Expose span locations on stable

[dtolnay]

In the way suggested by #165 (comment).

[mitsuhiko]

Currently fails like this with the feature enabled:

error[E0433]: failed to resolve: use of undeclared type or module `RefCell`
   --> /Users/mitsuhiko/.cargo/git/checkouts/proc-macro2-bc9077ffc3c20345/f24566c/src/fallback.rs:228:40
    |
228 |     static CODEMAP: RefCell<Codemap> = RefCell::new(Codemap {
    |                                        ^^^^^^^ use of undeclared type or module `RefCell`

error[E0412]: cannot find type `RefCell` in this scope
   --> /Users/mitsuhiko/.cargo/git/checkouts/proc-macro2-bc9077ffc3c20345/f24566c/src/fallback.rs:228:21
    |
228 |     static CODEMAP: RefCell<Codemap> = RefCell::new(Codemap {
    |                     ^^^^^^^ not found in this scope
help: possible candidate is found in another module, you can import it into scope
    |
5   | use std::cell::RefCell;
    |

error[E0412]: cannot find type `Codemap` in this scope
   --> /Users/mitsuhiko/.cargo/git/checkouts/proc-macro2-bc9077ffc3c20345/f24566c/src/fallback.rs:228:29
    |
228 |     static CODEMAP: RefCell<Codemap> = RefCell::new(Codemap {
    |                             ^^^^^^^ not found in this scope

error[E0422]: cannot find struct, variant or union type `Codemap` in this scope
   --> /Users/mitsuhiko/.cargo/git/checkouts/proc-macro2-bc9077ffc3c20345/f24566c/src/fallback.rs:228:53
    |
228 |     static CODEMAP: RefCell<Codemap> = RefCell::new(Codemap {
    |                                                     ^^^^^^^ not found in this scope

error[E0422]: cannot find struct, variant or union type `FileInfo` in this scope
   --> /Users/mitsuhiko/.cargo/git/checkouts/proc-macro2-bc9077ffc3c20345/f24566c/src/fallback.rs:231:21
    |
231 |         files: vec![FileInfo {
    |                     ^^^^^^^^ not found in this scope

error[E0609]: no field `lo` on type `&fallback::Span`
   --> /Users/mitsuhiko/.cargo/git/checkouts/proc-macro2-bc9077ffc3c20345/f24566c/src/fallback.rs:379:40
    |
379 |             fi.offset_line_column(self.lo as usize)
    |                                        ^^

error[E0609]: no field `hi` on type `&fallback::Span`
   --> /Users/mitsuhiko/.cargo/git/checkouts/proc-macro2-bc9077ffc3c20345/f24566c/src/fallback.rs:388:40
    |
388 |             fi.offset_line_column(self.hi as usize)
    |                                        ^^

error[E0560]: struct `fallback::Span` has no field named `lo`
   --> /Users/mitsuhiko/.cargo/git/checkouts/proc-macro2-bc9077ffc3c20345/f24566c/src/fallback.rs:233:26
    |
233 |             span: Span { lo: 0, hi: 0 },
    |                          ^^ `fallback::Span` does not have this field

error[E0560]: struct `fallback::Span` has no field named `hi`
   --> /Users/mitsuhiko/.cargo/git/checkouts/proc-macro2-bc9077ffc3c20345/f24566c/src/fallback.rs:233:33
    |
233 |             span: Span { lo: 0, hi: 0 },
    |                                 ^^ `fallback::Span` does not have this field

error: aborting due to 9 previous errors

[mitsuhiko]

Can confirm this works!

[dtolnay]

Tagging wip because this needs tests. Does not work when I tried it:

use proc_macro2::TokenTree;

fn main() {
    let sp = syn::parse_str::<TokenTree>("testing").unwrap().span();
    println!("start line={} column={}", sp.start().line, sp.start().column);
    println!("end line={} column={}", sp.end().line, sp.end().column);
}

start line=1 column=0
end line=1 column=0

I am not going to be able to get to this today. @mitsuhiko could you investigate?

[mitsuhiko]

Yes, will investigate. Information seems absent indeed.

[alexcrichton]

If we're going to offer this in a published stable version then I think we can probably go ahead and do this without a feature. The proc-macro2 crate can't have many major version bumps due to the degree of reliance on it through syn in the proc-macro ecosystem, so being stable is a big commitment. If we're willing to commit to this as a stable feature then I think it's worth providing by default.

I think that may also simplify the cfg story here (which I am always confused by every time I look at this!). That way outside of a proc-macro context you always have accurate information, but in a proc-macro context you'll only have accurate information if a nightly feature toggle is enabled.

[dtolnay]

I kept a feature because last time I checked there was a big performance hit for Span going from 0 bytes to 8 bytes. There are a lot of spans in the syntax tree...

[alexcrichton]

Ah ok that makes sense to me, and in that case seems fine to document as such as to why it's a feature

[mystor]

Hmm, I wonder if a span interning system could compress span sizes down even more, and if that would be helpful here.

[dtolnay]

I added a note in build.rs about the performance hit.

[dtolnay]

I think this works now.

I filed #167 to follow up on mitigating the performance impact.

[mitsuhiko]

Is this blocked on #167 or is it mergeable as such?

[dtolnay]

I don't think this needs to block on the performance work, because the increase in size of Span only happens if you opt in to the new feature. Thanks for the reminder!

[dtolnay]

Published in 0.4.27.