generated call to core::fmt::Arguments::new_const fails to constant fold

[lolbinarycat]

normal code:

pub fn main() {
    println!("hello");
}

code that should be equivalent, but is actually more efficient

#![feature(print_internals)]
#![feature(const_fmt_arguments_new)]

pub fn main() {
    ::std::io::_print(const { format_args!("hello\n") }); 
}

godbolt link for asm comparison

[lolbinarycat]

possible workaround: during the code generation at compiler/rustc_ast_lowering/src/format.rs:474, wrap the call expr in a ConstBlock expr.

[SkiFire13]

This is not a valid optimization in general, as soon as format_args captures a variable this becomes invalid.

#![feature(print_internals)]
#![feature(const_fmt_arguments_new)]

pub fn main() {
    let n = 42;
    // println!("hello {n}"); // Compiles fine
    ::std::io::_print(const { format_args!("hello {n}\n") }); // Error
}

[lolbinarycat]

yes, but there's already a special case in codegen for when there's no arguments (which happens after the implicit named argument desugaring). this special case is what calls new_const, but it doesn't actually get folded.

[veera-sivarajan]

@rustbot claim