Support negative numbers in Literal::from_str

[dtolnay]

proc_macro::Literal has allowed negative numbers in a single literal token ever since Rust 1.29, using https://doc.rust-lang.org/stable/proc_macro/struct.Literal.html#method.isize_unsuffixed and similar constructors.

let lit = proc_macro::Literal::isize_unsuffixed(-10);

However, the suite of constructors on Literal is not sufficient for all use cases, for example arbitrary precision floats, or custom suffixes in FFI macros.

let lit = proc_macro::Literal::f64_unsuffixed(0.101001000100001000001000000100000001); // :(
let lit = proc_macro::Literal::i???_suffixed(10ulong); // :(

For those, macros construct the literal using from_str instead, which preserves arbitrary precision, custom suffixes, base, and digit grouping.

let lit = "0.101001000100001000001000000100000001".parse::<Literal>().unwrap();
let lit = "10ulong".parse::<Literal>().unwrap();
let lit = "0b1000_0100_0010_0001".parse::<Literal>().unwrap();

However, until this PR it was not possible to construct a literal token that is both negative and preserving of arbitrary precision etc.

This PR fixes Literal::from_str to recognize negative integer and float literals.

[rust-highfive]

r? @LeSeulArtichaut

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[LeSeulArtichaut]

r? @Aaron1011 Do you want to take this?

[Aaron1011]

This looks like a pre-existing bug - this literal will not round-trip, since "- 10" will result in a parse error. Could you open an issue for this?

[dtolnay]

It's fixed in #87267.

[Aaron1011]

Sorry for the delay in getting to this.

@bors r+

[bors]

📌 Commit 55ff45a has been approved by Aaron1011

[bors]

⌛ Testing commit 55ff45a with merge e91405b...

[bors]

☀️ Test successful - checks-actions
Approved by: Aaron1011
Pushing e91405b to master...