make format! accept const vars as format-strings

[matthiaskrgr]

I wonder if we could make rustc accept code like this:

    const y: &str = "hello {}";
    let x = format!(y, "world");

Currently this errors with:

error: format argument must be a string literal
 --> src/main.rs:3:21
  |
3 |     let x = format!(y, "world");
  |                     ^
  |
help: you might be missing a string literal to format with
  |
3 |     let x = format!("{} {}", y, "world");
  |                     ^^^^^^^^

rustc 1.43.0-nightly (58b834344 2020-02-05)

[sfackler]

Macros run before name resolution, so there's no way for format! to find the contents of the string y.

[Centril]

Closing with @sfackler's explanation.

[vgsantoniazzi]

If you want to interpolate like that, you can use stringify!

https://doc.rust-lang.org/std/macro.stringify.html

[EricHe56]

maybe a inline function can help on it.

[dtolnay]

I wonder whether this can be reconsidered now after #106745, as processing the format string no longer occurs during macro expansion, so it could do name resolution.

@m-ou-se?

[m-ou-se]

Processing the string still happens during macro expansion actually. (The format string syntax is directly parsed, implicit arguments are directly resolved to their indexes, etc. etc.) It's just that instead of expanding to a fmt::Arguments::new(…) expression directly, it will expand to a special FormatArgs AST node first, which will be expanded to the call expression when lowering the AST to HIR.

(And if we want to be able to resolve names/constants, I think we'd need to postpone the parsing not only until the AST lowering, but even later in the HIR.)