Rollup merge of #94746 - notriddle:notriddle/method-rustc-on-unimplemented, r=davidtwco

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 #94581

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,
@@ -204,6 +204,10 @@ 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))));