Adding a hint on iterator type errors

library/core/src/iter/traits/iterator.rs

@@ -58,6 +58,11 @@ fn _assert_is_object_safe(_: &dyn Iterator<Item = ()>) {}
         note = "if you want to iterate between `start` until a value `end`, use the exclusive range \
               syntax `start..end` or the inclusive range syntax `start..=end`"
     ),
+    on(
+        _Self = "{float}",
+        note = "if you want to iterate between `start` until a value `end`, use the exclusive range \
+              syntax `start..end` or the inclusive range syntax `start..=end`"
+    ),
     label = "`{Self}` is not an iterator",
     message = "`{Self}` is not an iterator"
 )]

tests/ui/iterators/float_iterator_hint.rs

@@ -0,0 +1,15 @@
+// #106728
+
+fn main() {
+    for i in 0.2 {
+        //~^ ERROR `{float}` is not an iterator
+        //~| `{float}` is not an iterator
+        //~| NOTE in this expansion of desugaring of `for` loop
+        //~| NOTE in this expansion of desugaring of `for` loop
+        //~| NOTE in this expansion of desugaring of `for` loop
+        //~| NOTE in this expansion of desugaring of `for` loop
+        //~| NOTE if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
+        //~| NOTE required for `{float}` to implement `IntoIterator`
+        println!();
+    }
+}

tests/ui/iterators/float_iterator_hint.stderr

@@ -0,0 +1,13 @@
+error[E0277]: `{float}` is not an iterator
+  --> $DIR/float_iterator_hint.rs:4:14
+   |
+LL |     for i in 0.2 {
+   |              ^^^ `{float}` is not an iterator
+   |
+   = help: the trait `Iterator` is not implemented for `{float}`
+   = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
+   = note: required for `{float}` to implement `IntoIterator`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/iterators/integral.stderr

@@ -115,6 +115,7 @@ LL |     for _ in 42.0 {}
    |              ^^^^ `{float}` is not an iterator
    |
    = help: the trait `Iterator` is not implemented for `{float}`
+   = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
    = note: required for `{float}` to implement `IntoIterator`
 
 error: aborting due to 12 previous errors