Auto merge of rust-lang#83302 - camsteffen:write-piece-unchecked, r=dtolnay

Get piece unchecked in `write`

We already use specialized `zip`, but it seems like we can do a little better by not checking `pieces` length at all.

`Arguments` constructors are now unsafe. So the `format_args!` expansion now includes an `unsafe` block.

<details>
<summary>Local Bench Diff</summary>

```text
 name                        before ns/iter  after ns/iter  diff ns/iter   diff %  speedup
 fmt::write_str_macro1       22,967          19,718               -3,249  -14.15%   x 1.16
 fmt::write_str_macro2       35,527          32,654               -2,873   -8.09%   x 1.09
 fmt::write_str_macro_debug  571,953         575,973               4,020    0.70%   x 0.99
 fmt::write_str_ref          9,579           9,459                  -120   -1.25%   x 1.01
 fmt::write_str_value        9,573           9,572                    -1   -0.01%   x 1.00
 fmt::write_u128_max         176             173                      -3   -1.70%   x 1.02
 fmt::write_u128_min         138             134                      -4   -2.90%   x 1.03
 fmt::write_u64_max          139             136                      -3   -2.16%   x 1.02
 fmt::write_u64_min          129             135                       6    4.65%   x 0.96
 fmt::write_vec_macro1       24,401          22,273               -2,128   -8.72%   x 1.10
 fmt::write_vec_macro2       37,096          35,602               -1,494   -4.03%   x 1.04
 fmt::write_vec_macro_debug  588,291         589,575               1,284    0.22%   x 1.00
 fmt::write_vec_ref          9,568           9,732                   164    1.71%   x 0.98
 fmt::write_vec_value        9,516           9,625                   109    1.15%   x 0.99
```
</details>

Author: bors

clippy_utils/src/higher.rs

@@ -469,12 +469,28 @@ impl FormatArgsExpn<'tcx> {
             if let ExpnKind::Macro(_, name) = expr.span.ctxt().outer_expn_data().kind;
             let name = name.as_str();
             if name.ends_with("format_args") || name.ends_with("format_args_nl");
-            if let ExprKind::Call(_, args) = expr.kind;
-            if let Some((strs_ref, args, fmt_expr)) = match args {
+
+            if let ExprKind::Match(inner_match, [arm], _) = expr.kind;
+
+            // `match match`, if you will
+            if let ExprKind::Match(args, [inner_arm], _) = inner_match.kind;
+            if let ExprKind::Tup(value_args) = args.kind;
+            if let Some(value_args) = value_args
+                .iter()
+                .map(|e| match e.kind {
+                    ExprKind::AddrOf(_, _, e) => Some(e),
+                    _ => None,
+                })
+                .collect();
+            if let ExprKind::Array(args) = inner_arm.body.kind;
+
+            if let ExprKind::Block(Block { stmts: [], expr: Some(expr), .. }, _) = arm.body.kind;
+            if let ExprKind::Call(_, call_args) = expr.kind;
+            if let Some((strs_ref, fmt_expr)) = match call_args {
                 // Arguments::new_v1
-                [strs_ref, args] => Some((strs_ref, args, None)),
+                [strs_ref, _] => Some((strs_ref, None)),
                 // Arguments::new_v1_formatted
-                [strs_ref, args, fmt_expr] => Some((strs_ref, args, Some(fmt_expr))),
+                [strs_ref, _, fmt_expr] => Some((strs_ref, Some(fmt_expr))),
                 _ => None,
             };
             if let ExprKind::AddrOf(BorrowKind::Ref, _, strs_arr) = strs_ref.kind;
@@ -490,17 +506,6 @@ impl FormatArgsExpn<'tcx> {
                     None
                 })
                 .collect();
-            if let ExprKind::AddrOf(BorrowKind::Ref, _, args) = args.kind;
-            if let ExprKind::Match(args, [arm], _) = args.kind;
-            if let ExprKind::Tup(value_args) = args.kind;
-            if let Some(value_args) = value_args
-                .iter()
-                .map(|e| match e.kind {
-                    ExprKind::AddrOf(_, _, e) => Some(e),
-                    _ => None,
-                })
-                .collect();
-            if let ExprKind::Array(args) = arm.body.kind;
             then {
                 Some(FormatArgsExpn {
                     format_string_span: strs_ref.span,