E0191 suggestion correction, inserts turbofish without dyn (#91997)

compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs

@@ -12,9 +12,9 @@
 use rustc_errors::{
     codes::*, pluralize, struct_span_code_err, Applicability, Diag, ErrorGuaranteed,
 };
-use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_hir::{self as hir, Node};
 use rustc_middle::bug;
 use rustc_middle::query::Key;
 use rustc_middle::ty::print::{PrintPolyTraitRefExt as _, PrintTraitRefExt as _};
@@ -745,7 +745,15 @@ pub(crate) fn complain_about_missing_assoc_tys(
         if object_safety_violations {
             return;
         }
+
+        // related to issue #91997, turbofishes added only when in an expr or pat
+        let mut in_expr_or_pat = false;
         if let ([], [bound]) = (&potential_assoc_types[..], &trait_bounds) {
+            let grandparent = tcx.parent_hir_node(tcx.parent_hir_id(bound.trait_ref.hir_ref_id));
+            in_expr_or_pat = match grandparent {
+                Node::Expr(_) | Node::Pat(_) => true,
+                _ => false,
+            };
             match bound.trait_ref.path.segments {
                 // FIXME: `trait_ref.path.span` can point to a full path with multiple
                 // segments, even though `trait_ref.path.segments` is of length `1`. Work
@@ -901,6 +909,10 @@ pub(crate) fn complain_about_missing_assoc_tys(
                     // `Trait<'a, Item = Type>` while accounting for the `<'a>` in the
                     // suggestion.
                     format!("{}, {}>", &snippet[..snippet.len() - 1], types.join(", "))
+                } else if in_expr_or_pat {
+                    // The user wrote `Iterator`, so we don't have a type we can suggest, but at
+                    // least we can clue them to the correct syntax `Iterator::<Item = Type>`.
+                    format!("{}::<{}>", snippet, types.join(", "))
                 } else {
                     // The user wrote `Iterator`, so we don't have a type we can suggest, but at
                     // least we can clue them to the correct syntax `Iterator<Item = Type>`.

tests/ui/errors/dynless-turbofish-e0191-issue-91997.rs

@@ -0,0 +1,8 @@
+trait MyIterator : Iterator {}
+
+fn main() {
+    let _ = MyIterator::next;
+}
+//~^^ ERROR the value of the associated type `Item` in `Iterator` must be specified [E0191]
+//~| WARN trait objects without an explicit `dyn` are deprecated [bare_trait_objects]
+//~| WARN this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2021!

tests/ui/errors/dynless-turbofish-e0191-issue-91997.stderr

@@ -0,0 +1,23 @@
+warning: trait objects without an explicit `dyn` are deprecated
+  --> $DIR/dynless-turbofish-e0191-issue-91997.rs:4:13
+   |
+LL |     let _ = MyIterator::next;
+   |             ^^^^^^^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2021!
+   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/warnings-promoted-to-error.html>
+   = note: `#[warn(bare_trait_objects)]` on by default
+help: if this is an object-safe trait, use `dyn`
+   |
+LL |     let _ = <dyn MyIterator>::next;
+   |             ++++           +
+
+error[E0191]: the value of the associated type `Item` in `Iterator` must be specified
+  --> $DIR/dynless-turbofish-e0191-issue-91997.rs:4:13
+   |
+LL |     let _ = MyIterator::next;
+   |             ^^^^^^^^^^ help: specify the associated type: `MyIterator::<Item = Type>`
+
+error: aborting due to 1 previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0191`.

tests/ui/issues/issue-23024.stderr

@@ -23,7 +23,7 @@ error[E0191]: the value of the associated type `Output` in `FnOnce` must be spec
   --> $DIR/issue-23024.rs:8:39
    |
 LL |     println!("{:?}",(vfnfer[0] as dyn Fn)(3));
-   |                                       ^^ help: specify the associated type: `Fn<Output = Type>`
+   |                                       ^^ help: specify the associated type: `Fn::<Output = Type>`
 
 error: aborting due to 3 previous errors
 

tests/ui/issues/issue-28344.stderr

@@ -16,7 +16,7 @@ error[E0191]: the value of the associated type `Output` in `BitXor` must be spec
   --> $DIR/issue-28344.rs:4:17
    |
 LL |     let x: u8 = BitXor::bitor(0 as u8, 0 as u8);
-   |                 ^^^^^^ help: specify the associated type: `BitXor<Output = Type>`
+   |                 ^^^^^^ help: specify the associated type: `BitXor::<Output = Type>`
 
 error[E0599]: no function or associated item named `bitor` found for trait object `dyn BitXor<_>` in the current scope
   --> $DIR/issue-28344.rs:4:25
@@ -44,7 +44,7 @@ error[E0191]: the value of the associated type `Output` in `BitXor` must be spec
   --> $DIR/issue-28344.rs:10:13
    |
 LL |     let g = BitXor::bitor;
-   |             ^^^^^^ help: specify the associated type: `BitXor<Output = Type>`
+   |             ^^^^^^ help: specify the associated type: `BitXor::<Output = Type>`
 
 error[E0599]: no function or associated item named `bitor` found for trait object `dyn BitXor<_>` in the current scope
   --> $DIR/issue-28344.rs:10:21