Rollup merge of #128762 - fmease:use-more-slice-pats, r=compiler-errors

Use more slice patterns inside the compiler

Nothing super noteworthy. Just replacing the common 'fragile' pattern of "length check followed by indexing or unwrap" with slice patterns for legibility and 'robustness'.

r? ghost

compiler/rustc_builtin_macros/src/format.rs

@@ -180,8 +180,8 @@ fn make_format_args(
                     Ok((mut err, suggested)) => {
                         if !suggested {
                             if let ExprKind::Block(block, None) = &efmt.kind
-                                && block.stmts.len() == 1
-                                && let StmtKind::Expr(expr) = &block.stmts[0].kind
+                                && let [stmt] = block.stmts.as_slice()
+                                && let StmtKind::Expr(expr) = &stmt.kind
                                 && let ExprKind::Path(None, path) = &expr.kind
                                 && path.is_potential_trivial_const_arg()
                             {
@@ -196,8 +196,8 @@ fn make_format_args(
                             } else {
                                 let sugg_fmt = match args.explicit_args().len() {
                                     0 => "{}".to_string(),
-                                    _ => {
-                                        format!("{}{{}}", "{} ".repeat(args.explicit_args().len()))
+                                    count => {
+                                        format!("{}{{}}", "{} ".repeat(count))
                                     }
                                 };
                                 err.span_suggestion(