Auto merge of #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>

src/test/ui/attributes/key-value-expansion.stderr

@@ -17,12 +17,16 @@ LL | bug!();
 
 error: unexpected token: `{
     let res =
-        ::alloc::fmt::format(::core::fmt::Arguments::new_v1(&[""],
-                                                            &match (&"u8",) {
-                                                                 (arg0,) =>
-                                                                 [::core::fmt::ArgumentV1::new(arg0,
-                                                                                               ::core::fmt::Display::fmt)],
-                                                             }));
+        ::alloc::fmt::format(match match (&"u8",) {
+                                       (arg0,) =>
+                                       [::core::fmt::ArgumentV1::new(arg0,
+                                                                     ::core::fmt::Display::fmt)],
+                                   } {
+                                 ref args => unsafe {
+                                     ::core::fmt::Arguments::new_v1(&[""],
+                                                                    args)
+                                 }
+                             });
     res
 }.as_str()`
   --> $DIR/key-value-expansion.rs:48:23

src/test/ui/unsafe/unsafe-around-compiler-generated-unsafe.mir.stderr

@@ -10,5 +10,11 @@ note: the lint level is defined here
 LL | #![deny(unused_unsafe)]
    |         ^^^^^^^^^^^^^
 
-error: aborting due to previous error
+error: unnecessary `unsafe` block
+  --> $DIR/unsafe-around-compiler-generated-unsafe.rs:13:5
+   |
+LL |     unsafe { println!("foo"); }
+   |     ^^^^^^ unnecessary `unsafe` block
+
+error: aborting due to 2 previous errors
 

src/test/ui/unsafe/unsafe-around-compiler-generated-unsafe.rs

@@ -8,4 +8,7 @@ fn main() {
     let _ = async {
         unsafe { async {}.await; } //~ ERROR unnecessary `unsafe`
     };
+
+    // `format_args!` expands with a compiler-generated unsafe block
+    unsafe { println!("foo"); } //~ ERROR unnecessary `unsafe`
 }

src/test/ui/unsafe/unsafe-around-compiler-generated-unsafe.thir.stderr

@@ -10,5 +10,11 @@ note: the lint level is defined here
 LL | #![deny(unused_unsafe)]
    |         ^^^^^^^^^^^^^
 
-error: aborting due to previous error
+error: unnecessary `unsafe` block
+  --> $DIR/unsafe-around-compiler-generated-unsafe.rs:13:5
+   |
+LL |     unsafe { println!("foo"); }
+   |     ^^^^^^ unnecessary `unsafe` block
+
+error: aborting due to 2 previous errors