Mention Range when Step trait bound is unmet

Author: estebank

library/core/src/iter/range.rs

@@ -21,6 +21,11 @@ unsafe_impl_trusted_step![AsciiChar char i8 i16 i32 i64 i128 isize u8 u16 u32 u6
 /// The *successor* operation moves towards values that compare greater.
 /// The *predecessor* operation moves towards values that compare lesser.
 #[rustc_diagnostic_item = "range_step"]
+#[rustc_on_unimplemented(
+    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"
+)]
 #[unstable(feature = "step_trait", issue = "42168")]
 pub trait Step: Clone + PartialOrd + Sized {
     /// Returns the bounds on the number of *successor* steps required to get from `start` to `end`

tests/ui/range/range-1.stderr

@@ -10,6 +10,7 @@ error[E0277]: the trait bound `bool: Step` is not satisfied
 LL |     for i in false..true {}
    |              ^^^^^^^^^^^ 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
    = note: required for `std::ops::Range<bool>` to implement `Iterator`
    = note: required for `std::ops::Range<bool>` to implement `IntoIterator`
 

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

@@ -22,6 +22,7 @@ error[E0277]: the trait bound `T: Step` is not satisfied
 LL |     for _ in t {}
    |              ^ the nightly-only, unstable trait `Step` is not implemented for `T`
    |
+   = 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`
    = note: required for `std::ops::Range<T>` to implement `IntoIterator`
 help: consider restricting type parameter `T` with unstable trait `Step`