,
 }
 
-impl<'tcx> TypeFolder<TyCtxt<'tcx>> for ClosureEraser<'tcx> {
+impl<'a, 'tcx> ClosureEraser<'a, 'tcx> {
+    fn new_infer(&mut self) -> Ty<'tcx> {
+        self.infcx.next_ty_var(DUMMY_SP)
+    }
+}
+
+impl<'a, 'tcx> TypeFolder<TyCtxt<'tcx>> for ClosureEraser<'a, 'tcx> {
     fn cx(&self) -> TyCtxt<'tcx> {
-        self.tcx
+        self.infcx.tcx
     }
 
     fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
         match ty.kind() {
             ty::Closure(_, args) => {
+                // For a closure type, we turn it into a function pointer so that it gets rendered
+                // as `fn(args) -> Ret`.
                 let closure_sig = args.as_closure().sig();
                 Ty::new_fn_ptr(
-                    self.tcx,
-                    self.tcx.signature_unclosure(closure_sig, hir::Safety::Safe),
+                    self.cx(),
+                    self.cx().signature_unclosure(closure_sig, hir::Safety::Safe),
                 )
             }
-            _ => ty.super_fold_with(self),
+            ty::Adt(_, args) if !args.iter().any(|a| a.has_infer()) => {
+                // We have a type that doesn't have any inference variables, so we replace
+                // the whole thing with `_`. The type system already knows about this type in
+                // its entirety and it is redundant to specify it for the user. The user only
+                // needs to specify the type parameters that we *couldn't* figure out.
+                self.new_infer()
+            }
+            ty::Adt(def, args) => {
+                let generics = self.cx().generics_of(def.did());
+                let generics: Vec<bool> = generics
+                    .own_params
+                    .iter()
+                    .map(|param| param.default_value(self.cx()).is_some())
+                    .collect();
+                let ty = Ty::new_adt(
+                    self.cx(),
+                    *def,
+                    self.cx().mk_args_from_iter(generics.into_iter().zip(args.iter()).map(
+                        |(has_default, arg)| {
+                            if arg.has_infer() {
+                                // This param has an unsubstituted type variable, meaning that this
+                                // type has a (potentially deeply nested) type parameter from the
+                                // corresponding type's definition. We have explicitly asked this
+                                // type to not be hidden. In either case, we keep the type and don't
+                                // substitute with `_` just yet.
+                                arg.fold_with(self)
+                            } else if has_default {
+                                // We have a type param that has a default type, like the allocator
+                                // in Vec. We decided to show `Vec` itself, because it hasn't yet
+                                // been replaced by an `_` `Infer`, but we want to ensure that the
+                                // type parameter with default types does *not* get replaced with
+                                // `_` because then we'd end up with `Vec<_, _>`, instead of
+                                // `Vec<_>`.
+                                arg
+                            } else if let GenericArgKind::Type(_) = arg.kind() {
+                                // We don't replace lifetime or const params, only type params.
+                                self.new_infer().into()
+                            } else {
+                                arg.fold_with(self)
+                            }
+                        },
+                    )),
+                );
+                ty
+            }
+            _ if ty.has_infer() => {
+                // This type has a (potentially nested) type parameter that we couldn't figure out.
+                // We will print this depth of type, so at least the type name and at least one of
+                // its type parameters.
+                ty.super_fold_with(self)
+            }
+            // We don't have an unknown type parameter anywhere, replace with `_`.
+            _ => self.new_infer(),
         }
     }
+
+    fn fold_const(&mut self, c: ty::Const<'tcx>) -> ty::Const<'tcx> {
+        // Avoid accidentally erasing the type of the const.
+        c
+    }
 }
 
 fn fmt_printer<'a, 'tcx>(infcx: &'a InferCtxt<'tcx>, ns: Namespace) -> FmtPrinter<'a, 'tcx> {
@@ -219,9 +286,9 @@ fn ty_to_string<'tcx>(
 ) -> String {
     let mut printer = fmt_printer(infcx, Namespace::TypeNS);
     let ty = infcx.resolve_vars_if_possible(ty);
-    // We use `fn` ptr syntax for closures, but this only works when the closure
-    // does not capture anything.
-    let ty = ty.fold_with(&mut ClosureEraser { tcx: infcx.tcx });
+    // We use `fn` ptr syntax for closures, but this only works when the closure does not capture
+    // anything. We also remove all type parameters that are fully known to the type system.
+    let ty = ty.fold_with(&mut ClosureEraser { infcx });
 
