Auto merge of rust-lang#139131 - m-ou-se:format-args-struct-expr, r=Mark-Simulacrum

Simplify expansion for format_args!().

Instead of calling `Placeholder::new()`, we can just use a struct expression directly.

Before:

```rust
        Placeholder::new(…, …, …, …)
```

After:

```rust
        Placeholder {
                position: …,
                flags: …,
                width: …,
                precision: …,
        }
```

(I originally avoided the struct expression, because `Placeholder` had a lot of fields. But now that rust-lang#136974 is merged, it only has four fields left.)

This will make the `fmt` argument to `fmt::Arguments::new_v1_formatted()` a candidate for const promotion, which is important if we ever hope to tackle rust-lang#92698 (It doesn't change anything yet though, because the `args` argument to `fmt::Arguments::new_v1_formatted()` is not const-promotable.)

Author: bors

core/src/fmt/rt.rs

@@ -20,8 +20,8 @@ pub struct Placeholder {
     pub width: Count,
 }
 
+#[cfg(bootstrap)]
 impl Placeholder {
-    #[cfg(bootstrap)]
     #[inline]
     pub const fn new(
         position: usize,
@@ -33,12 +33,6 @@ impl Placeholder {
     ) -> Self {
         Self { position, fill, align, flags, precision, width }
     }
-
-    #[cfg(not(bootstrap))]
-    #[inline]
-    pub const fn new(position: usize, flags: u32, precision: Count, width: Count) -> Self {
-        Self { position, flags, precision, width }
-    }
 }
 
 #[cfg(bootstrap)]