Auto merge of #148789 - m-ou-se:new-fmt-args-alt, r=wafflelapkin,jdonszelmann

New format_args!() and fmt::Arguments implementation

Part of #99012

This is a new implementation of `format_args!()` and `fmt::Arguments`. With this implementation, `fmt::Arguments` is only two pointers in size. (Instead of six, before.) This makes it the same size as a `&str` and makes it fit in a register pair.

---

This `fmt::Arguments` can store a `&'static str` _without any indirection_ or additional storage. This means that simple cases like `print_fmt(format_args!("hello"))` are now just as efficient for the caller as `print_str("hello")`, as shown by this example:

> code:
> ```rust
> fn main() {
>     println!("Hello, world!");
> }
> ```
>
> before:
> ```asm
> main:
>  sub     rsp, 56
>  lea     rax, [rip + .Lanon_hello_world]
>  mov     qword ptr [rsp + 8], rax
>  mov     qword ptr [rsp + 16], 1
>  mov     qword ptr [rsp + 24], 8
>  xorps   xmm0, xmm0
>  movups  xmmword ptr [rsp + 32], xmm0
>  lea     rdi, [rsp + 8]
>  call    qword ptr [rip + std::io::stdio::_print]
>  add     rsp, 56
>  ret
> ```
>
> after:
> ```asm
> main:
>  lea     rsi, [rip + .Lanon_hello_world]
>  mov     edi, 29
>  jmp     qword ptr [rip + std::io::stdio::_print]
> ```

(`panic!("Hello, world!");` shows a similar change.)

---

This implementation stores all static information as just a single (byte) string, without any indirection:

> code:
> ```rust
> format_args!("Hello, {name:-^20}!")
> ```
>
> lowering before:
> ```rust
> fmt::Arguments::new_v1_formatted(
>     &["Hello, ", "!\n"],
>     &args,
>     &[
>         Placeholder {
>             position: 0usize,
>             flags: 3355443245u32,
>             precision: format_count::Implied,
>             width: format_count::Is(20u16),
>         },
>     ],
> )
> ```
>
> lowering after:
> ```rust
> fmt::Arguments::new(
>     b"\x07Hello, \xc3-\x00\x00\xc8\x14\x00\x02!\n\x00",
>     &args,
> )
> ```

This saves a ton of pointers and simplifies the expansion significantly, but does mean that individual pieces (e.g. `"Hello, "` and `"!\n"`) cannot be reused. (Those pieces are often smaller than a pointer to them, though, in which case reusing them is useless.)

---

The details of the new representation are documented in [library/core/src/fmt/mod.rs](https://github.com/m-ou-se/rust/blob/new-fmt-args-alt/library/core/src/fmt/mod.rs#L609-L712).

Author: bors

compiler/rustc_ast_lowering/src/expr.rs

@@ -13,7 +13,7 @@ use rustc_middle::span_bug;
 use rustc_middle::ty::TyCtxt;
 use rustc_session::errors::report_lit_error;
 use rustc_span::source_map::{Spanned, respan};
-use rustc_span::{DUMMY_SP, DesugaringKind, Ident, Span, Symbol, sym};
+use rustc_span::{ByteSymbol, DUMMY_SP, DesugaringKind, Ident, Span, Symbol, sym};
 use thin_vec::{ThinVec, thin_vec};
 use visit::{Visitor, walk_expr};
 
@@ -924,7 +924,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     arena_vec![self; new_unchecked, get_context],
                 ),
             };
-            self.arena.alloc(self.expr_unsafe(call))
+            self.arena.alloc(self.expr_unsafe(span, call))
         };
 
         // `::std::task::Poll::Ready(result) => break result`
@@ -1832,7 +1832,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         arena_vec![self; iter],
                     ));
                     // `unsafe { ... }`
-                    let iter = self.arena.alloc(self.expr_unsafe(iter));
+                    let iter = self.arena.alloc(self.expr_unsafe(head_span, iter));
                     let kind = self.make_lowered_await(head_span, iter, FutureKind::AsyncIterator);
                     self.arena.alloc(hir::Expr { hir_id: self.next_id(), kind, span: head_span })
                 }
@@ -1887,7 +1887,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     arena_vec![self; iter],
                 ));
                 // `unsafe { ... }`
-                let iter = self.arena.alloc(self.expr_unsafe(iter));
+                let iter = self.arena.alloc(self.expr_unsafe(head_span, iter));
                 let inner_match_expr = self.arena.alloc(self.expr_match(
                     for_span,
                     iter,
@@ -2103,30 +2103,18 @@ impl<'hir> LoweringContext<'_, 'hir> {
         self.arena.alloc(self.expr(sp, hir::ExprKind::Tup(&[])))
     }
 
-    fn expr_uint(&mut self, sp: Span, ty: ast::UintTy, value: u128) -> hir::Expr<'hir> {
+    pub(super) fn expr_str(&mut self, sp: Span, value: Symbol) -> hir::Expr<'hir> {
         let lit = hir::Lit {
             span: self.lower_span(sp),
-            node: ast::LitKind::Int(value.into(), ast::LitIntType::Unsigned(ty)),
+            node: ast::LitKind::Str(value, ast::StrStyle::Cooked),
         };
         self.expr(sp, hir::ExprKind::Lit(lit))
     }
 
-    pub(super) fn expr_usize(&mut self, sp: Span, value: usize) -> hir::Expr<'hir> {
-        self.expr_uint(sp, ast::UintTy::Usize, value as u128)
-    }
-
-    pub(super) fn expr_u32(&mut self, sp: Span, value: u32) -> hir::Expr<'hir> {
-        self.expr_uint(sp, ast::UintTy::U32, value as u128)
-    }
-
-    pub(super) fn expr_u16(&mut self, sp: Span, value: u16) -> hir::Expr<'hir> {
-        self.expr_uint(sp, ast::UintTy::U16, value as u128)
-    }
-
-    pub(super) fn expr_str(&mut self, sp: Span, value: Symbol) -> hir::Expr<'hir> {
+    pub(super) fn expr_byte_str(&mut self, sp: Span, value: ByteSymbol) -> hir::Expr<'hir> {
         let lit = hir::Lit {
             span: self.lower_span(sp),
-            node: ast::LitKind::Str(value, ast::StrStyle::Cooked),
+            node: ast::LitKind::ByteStr(value, ast::StrStyle::Cooked),
         };
         self.expr(sp, hir::ExprKind::Lit(lit))
     }
@@ -2262,9 +2250,12 @@ impl<'hir> LoweringContext<'_, 'hir> {
         self.expr(span, expr_path)
     }
 
-    fn expr_unsafe(&mut self, expr: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
+    pub(super) fn expr_unsafe(
+        &mut self,
+        span: Span,
+        expr: &'hir hir::Expr<'hir>,
+    ) -> hir::Expr<'hir> {
         let hir_id = self.next_id();
-        let span = expr.span;
         self.expr(
             span,
             hir::ExprKind::Block(
@@ -2302,15 +2293,6 @@ impl<'hir> LoweringContext<'_, 'hir> {
         self.arena.alloc(self.expr_block(b))
     }
 
-    pub(super) fn expr_array_ref(
-        &mut self,
-        span: Span,
-        elements: &'hir [hir::Expr<'hir>],
-    ) -> hir::Expr<'hir> {
-        let array = self.arena.alloc(self.expr(span, hir::ExprKind::Array(elements)));
-        self.expr_ref(span, array)
-    }
-
     pub(super) fn expr_ref(&mut self, span: Span, expr: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
         self.expr(span, hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Not, expr))
     }