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New format_args!() and fmt::Arguments implementation Part of https://github.com/rust-lang/rust/issues/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).
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