Be more lax in .into_iter() suggestion when encountering Iterator methods on non-Iterator

```
error[E0599]: no method named `map` found for struct `Vec<bool>` in the current scope
  --> $DIR/vec-on-unimplemented.rs:3:23
   |
LL |     vec![true, false].map(|v| !v).collect::<Vec<_>>();
   |                       ^^^ `Vec<bool>` is not an iterator
   |
help: call `.into_iter()` first
   |
LL |     vec![true, false].into_iter().map(|v| !v).collect::<Vec<_>>();
   |                       ++++++++++++
```

We used to provide some help through `rustc_on_unimplemented` on non-`impl Trait` and non-type-params, but this lets us get rid of some otherwise unnecessary conditions in the annotation on `Iterator`.

compiler/rustc_hir_typeck/src/method/suggest.rs

@@ -166,7 +166,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             return false;
         }
 
-        match ty.kind() {
+        match ty.peel_refs().kind() {
             ty::Param(param) => {
                 let generics = self.tcx.generics_of(self.body_id);
                 let generic_param = generics.type_param(&param, self.tcx);
@@ -184,7 +184,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     }
                 }
             }
-            ty::Alias(ty::AliasKind::Opaque, _) => {
+            ty::Slice(..) | ty::Adt(..) | ty::Alias(ty::AliasKind::Opaque, _) => {
                 for unsatisfied in unsatisfied_predicates.iter() {
                     if is_iterator_predicate(unsatisfied.0, self.tcx) {
                         return true;

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

@@ -34,15 +34,6 @@ fn _assert_is_object_safe(_: &dyn Iterator<Item = ()>) {}
         _Self = "core::ops::range::RangeToInclusive<Idx>",
         note = "you might have meant to use a bounded `RangeInclusive`"
     ),
-    on(
-        _Self = "[]",
-        label = "`{Self}` is not an iterator; try calling `.into_iter()` or `.iter()`"
-    ),
-    on(_Self = "&[]", label = "`{Self}` is not an iterator; try calling `.iter()`"),
-    on(
-        _Self = "alloc::vec::Vec<T, A>",
-        label = "`{Self}` is not an iterator; try calling `.into_iter()` or `.iter()`"
-    ),
     label = "`{Self}` is not an iterator",
     message = "`{Self}` is not an iterator"
 )]

tests/ui/iterators/vec-on-unimplemented.fixed

@@ -0,0 +1,5 @@
+//@ run-rustfix
+fn main() {
+    let _ = vec![true, false].into_iter().map(|v| !v).collect::<Vec<_>>();
+    //~^ ERROR no method named `map` found for struct `Vec<bool>` in the current scope
+}

tests/ui/iterators/vec-on-unimplemented.rs

@@ -1,4 +1,5 @@
+//@ run-rustfix
 fn main() {
-    vec![true, false].map(|v| !v).collect::<Vec<_>>();
-    //~^ ERROR `Vec<bool>` is not an iterator
+    let _ = vec![true, false].map(|v| !v).collect::<Vec<_>>();
+    //~^ ERROR no method named `map` found for struct `Vec<bool>` in the current scope
 }

tests/ui/iterators/vec-on-unimplemented.stderr

@@ -1,14 +1,13 @@
-error[E0599]: `Vec<bool>` is not an iterator
-  --> $DIR/vec-on-unimplemented.rs:2:23
+error[E0599]: no method named `map` found for struct `Vec<bool>` in the current scope
+  --> $DIR/vec-on-unimplemented.rs:3:31
    |
-LL |     vec![true, false].map(|v| !v).collect::<Vec<_>>();
-   |                       ^^^ `Vec<bool>` is not an iterator; try calling `.into_iter()` or `.iter()`
+LL |     let _ = vec![true, false].map(|v| !v).collect::<Vec<_>>();
+   |                               ^^^ `Vec<bool>` is not an iterator
    |
-   = note: the following trait bounds were not satisfied:
-           `Vec<bool>: Iterator`
-           which is required by `&mut Vec<bool>: Iterator`
-           `[bool]: Iterator`
-           which is required by `&mut [bool]: Iterator`
+help: call `.into_iter()` first
+   |
+LL |     let _ = vec![true, false].into_iter().map(|v| !v).collect::<Vec<_>>();
+   |                               ++++++++++++
 