     match (ty.kind(), called_method_def_id) {
         // We don't want the regular output for `fn`s because it includes its path in

tests/ui/inference/cannot-infer-closure-circular.stderr

@@ -9,8 +9,8 @@ LL |         Ok(v)
    |
 help: consider giving this closure parameter an explicit type, where the type for type parameter `E` is specified
    |
-LL |     let x = |r: Result<(), E>| {
-   |               +++++++++++++++
+LL |     let x = |r: Result<_, E>| {
+   |               ++++++++++++++
 
 error: aborting due to 1 previous error
 

tests/ui/inference/erase-type-params-in-label.stderr

@@ -12,8 +12,8 @@ LL | fn foo<T, K, W: Default, Z: Default>(t: T, k: K) -> Foo<T, K, W, Z> {
    |                 ^^^^^^^ required by this bound in `foo`
 help: consider giving `foo` an explicit type, where the type for type parameter `W` is specified
    |
-LL |     let foo: Foo<i32, &str, W, Z> = foo(1, "");
-   |            ++++++++++++++++++++++
+LL |     let foo: Foo<_, &_, W, Z> = foo(1, "");
+   |            ++++++++++++++++++
 
 error[E0283]: type annotations needed for `Bar<i32, &str, _>`
   --> $DIR/erase-type-params-in-label.rs:5:9
@@ -29,8 +29,8 @@ LL | fn bar<T, K, Z: Default>(t: T, k: K) -> Bar<T, K, Z> {
    |                 ^^^^^^^ required by this bound in `bar`
 help: consider giving `bar` an explicit type, where the type for type parameter `Z` is specified
    |
-LL |     let bar: Bar<i32, &str, Z> = bar(1, "");
-   |            +++++++++++++++++++
+LL |     let bar: Bar<_, &_, Z> = bar(1, "");
+   |            +++++++++++++++
 
 error: aborting due to 2 previous errors
 

tests/ui/inference/issue-104649.stderr

@@ -6,8 +6,8 @@ LL |     let a = A(Result::Ok(Result::Ok(())));
    |
 help: consider giving `a` an explicit type, where the type for type parameter `E` is specified
    |
-LL |     let a: A<std::result::Result<std::result::Result<(), E>, Error>> = A(Result::Ok(Result::Ok(())));
-   |          +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+LL |     let a: A<std::result::Result<std::result::Result<_, E>, _>> = A(Result::Ok(Result::Ok(())));
+   |          ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 
 error: aborting due to 1 previous error
 

tests/ui/inference/issue-83606.stderr

@@ -11,8 +11,8 @@ LL | fn foo<const N: usize>(_: impl std::fmt::Display) -> [usize; N] {
    |        ^^^^^^^^^^^^^^ required by this const generic parameter in `foo`
 help: consider giving this pattern a type, where the value of const parameter `N` is specified
    |
-LL |     let _: [usize; N] = foo("foo");
-   |          ++++++++++++
+LL |     let _: [_; N] = foo("foo");
+   |          ++++++++
 
 error: aborting due to 1 previous error
 

tests/ui/inference/really-long-type-in-let-binding-without-sufficient-type-info.rs

@@ -0,0 +1,10 @@
+type A = (i32, i32, i32, i32);
+type B = (A, A, A, A);
+type C = (B, B, B, B);
+type D = (C, C, C, C);
+
+fn foo(x: D) {
+    let y = Err(x); //~ ERROR type annotations needed for `Result<_
+}
+
+fn main() {}

tests/ui/inference/really-long-type-in-let-binding-without-sufficient-type-info.stderr

@@ -0,0 +1,14 @@
+error[E0282]: type annotations needed for `Result<_, ((((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32))), (((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32))), (((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32))), (((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32)), ((i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32), (i32, i32, i32, i32))))>`
+  --> $DIR/really-long-type-in-let-binding-without-sufficient-type-info.rs:7:9
+   |
+LL |     let y = Err(x);
+   |         ^   ------ type must be known at this point
+   |
+help: consider giving `y` an explicit type, where the type for type parameter `T` is specified
+   |
+LL |     let y: Result<T, _> = Err(x);
+   |          ++++++++++++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0282`.

tests/ui/type-inference/or_else-multiple-type-params.stderr

@@ -6,8 +6,8 @@ LL |         .or_else(|err| {
    |
 help: try giving this closure an explicit return type
    |
-LL |         .or_else(|err| -> Result<Child, F> {
-   |                        +++++++++++++++++++
+LL |         .or_else(|err| -> Result<_, F> {
+   |                        +++++++++++++++
 
 error: aborting due to 1 previous error