diagnostics: use rustc_on_unimplemented to recommend [].iter()

To make this work, the `#[rustc_on_unimplemented]` data needs to be used to
report method resolution errors, which is most of what this commit does.

Fixes rust-lang#94581

compiler/rustc_middle/src/ty/sty.rs

@@ -1887,6 +1887,15 @@ impl<'tcx> Ty<'tcx> {
         }
     }
 
+    #[inline]
+    pub fn is_array_slice(self) -> bool {
+        match self.kind() {
+            Slice(_) => true,
+            RawPtr(TypeAndMut { ty, .. }) | Ref(_, ty, _) => matches!(ty.kind(), Slice(_)),
+            _ => false,
+        }
+    }
+
     #[inline]
     pub fn is_array(self) -> bool {
         matches!(self.kind(), Array(..))

compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs

@@ -11,7 +11,7 @@ use std::iter;
 
 use super::InferCtxtPrivExt;
 
-crate trait InferCtxtExt<'tcx> {
+pub trait InferCtxtExt<'tcx> {
     /*private*/
     fn impl_similar_to(
         &self,
@@ -202,6 +202,10 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                 flags.push((sym::_Self, Some("{integral}".to_owned())));
             }
 
+            if self_ty.is_array_slice() {
+                flags.push((sym::_Self, Some("&[]".to_owned())));
+            }
+
             if let ty::Array(aty, len) = self_ty.kind() {
                 flags.push((sym::_Self, Some("[]".to_owned())));
                 flags.push((sym::_Self, Some(format!("[{}]", aty))));

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

@@ -34,6 +34,11 @@ fn _assert_is_object_safe(_: &dyn Iterator<Item = ()>) {}
         note = "`..=end` is a `RangeToInclusive`, which cannot be iterated on; you might have meant \
               to have a bounded `RangeInclusive`: `0..=end`"
     ),
+    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 = "&str",
         label = "`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`"

src/test/ui/issues/issue-21596.stderr

@@ -1,8 +1,8 @@
-error[E0599]: the method `to_string` exists for raw pointer `*const u8`, but its trait bounds were not satisfied
+error[E0599]: `*const u8` doesn't implement `std::fmt::Display`
   --> $DIR/issue-21596.rs:4:22
    |
 LL |     println!("{}", z.to_string());
-   |                      ^^^^^^^^^ method cannot be called on `*const u8` due to unsatisfied trait bounds
+   |                      ^^^^^^^^^ `*const u8` cannot be formatted with the default formatter
    |
    = note: try using `<*const T>::as_ref()` to get a reference to the type behind the pointer: https://doc.rust-lang.org/std/primitive.pointer.html#method.as_ref
    = note: using `<*const T>::as_ref()` on a pointer which is unaligned or points to invalid or uninitialized memory is undefined behavior

src/test/ui/issues/issue-31173.stderr

@@ -18,16 +18,16 @@ error[E0599]: the method `collect` exists for struct `Cloned<TakeWhile<&mut std:
 LL |         .collect();
    |          ^^^^^^^ method cannot be called on `Cloned<TakeWhile<&mut std::vec::IntoIter<u8>, [closure@$DIR/issue-31173.rs:6:39: 9:6]>>` due to unsatisfied trait bounds
    |
-  ::: $SRC_DIR/core/src/iter/adapters/cloned.rs:LL:COL
-   |
-LL | pub struct Cloned<I> {
-   | -------------------- doesn't satisfy `_: Iterator`
-   |
   ::: $SRC_DIR/core/src/iter/adapters/take_while.rs:LL:COL
    |
 LL | pub struct TakeWhile<I, P> {
    | -------------------------- doesn't satisfy `<_ as Iterator>::Item = &_`
    |
+  ::: $SRC_DIR/core/src/iter/adapters/cloned.rs:LL:COL
+   |
+LL | pub struct Cloned<I> {
+   | -------------------- doesn't satisfy `_: Iterator`
+   |
    = note: the following trait bounds were not satisfied:
            `<TakeWhile<&mut std::vec::IntoIter<u8>, [closure@$DIR/issue-31173.rs:6:39: 9:6]> as Iterator>::Item = &_`
            which is required by `Cloned<TakeWhile<&mut std::vec::IntoIter<u8>, [closure@$DIR/issue-31173.rs:6:39: 9:6]>>: Iterator`

