Mention the type in the label, to avoid confusion at the cost of redundancy

library/core/src/iter/range.rs

@@ -23,7 +23,7 @@ unsafe_impl_trusted_step![AsciiChar char i8 i16 i32 i64 i128 isize u8 u16 u32 u6
 #[rustc_diagnostic_item = "range_step"]
 #[rustc_on_unimplemented(
     message = "`std::ops::Range<{Self}>` is not an iterator",
-    label = "not an iterator",
+    label = "`Range<{Self}>` is not an iterator",
     note = "`Range` only implements `Iterator` for select types in the standard library, \
             particularly integers; to see the full list of types, see the documentation for the \
             unstable `Step` trait"

tests/run-make/missing-unstable-trait-bound/missing-bound.stderr

@@ -2,7 +2,7 @@ error[E0277]: `std::ops::Range<T>` is not an iterator
  --> missing-bound.rs:2:14
   |
 2 |     for _ in t {}
-  |              ^ not an iterator
+  |              ^ `Range<T>` is not an iterator
   |
   = note: `Range` only implements `Iterator` for select types in the standard library, particularly integers; to see the full list of types, see the documentation for the unstable `Step` trait
   = note: required for `std::ops::Range<T>` to implement `Iterator`

tests/ui/range/range-1.stderr

@@ -8,7 +8,7 @@ error[E0277]: `std::ops::Range<bool>` is not an iterator
   --> $DIR/range-1.rs:9:14
    |
 LL |     for i in false..true {}
-   |              ^^^^^^^^^^^ not an iterator
+   |              ^^^^^^^^^^^ `Range<bool>` is not an iterator
    |
    = help: the nightly-only, unstable trait `Step` is not implemented for `bool`
    = note: `Range` only implements `Iterator` for select types in the standard library, particularly integers; to see the full list of types, see the documentation for the unstable `Step` trait

tests/ui/trait-bounds/unstable-trait-suggestion.stderr

@@ -20,7 +20,7 @@ error[E0277]: `std::ops::Range<T>` is not an iterator
   --> $DIR/unstable-trait-suggestion.rs:17:14
    |
 LL |     for _ in t {}
-   |              ^ not an iterator
+   |              ^ `Range<T>` is not an iterator
    |
    = note: `Range` only implements `Iterator` for select types in the standard library, particularly integers; to see the full list of types, see the documentation for the unstable `Step` trait
    = note: required for `std::ops::Range<T>` to implement `Iterator`