serde no longer builds with latest rustc (master)

[RalfJung]

Looks like something that landed in the commit range 0b680cf...d4e1565 broke building serde:

error[E0433]: failed to resolve: unresolved import
   --> /home/travis/.cargo/registry/src/github.com-1ecc6299db9ec823/serde_derive-1.0.94/src/internals/ast.rs:142:44
    |
142 |     variants: &'a Punctuated<syn::Variant, Token![,]>,
    |                                            ^^^^^^^^^
    |                                            |
    |                                            unresolved import
    |                                            help: a similar path exists: `syn::token`
    |
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
error[E0433]: failed to resolve: unresolved import
   --> /home/travis/.cargo/registry/src/github.com-1ecc6299db9ec823/serde_derive-1.0.94/src/internals/ast.rs:187:40
    |
187 |     fields: &'a Punctuated<syn::Field, Token![,]>,
    |                                        ^^^^^^^^^
    |                                        |
    |                                        unresolved import
    |                                        help: a similar path exists: `syn::token`
    |
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

I would try to reproduce locally but rustup-toolchain-install-master takes many hours to download rustc+libstd currently. (This bug is not yet on nightly, just in master.)

Here's a prior succeeding build for reference.

[Mark-Simulacrum]

This is #62393 almost certainly. @petrochenkov plans to investigate.

[petrochenkov]

Minimized and reproduced.

#62393 broke something unexpected - macro encoding in metadata, which is also done through pretty-printing at the moment (note that this is not a proper way to encode macro definitions, we don't have cross-crate hygiene due to this).

Nested macros in particular become broken in metadata:

macro_rules! generate_exported { () => {
    #[macro_export]
    macro_rules! exported {
        () => ($crate::S)
    }
}}

generate_exported!();

I still need to investigate why exactly this happens though, printing for metadata is done by tts_to_string which was supposed to print $crate as $crate without any conversions.

[petrochenkov]

Fixed in #62574.

[gbenattar]

Can we get an update?

[dtolnay]

It will take a day or two for the bug to be fixed in nightly. I would recommend using an older nightly for now; nightly-2019-07-10 or older should work.

[AlterionX]

serde-rs/serde#1568 actually, one more day before that, nightly-2019-07-09

[dtolnay]

nightly-2019-07-10 does work. The version reported by the compiler binary is off by one. The first broken nightly is nightly-2019-07-11 which contains "rustc 1.38.0-nightly (cd2cd4c 2019-07-10)".

[AlterionX]

Ah, okay, my bad.

[pnkfelix]

triage: P-high, has PR (#62574), assigning to @petrochenkov and removing nomination.