 error: aborting due to 1 previous error
 

tests/ui/methods/issues/issue-94581.fixed

@@ -0,0 +1,8 @@
+//@ run-rustfix
+fn get_slice() -> &'static [i32] {
+    &[1, 2, 3, 4]
+}
+
+fn main() {
+    let _sqsum: i32 = get_slice().into_iter().map(|i| i * i).sum(); //~ ERROR [E0599]
+}

tests/ui/methods/issues/issue-94581.rs

@@ -1,7 +1,8 @@
+//@ run-rustfix
 fn get_slice() -> &'static [i32] {
     &[1, 2, 3, 4]
 }
 
 fn main() {
-    let sqsum = get_slice().map(|i| i * i).sum(); //~ ERROR [E0599]
+    let _sqsum: i32 = get_slice().map(|i| i * i).sum(); //~ ERROR [E0599]
 }

tests/ui/methods/issues/issue-94581.stderr

@@ -1,14 +1,13 @@
-error[E0599]: `&'static [i32]` is not an iterator
-  --> $DIR/issue-94581.rs:6:29
+error[E0599]: no method named `map` found for reference `&'static [i32]` in the current scope
+  --> $DIR/issue-94581.rs:7:35
    |
-LL |     let sqsum = get_slice().map(|i| i * i).sum();
-   |                             ^^^ `&'static [i32]` is not an iterator; try calling `.iter()`
+LL |     let _sqsum: i32 = get_slice().map(|i| i * i).sum();
+   |                                   ^^^ `&'static [i32]` is not an iterator
    |
-   = note: the following trait bounds were not satisfied:
-           `&'static [i32]: Iterator`
-           which is required by `&mut &'static [i32]: Iterator`
-           `[i32]: Iterator`
-           which is required by `&mut [i32]: Iterator`
+help: call `.into_iter()` first
+   |
+LL |     let _sqsum: i32 = get_slice().into_iter().map(|i| i * i).sum();
+   |                                   ++++++++++++
 
 error: aborting due to 1 previous error
 

tests/ui/proc-macro/issue-104884-trait-impl-sugg-err.rs

@@ -14,7 +14,7 @@ struct PriorityQueueEntry<T> {
 //~^ ERROR can't compare `PriorityQueue<T>` with `PriorityQueue<T>`
 //~| ERROR the trait bound `PriorityQueue<T>: Eq` is not satisfied
 //~| ERROR can't compare `T` with `T`
-//~| ERROR `BinaryHeap<PriorityQueueEntry<T>>` is not an iterator
+//~| ERROR no method named `cmp` found for struct `BinaryHeap<PriorityQueueEntry<T>>`
 //~| ERROR no field `height` on type `&PriorityQueue<T>`
 
 struct PriorityQueue<T>(BinaryHeap<PriorityQueueEntry<T>>);

tests/ui/proc-macro/issue-104884-trait-impl-sugg-err.stderr

@@ -46,15 +46,12 @@ LL | struct PriorityQueue<T>(BinaryHeap<PriorityQueueEntry<T>>);
    = help: the trait `PartialOrd<_>` is not implemented for `BinaryHeap<PriorityQueueEntry<T>>`
    = note: this error originates in the derive macro `PartialOrd` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error[E0599]: `BinaryHeap<PriorityQueueEntry<T>>` is not an iterator
+error[E0599]: no method named `cmp` found for struct `BinaryHeap<PriorityQueueEntry<T>>` in the current scope
   --> $DIR/issue-104884-trait-impl-sugg-err.rs:13:22
    |
 LL | #[derive(PartialOrd, AddImpl)]
    |                      ^^^^^^^ `BinaryHeap<PriorityQueueEntry<T>>` is not an iterator
    |
-   = note: the following trait bounds were not satisfied:
-           `BinaryHeap<PriorityQueueEntry<T>>: Iterator`
-           which is required by `&mut BinaryHeap<PriorityQueueEntry<T>>: Iterator`
    = note: this error originates in the derive macro `AddImpl` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0609]: no field `height` on type `&PriorityQueue<T>`