Auto merge of #136974 - m-ou-se:fmt-options-64-bit, r=scottmcm

Reduce FormattingOptions to 64 bits

This is part of https://github.com/rust-lang/rust/issues/99012

This reduces FormattingOptions from 6-7 machine words (384 bits on 64-bit platforms, 224 bits on 32-bit platforms) to just 64 bits (a single register on 64-bit platforms).

Before:

```rust
pub struct FormattingOptions {
    flags: u32, // only 6 bits used
    fill: char,
    align: Option<Alignment>,
    width: Option<usize>,
    precision: Option<usize>,
}
```

After:

```rust
pub struct FormattingOptions {
    /// Bits:
    ///  - 0-20: fill character (21 bits, a full `char`)
    ///  - 21: `+` flag
    ///  - 22: `-` flag
    ///  - 23: `#` flag
    ///  - 24: `0` flag
    ///  - 25: `x?` flag
    ///  - 26: `X?` flag
    ///  - 27: Width flag (if set, the width field below is used)
    ///  - 28: Precision flag (if set, the precision field below is used)
    ///  - 29-30: Alignment (0: Left, 1: Right, 2: Center, 3: Unknown)
    ///  - 31: Always set to 1
    flags: u32,
    /// Width if width flag above is set. Otherwise, always 0.
    width: u16,
    /// Precision if precision flag above is set. Otherwise, always 0.
    precision: u16,
}
```

compiler/rustc_ast_lowering/src/expr.rs

@@ -2150,11 +2150,6 @@ impl<'hir> LoweringContext<'_, 'hir> {
         self.expr_uint(sp, ast::UintTy::U16, value as u128)
     }
 
-    pub(super) fn expr_char(&mut self, sp: Span, value: char) -> hir::Expr<'hir> {
-        let lit = self.arena.alloc(hir::Lit { span: sp, node: ast::LitKind::Char(value) });
-        self.expr(sp, hir::ExprKind::Lit(lit))
-    }
-
     pub(super) fn expr_str(&mut self, sp: Span, value: Symbol) -> hir::Expr<'hir> {
         let lit = self
             .arena

compiler/rustc_ast_lowering/src/format.rs

@@ -361,24 +361,26 @@ fn make_format_spec<'hir>(
         zero_pad,
         debug_hex,
     } = &placeholder.format_options;
-    let fill = ctx.expr_char(sp, fill.unwrap_or(' '));
-    let align = ctx.expr_lang_item_type_relative(
-        sp,
-        hir::LangItem::FormatAlignment,
-        match alignment {
-            Some(FormatAlignment::Left) => sym::Left,
-            Some(FormatAlignment::Right) => sym::Right,
-            Some(FormatAlignment::Center) => sym::Center,
-            None => sym::Unknown,
-        },
-    );
-    // This needs to match `Flag` in library/core/src/fmt/rt.rs.
-    let flags: u32 = ((sign == Some(FormatSign::Plus)) as u32)
-        | ((sign == Some(FormatSign::Minus)) as u32) << 1
-        | (alternate as u32) << 2
-        | (zero_pad as u32) << 3
-        | ((debug_hex == Some(FormatDebugHex::Lower)) as u32) << 4
-        | ((debug_hex == Some(FormatDebugHex::Upper)) as u32) << 5;
+    let fill = fill.unwrap_or(' ');
+    // These need to match the constants in library/core/src/fmt/rt.rs.
+    let align = match alignment {
+        Some(FormatAlignment::Left) => 0,
+        Some(FormatAlignment::Right) => 1,
+        Some(FormatAlignment::Center) => 2,
+        None => 3,
+    };
+    // This needs to match the constants in library/core/src/fmt/rt.rs.
+    let flags: u32 = fill as u32
+        | ((sign == Some(FormatSign::Plus)) as u32) << 21
+        | ((sign == Some(FormatSign::Minus)) as u32) << 22
+        | (alternate as u32) << 23
+        | (zero_pad as u32) << 24
+        | ((debug_hex == Some(FormatDebugHex::Lower)) as u32) << 25
+        | ((debug_hex == Some(FormatDebugHex::Upper)) as u32) << 26
+        | (width.is_some() as u32) << 27
+        | (precision.is_some() as u32) << 28
+        | align << 29
+        | 1 << 31; // Highest bit always set.
     let flags = ctx.expr_u32(sp, flags);
     let precision = make_count(ctx, sp, precision, argmap);
     let width = make_count(ctx, sp, width, argmap);
@@ -387,7 +389,7 @@ fn make_format_spec<'hir>(
         hir::LangItem::FormatPlaceholder,
         sym::new,
     ));
-    let args = ctx.arena.alloc_from_iter([position, fill, align, flags, precision, width]);
+    let args = ctx.arena.alloc_from_iter([position, flags, precision, width]);
     ctx.expr_call_mut(sp, format_placeholder_new, args)
 }
 

compiler/rustc_hir/src/lang_items.rs

@@ -322,7 +322,6 @@ language_item_table! {
     BeginPanic,              sym::begin_panic,         begin_panic_fn,             Target::Fn,             GenericRequirement::None;
 
     // Lang items needed for `format_args!()`.
-    FormatAlignment,         sym::format_alignment,    format_alignment,           Target::Enum,           GenericRequirement::None;
     FormatArgument,          sym::format_argument,     format_argument,            Target::Struct,         GenericRequirement::None;
     FormatArguments,         sym::format_arguments,    format_arguments,           Target::Struct,         GenericRequirement::None;
     FormatCount,             sym::format_count,        format_count,               Target::Enum,           GenericRequirement::None;

compiler/rustc_span/src/symbol.rs

@@ -1003,7 +1003,6 @@ symbols! {
         forbid,
         forget,
         format,
-        format_alignment,
         format_args,
         format_args_capture,
         format_args_macro,