Proc macro hygiene regression

[SimonSapin]

Something in the range bb42071...f9b0897, likely #46343 (CC @jseyfried), broke Servo.

We have a #[dom_struct] attribute implemented in a dom_struct crate like this:

#[proc_macro_attribute]
pub fn dom_struct(args: TokenStream, input: TokenStream) -> TokenStream {
    if !args.is_empty() {
        panic!("#[dom_struct] takes no arguments");
    }
    let attributes = quote! {
        #[derive(DenyPublicFields, DomObject, JSTraceable, MallocSizeOf)]
        #[must_root]
        #[repr(C)]
    };
    iter::once(attributes).chain(iter::once(input)).collect()
}

Each of the derives is defined in a respective crate. The script crate depends on all of these crates and uses #[dom_struct]. Some of the derives generate code that reference items defined in script. For example, #[derive(DomObject)] implements the script::dom::bindings::refector::DomObject trait.

Since rustc 1.24.0-nightly (f9b0897 2017-12-02), every use of #[dom_struct] fails with:

error[E0433]: failed to resolve. Could not find `js` in `{{root}}`
  --> components/script/dom/attr.rs:28:1
   |
28 | #[dom_struct]
   | ^^^^^^^^^^^^^ Could not find `js` in `{{root}}`

error[E0433]: failed to resolve. Could not find `dom` in `{{root}}`
  --> components/script/dom/attr.rs:28:1
   |
28 | #[dom_struct]
   | ^^^^^^^^^^^^^ Could not find `dom` in `{{root}}`

error[E0433]: failed to resolve. Could not find `js` in `{{root}}`
  --> components/script/dom/attr.rs:28:1
   |
28 | #[dom_struct]
   | ^^^^^^^^^^^^^ Could not find `js` in `{{root}}`

error[E0433]: failed to resolve. Could not find `malloc_size_of` in `{{root}}`
  --> components/script/dom/attr.rs:28:1
   |
28 | #[dom_struct]
   | ^^^^^^^^^^^^^ Could not find `malloc_size_of` in `{{root}}`

I suppose that these errors come from code generated by the derives, and that {{root}} refers to the root of the dom_struct crate where the #[derive(…)] tokens come from. Indeed, the js and dom are not an cannot be available there, they’re in the script crate which depends on dom_struct.

We can work around this by erasing hygiene data in the #[derive(…)] tokens:

--- a/components/dom_struct/lib.rs
+++ b/components/dom_struct/lib.rs
@@ -17,7 +17,9 @@ pub fn dom_struct(args: TokenStream, input: TokenStream) -> TokenStream {
     let attributes = quote! {
         #[derive(DenyPublicFields, DomObject, JSTraceable, MallocSizeOf)]
         #[must_root]
         #[repr(C)]
     };
+    let attributes = attributes.to_string().parse().unwrap();
     iter::once(attributes).chain(iter::once(input)).collect()
 }

… but this seems like a valid pattern that shouldn’t necessitate such hacks.

[Mark-Simulacrum]

This also broke perf.rlo (due to breaking servo).

[SimonSapin]

rust-lang/rustc-perf#171 applies the work around to the servo copy in perf.rlo.

[nrc]

This seems like correct behaviour. The derive names must be in scope where the macro is defined, not where it is used. I don't recall how derives are looked up though, so I'm not sure if that mechanism works correctly with the proper hygiene here - we should re-open or open another issue if that doesn't work.

Stripping the hygiene info as in the work-around (which is actually setting all the hygiene info to the macro use site) is not recommended.

[nrc]

Urgh, sorry, I misunderstood what is going on here - it seems that we're looking up and expanding the derives fine. Given that the derives (unhygienic) are being expanded inside a hygienic context, I'm not even sure where I would expect them to look for their names.

[SimonSapin]

is not recommended

I do realize this is a hack that I’d rather was not needed, but this is the only way I’ve found to make this existing code compile with a newer compiler, which unblocks unrelated work.

[victorporof]

This broke rsx. Latest nightly that works is from Dec 1.

This doesn't work anymore:

let foo = 42;
...
my_proc_macro! { foo }

[victorporof]

A more explicative use case for this in case of rsx would be the following:

let stylesheet = ...;
let node = rsx! {
  <view style={stylesheet.get(".root")}>
      Hello world!
  </view>
};

where rsx! is a procedural macro.