Use more slice patterns inside the compiler

compiler/rustc_parse/src/parser/path.rs

@@ -826,7 +826,8 @@ impl<'a> Parser<'a> {
             // We can only resolve single-segment paths at the moment, because multi-segment paths
             // require type-checking: see `visit_generic_arg` in `src/librustc_resolve/late.rs`.
             ast::ExprKind::Path(None, path)
-                if path.segments.len() == 1 && path.segments[0].args.is_none() =>
+                if let [segment] = path.segments.as_slice()
+                    && segment.args.is_none() =>
             {
                 true
             }