Add unstable Literal::subspan().

[SergioBenitez]

Take 2 of #55971. Still ~wrong, but now with a comment! (and less of a surface) Unblocks #49219.

r? @alexcrichton

[alexcrichton]

@bors: r+

This is unstable, helps unblock other PRs, is within the ballpark of what we'd like to stabilize long-term, and looks solid enough for immediate use. Sounds good to me!

[bors]

📌 Commit 09e7051 has been approved by alexcrichton

[eddyb]

I don't think this is correct. Literal::to_string should show the exact source contents, if the literal isn't synthetic.

[SergioBenitez]

This is correct. Try:

externcrate proc_macro;usecrate::proc_macro::Literal;usecrate::proc_macro::TokenStream;#[proc_macro]pubfngo(_:TokenStream) -> TokenStream{let literal = Literal::string("hel\u{6c}o");println!("literal: {}", literal.to_string());let literal = Literal::character('c');println!("literal: {}", literal.to_string());let literal = Literal::character(100aschar);println!("literal: {}", literal.to_string());let literal = Literal::byte_string(b"hello");println!("literal: {}", literal.to_string());let literal = Literal::byte_string(b"he\x42");println!("literal: {}", literal.to_string());let literal = Literal::byte_string(&['h'asu8,'e'asu8,]);println!("literal: {}", literal.to_string());let literal = Literal::f32_unsuffixed(1.0);println!("literal: {}", literal.to_string());let literal = Literal::f32_unsuffixed(1.134100000022);println!("literal: {}", literal.to_string());TokenStream::new()}

Edit: This may be different if the Literal comes from a TokenStream parsed from source. Nevertheless, this method needs to be correct regardless of where the Literal comes from.

[eddyb]

If it's synthetic then the span doesn't matter, I was talking about Literals from the source. But also, we shouldn't be using escape_unicode like that.