Reword rustc_on_unimplemented errors for Iterator

 - Detect one element array of `Range` type, which is potentially a typo:
   `for _ in [0..10] {}` where iterating between `0` and `10` was intended.
   (rust-lang#23141)
 - Suggest `.bytes()` and `.chars()` for `String`.
 - Suggest borrowing or `.iter()` on arrays (rust-lang#36391)
 - Suggest using range literal when iterating on integers (rust-lang#34353)
 - Do not suggest `.iter()` by default (rust-lang#50773, rust-lang#46806)

Author: estebank

src/libcore/iter/iterator.rs

@@ -30,11 +30,30 @@ fn _assert_is_object_safe(_: &dyn Iterator<Item=()>) {}
 /// [impl]: index.html#implementing-iterator
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_on_unimplemented(
+    on(
+        _Self="[std::ops::Range<Idx>; 1]",
+        label="if you meant to iterate between two values, remove the square brackets",
+        note="`[start..end]` is an array of one `Range`, you might have meant to have a `Range`: `start..end`"
+    ),
     on(
         _Self="&str",
         label="`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`"
     ),
-    label="`{Self}` is not an iterator; maybe try calling `.iter()` or a similar method"
+    on(
+        _Self="std::string::String",
+        label="`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`"
+    ),
+    on(
+        _Self="[]",
+        label="borrow the array with `&` or call `.iter()` on it to iterate over it",
+        note="arrays are not an iterators, but slices like the following are: `&[1, 2, 3]`"
+    ),
+    on(
+        _Self="{integral}",
+        note="if you want to iterate between `0` until a value `end`, use the range syntax: `0..end`"
+    ),
+    label="`{Self}` is not an iterator",
+    message="`{Self}` is not an iterator"
 )]
 #[doc(spotlight)]
 pub trait Iterator {

src/test/ui/conservative_impl_trait.rs

@@ -11,7 +11,7 @@
 // #39872, #39553
 
 fn will_ice(something: &u32) -> impl Iterator<Item = &u32> {
-    //~^ ERROR the trait bound `(): std::iter::Iterator` is not satisfied [E0277]
+    //~^ ERROR `()` is not an iterator
 }
 
 fn main() {}

src/test/ui/conservative_impl_trait.stderr

@@ -1,8 +1,8 @@
-error[E0277]: the trait bound `(): std::iter::Iterator` is not satisfied
+error[E0277]: `()` is not an iterator
   --> $DIR/conservative_impl_trait.rs:13:33
    |
 LL | fn will_ice(something: &u32) -> impl Iterator<Item = &u32> {
-   |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^ `()` is not an iterator; maybe try calling `.iter()` or a similar method
+   |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^ `()` is not an iterator
    |
    = help: the trait `std::iter::Iterator` is not implemented for `()`
    = note: the return type of a function must have a statically known size

src/test/ui/feature-gates/feature-gate-trivial_bounds.stderr

@@ -75,15 +75,16 @@ LL | | }
    = help: see issue #48214
    = help: add #![feature(trivial_bounds)] to the crate attributes to enable
 
-error[E0277]: the trait bound `i32: std::iter::Iterator` is not satisfied
+error[E0277]: `i32` is not an iterator
   --> $DIR/feature-gate-trivial_bounds.rs:50:1
    |
 LL | / fn use_for() where i32: Iterator { //~ ERROR
 LL | |     for _ in 2i32 {}
 LL | | }
-   | |_^ `i32` is not an iterator; maybe try calling `.iter()` or a similar method
+   | |_^ `i32` is not an iterator
    |
    = help: the trait `std::iter::Iterator` is not implemented for `i32`
+   = note: if you want to iterate between `0` until a value `end`, use the range syntax: `0..end`
    = help: see issue #48214
    = help: add #![feature(trivial_bounds)] to the crate attributes to enable
 

src/test/ui/for/for-c-in-str.rs

@@ -12,7 +12,7 @@
 
 fn main() {
     for c in "asdf" {
-    //~^ ERROR the trait bound `&str: std::iter::Iterator` is not satisfied
+    //~^ ERROR `&str` is not an iterator
     //~| NOTE `&str` is not an iterator
     //~| HELP the trait `std::iter::Iterator` is not implemented for `&str`
     //~| NOTE required by `std::iter::IntoIterator::into_iter`

src/test/ui/for/for-c-in-str.stderr

@@ -1,4 +1,4 @@
-error[E0277]: the trait bound `&str: std::iter::Iterator` is not satisfied
+error[E0277]: `&str` is not an iterator
   --> $DIR/for-c-in-str.rs:14:14
    |
 LL |     for c in "asdf" {

src/test/ui/for/for-loop-bogosity.rs

@@ -24,7 +24,8 @@ pub fn main() {
         x: 1,
         y: 2,
     };
-    for x in bogus { //~ ERROR `MyStruct: std::iter::Iterator` is not satisfied
+    for x in bogus {
+    //~^ ERROR `MyStruct` is not an iterator
         drop(x);
     }
 }