src/test/ui/methods/issues/issue-94581.rs

@@ -0,0 +1,7 @@
+fn get_slice() -> &'static [i32] {
+    &[1, 2, 3, 4]
+}
+
+fn main() {
+    let sqsum = get_slice().map(|i| i * i).sum(); //~ ERROR [E0599]
+}

src/test/ui/methods/issues/issue-94581.stderr

@@ -0,0 +1,15 @@
+error[E0599]: `&'static [i32]` is not an iterator
+  --> $DIR/issue-94581.rs:6:29
+   |
+LL |     let sqsum = get_slice().map(|i| i * i).sum();
+   |                             ^^^ `&'static [i32]` is not an iterator; try calling `.iter()`
+   |
+   = 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`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0599`.

src/test/ui/methods/method-call-err-msg.rs

@@ -16,7 +16,7 @@ fn main() {
 
     let y = Foo;
     y.zero()
-     .take()    //~ ERROR the method
+     .take()    //~ ERROR not an iterator
      .one(0);
     y.three::<usize>(); //~ ERROR this function takes 3 arguments but 0 arguments were supplied
 }

src/test/ui/methods/method-call-err-msg.stderr

@@ -40,7 +40,7 @@ note: associated function defined here
 LL |     fn two(self, _: isize, _: isize) -> Foo { self }
    |        ^^^ ----  --------  --------
 
-error[E0599]: the method `take` exists for struct `Foo`, but its trait bounds were not satisfied
+error[E0599]: `Foo` is not an iterator
   --> $DIR/method-call-err-msg.rs:19:7
    |
 LL | pub struct Foo;
@@ -50,7 +50,7 @@ LL | pub struct Foo;
    | doesn't satisfy `Foo: Iterator`
 ...
 LL |      .take()
-   |       ^^^^ method cannot be called on `Foo` due to unsatisfied trait bounds
+   |       ^^^^ `Foo` is not an iterator
    |
    = note: the following trait bounds were not satisfied:
            `Foo: Iterator`

src/test/ui/mismatched_types/issue-36053-2.rs

@@ -5,6 +5,6 @@
 use std::iter::once;
 fn main() {
     once::<&str>("str").fuse().filter(|a: &str| true).count();
-    //~^ ERROR the method
+    //~^ ERROR not an iterator
     //~| ERROR type mismatch in closure arguments
 }

src/test/ui/mismatched_types/issue-36053-2.stderr

@@ -12,11 +12,11 @@ note: required by a bound in `filter`
 LL |         P: FnMut(&Self::Item) -> bool,
    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `filter`
 
-error[E0599]: the method `count` exists for struct `Filter<Fuse<std::iter::Once<&str>>, [closure@$DIR/issue-36053-2.rs:7:39: 7:53]>`, but its trait bounds were not satisfied
+error[E0599]: `Filter<Fuse<std::iter::Once<&str>>, [closure@$DIR/issue-36053-2.rs:7:39: 7:53]>` is not an iterator
   --> $DIR/issue-36053-2.rs:7:55
    |
 LL |     once::<&str>("str").fuse().filter(|a: &str| true).count();
-   |                                       --------------  ^^^^^ method cannot be called on `Filter<Fuse<std::iter::Once<&str>>, [closure@$DIR/issue-36053-2.rs:7:39: 7:53]>` due to unsatisfied trait bounds
+   |                                       --------------  ^^^^^ `Filter<Fuse<std::iter::Once<&str>>, [closure@$DIR/issue-36053-2.rs:7:39: 7:53]>` is not an iterator
    |                                       |
    |                                       doesn't satisfy `<_ as FnOnce<(&&str,)>>::Output = bool`
    |                                       doesn't satisfy `_: FnMut<(&&str,)>`