Rollup merge of rust-lang#135677 - yotamofek:resolve-cleanups2, r=compiler-errors

Small `rustc_resolve` cleanups

1. Don't open-code `Reverse`
2. Use slice patterns where possible

Author: jhpratt

compiler/rustc_resolve/src/diagnostics.rs

@@ -1127,7 +1127,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         });
 
         // Make sure error reporting is deterministic.
-        suggestions.sort_by(|a, b| a.candidate.as_str().partial_cmp(b.candidate.as_str()).unwrap());
+        suggestions.sort_by(|a, b| a.candidate.as_str().cmp(b.candidate.as_str()));
 
         match find_best_match_for_name(
             &suggestions.iter().map(|suggestion| suggestion.candidate).collect::<Vec<Symbol>>(),
@@ -2256,14 +2256,15 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         mut path: Vec<Segment>,
         parent_scope: &ParentScope<'ra>,
     ) -> Option<(Vec<Segment>, Option<String>)> {
-        match (path.get(0), path.get(1)) {
+        match path[..] {
             // `{{root}}::ident::...` on both editions.
             // On 2015 `{{root}}` is usually added implicitly.
-            (Some(fst), Some(snd))
-                if fst.ident.name == kw::PathRoot && !snd.ident.is_path_segment_keyword() => {}
+            [first, second, ..]
+                if first.ident.name == kw::PathRoot && !second.ident.is_path_segment_keyword() => {}
             // `ident::...` on 2018.
-            (Some(fst), _)
-                if fst.ident.span.at_least_rust_2018() && !fst.ident.is_path_segment_keyword() =>
+            [first, ..]
+                if first.ident.span.at_least_rust_2018()
+                    && !first.ident.is_path_segment_keyword() =>
             {
                 // Insert a placeholder that's later replaced by `self`/`super`/etc.
                 path.insert(0, Segment::from_ident(Ident::empty()));
@@ -2374,7 +2375,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         // 2) `std` suggestions before `core` suggestions.
         let mut extern_crate_names =
             self.extern_prelude.keys().map(|ident| ident.name).collect::<Vec<_>>();
-        extern_crate_names.sort_by(|a, b| b.as_str().partial_cmp(a.as_str()).unwrap());
+        extern_crate_names.sort_by(|a, b| b.as_str().cmp(a.as_str()));
 
         for name in extern_crate_names.into_iter() {
             // Replace first ident with a crate name and check if that is valid.