src/test/ui/for/for-loop-bogosity.stderr

@@ -1,8 +1,8 @@
-error[E0277]: the trait bound `MyStruct: std::iter::Iterator` is not satisfied
+error[E0277]: `MyStruct` is not an iterator
   --> $DIR/for-loop-bogosity.rs:27:14
    |
-LL |     for x in bogus { //~ ERROR `MyStruct: std::iter::Iterator` is not satisfied
-   |              ^^^^^ `MyStruct` is not an iterator; maybe try calling `.iter()` or a similar method
+LL |     for x in bogus {
+   |              ^^^^^ `MyStruct` is not an iterator
    |
    = help: the trait `std::iter::Iterator` is not implemented for `MyStruct`
    = note: required by `std::iter::IntoIterator::into_iter`

src/test/ui/issues/issue-28098.rs

@@ -10,13 +10,13 @@
 
 fn main() {
     let _ = Iterator::next(&mut ());
-    //~^ ERROR `(): std::iter::Iterator` is not satisfied
+    //~^ ERROR `()` is not an iterator
 
     for _ in false {}
-    //~^ ERROR `bool: std::iter::Iterator` is not satisfied
+    //~^ ERROR `bool` is not an iterator
 
     let _ = Iterator::next(&mut ());
-    //~^ ERROR `(): std::iter::Iterator` is not satisfied
+    //~^ ERROR `()` is not an iterator
 
     other()
 }
@@ -25,11 +25,11 @@ pub fn other() {
     // check errors are still reported globally
 
     let _ = Iterator::next(&mut ());
-    //~^ ERROR `(): std::iter::Iterator` is not satisfied
+    //~^ ERROR `()` is not an iterator
 
     let _ = Iterator::next(&mut ());
-    //~^ ERROR `(): std::iter::Iterator` is not satisfied
+    //~^ ERROR `()` is not an iterator
 
     for _ in false {}
-    //~^ ERROR `bool: std::iter::Iterator` is not satisfied
+    //~^ ERROR `bool` is not an iterator
 }

src/test/ui/issues/issue-28098.stderr

@@ -1,53 +1,53 @@
-error[E0277]: the trait bound `(): std::iter::Iterator` is not satisfied
+error[E0277]: `()` is not an iterator
   --> $DIR/issue-28098.rs:12:13
    |
 LL |     let _ = Iterator::next(&mut ());
-   |             ^^^^^^^^^^^^^^ `()` is not an iterator; maybe try calling `.iter()` or a similar method
+   |             ^^^^^^^^^^^^^^ `()` is not an iterator
    |
    = help: the trait `std::iter::Iterator` is not implemented for `()`
    = note: required by `std::iter::Iterator::next`
 
-error[E0277]: the trait bound `bool: std::iter::Iterator` is not satisfied
+error[E0277]: `bool` is not an iterator
   --> $DIR/issue-28098.rs:15:14
    |
 LL |     for _ in false {}
-   |              ^^^^^ `bool` is not an iterator; maybe try calling `.iter()` or a similar method
+   |              ^^^^^ `bool` is not an iterator
    |
    = help: the trait `std::iter::Iterator` is not implemented for `bool`
    = note: required by `std::iter::IntoIterator::into_iter`
 
-error[E0277]: the trait bound `(): std::iter::Iterator` is not satisfied
+error[E0277]: `()` is not an iterator
   --> $DIR/issue-28098.rs:18:13
    |
 LL |     let _ = Iterator::next(&mut ());
-   |             ^^^^^^^^^^^^^^ `()` is not an iterator; maybe try calling `.iter()` or a similar method
+   |             ^^^^^^^^^^^^^^ `()` is not an iterator
    |
    = help: the trait `std::iter::Iterator` is not implemented for `()`
    = note: required by `std::iter::Iterator::next`
 
-error[E0277]: the trait bound `(): std::iter::Iterator` is not satisfied
+error[E0277]: `()` is not an iterator
   --> $DIR/issue-28098.rs:27:13
    |
 LL |     let _ = Iterator::next(&mut ());
-   |             ^^^^^^^^^^^^^^ `()` is not an iterator; maybe try calling `.iter()` or a similar method
+   |             ^^^^^^^^^^^^^^ `()` is not an iterator
    |
    = help: the trait `std::iter::Iterator` is not implemented for `()`
    = note: required by `std::iter::Iterator::next`
 
-error[E0277]: the trait bound `(): std::iter::Iterator` is not satisfied
+error[E0277]: `()` is not an iterator
   --> $DIR/issue-28098.rs:30:13
    |
 LL |     let _ = Iterator::next(&mut ());
-   |             ^^^^^^^^^^^^^^ `()` is not an iterator; maybe try calling `.iter()` or a similar method
+   |             ^^^^^^^^^^^^^^ `()` is not an iterator
    |
    = help: the trait `std::iter::Iterator` is not implemented for `()`
    = note: required by `std::iter::Iterator::next`
 
-error[E0277]: the trait bound `bool: std::iter::Iterator` is not satisfied
+error[E0277]: `bool` is not an iterator
   --> $DIR/issue-28098.rs:33:14
    |
 LL |     for _ in false {}
-   |              ^^^^^ `bool` is not an iterator; maybe try calling `.iter()` or a similar method
+   |              ^^^^^ `bool` is not an iterator
    |
    = help: the trait `std::iter::Iterator` is not implemented for `bool`
    = note: required by `std::iter::IntoIterator::into_iter`