Auto merge of #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 https://github.com/rust-lang/rust/pull/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 https://github.com/rust-lang/rust/issues/92698 (It doesn't change anything yet though, because the `args` argument to `fmt::Arguments::new_v1_formatted()` is not const-promotable.)

3 files changed, 18 insertions, 22 deletions

diff --git a/compiler/rustc_ast_lowering/src/format.rs b/compiler/rustc_ast_lowering/src/format.rs
index 343895984ca..0de0319c667 100644
--- a/compiler/rustc_ast_lowering/src/format.rs
+++ b/compiler/rustc_ast_lowering/src/format.rs
@@ -323,14 +323,12 @@ fn make_count<'hir>(
 /// Generates
 ///
 /// ```text
-///     <core::fmt::rt::Placeholder::new(
-///         …usize, // position
-///         '…', // fill
-///         <core::fmt::rt::Alignment>::…, // alignment
-///         …u32, // flags
-///         <core::fmt::rt::Count::…>, // width
-///         <core::fmt::rt::Count::…>, // precision
-///     )
+///     <core::fmt::rt::Placeholder {
+///         position: …usize,
+///         flags: …u32,
+///         precision: <core::fmt::rt::Count::…>,
+///         width: <core::fmt::rt::Count::…>,
+///     }
 /// ```
 fn make_format_spec<'hir>(
     ctx: &mut LoweringContext<'_, 'hir>,
@@ -384,13 +382,13 @@ fn make_format_spec<'hir>(
     let flags = ctx.expr_u32(sp, flags);
     let precision = make_count(ctx, sp, precision, argmap);
     let width = make_count(ctx, sp, width, argmap);
-    let format_placeholder_new = ctx.arena.alloc(ctx.expr_lang_item_type_relative(
-        sp,
-        hir::LangItem::FormatPlaceholder,
-        sym::new,
-    ));
-    let args = ctx.arena.alloc_from_iter([position, flags, precision, width]);
-    ctx.expr_call_mut(sp, format_placeholder_new, args)
+    let position = ctx.expr_field(Ident::new(sym::position, sp), ctx.arena.alloc(position), sp);
+    let flags = ctx.expr_field(Ident::new(sym::flags, sp), ctx.arena.alloc(flags), sp);
+    let precision = ctx.expr_field(Ident::new(sym::precision, sp), ctx.arena.alloc(precision), sp);
+    let width = ctx.expr_field(Ident::new(sym::width, sp), ctx.arena.alloc(width), sp);
+    let placeholder = ctx.arena.alloc(hir::QPath::LangItem(hir::LangItem::FormatPlaceholder, sp));
+    let fields = ctx.arena.alloc_from_iter([position, flags, precision, width]);
+    ctx.expr(sp, hir::ExprKind::Struct(placeholder, fields, hir::StructTailExpr::None))
 }
 
 fn expand_format_args<'hir>(
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index 6807f96e712..cc4c9860bc2 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -986,6 +986,7 @@ symbols! {
         field_init_shorthand,
         file,
         file_options,
+        flags,
         float,
         float_to_int_unchecked,
         floorf128,
@@ -1570,6 +1571,7 @@ symbols! {
         pointer_like,
         poll,
         poll_next,
+        position,
         post_dash_lto: "post-lto",
         postfix_match,
         powerpc_target_feature,
@@ -1585,6 +1587,7 @@ symbols! {
         precise_capturing,
         precise_capturing_in_traits,
         precise_pointer_size_matching,
+        precision,
         pref_align_of,
         prefetch_read_data,
         prefetch_read_instruction,
@@ -2274,6 +2277,7 @@ symbols! {
         wasm_target_feature,
         where_clause_attrs,
         while_let,
+        width,
         windows,
         windows_subsystem,
         with_negative_coherence,
diff --git a/library/core/src/fmt/rt.rs b/library/core/src/fmt/rt.rs
index d27f7e6e0d8..0b04ebccae2 100644
--- a/library/core/src/fmt/rt.rs
+++ b/library/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)]