Rollup merge of #95260 - compiler-errors:fn, r=davidtwco

Better suggestions for `Fn`-family trait selection errors

1. Suppress suggestions to add `std::ops::Fn{,Mut,Once}` bounds when a type already implements `Fn{,Mut,Once}`
2. Add a note that points out that a type does in fact implement `Fn{,Mut,Once}`, but the arguments vary (either by number or by actual arguments)
3. Add a note that points out that a type does in fact implement `Fn{,Mut,Once}`, but not the right one (e.g. implements `FnMut`, but `Fn` is required).

Fixes #95147

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -1091,7 +1091,11 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
 
     /// Compares two given types, eliding parts that are the same between them and highlighting
     /// relevant differences, and return two representation of those types for highlighted printing.
-    fn cmp(&self, t1: Ty<'tcx>, t2: Ty<'tcx>) -> (DiagnosticStyledString, DiagnosticStyledString) {
+    pub fn cmp(
+        &self,
+        t1: Ty<'tcx>,
+        t2: Ty<'tcx>,
+    ) -> (DiagnosticStyledString, DiagnosticStyledString) {
         debug!("cmp(t1={}, t1.kind={:?}, t2={}, t2.kind={:?})", t1, t1.kind(), t2, t2.kind());
 
         // helper functions