unresolved import

[wcampbell0x2a]

having the following problem when compiling this library:

   Compiling rustyline-derive v0.3.1
error[E0432]: unresolved import `proc_macro`
 --> /home/$USER/.cargo/registry/src/github.com-1ecc6299db9ec823/rustyline-derive-0.3.1/src/lib.rs:1:5
  |
1 | use proc_macro::TokenStream;
  |     ^^^^^^^^^^ help: a similar path exists: `syn::proc_macro`
error: aborting due to previous error
For more information about this error, try `rustc --explain E0432`.
error: could not compile `rustyline-derive`.
To learn more, run the command again with --verbose.

[bjorn3]

Which version of rust are you using? (rustc -V)

[gwenn]

https://doc.rust-lang.org/proc_macro/index.html

1.15.0

[bjorn3]

rustyline-derive requires the 2018 edition of rust, which was introduced in rustc 1.31.

[gwenn]

@bjorn3 Sorry, you are right.

[hibachrach]

I'm encountering this after cloning and running examples. Normal cargo build works fine. Sorry, that's just for the top level crate, not for rustyline-derive.

$ rustc -V
rustc 1.41.0
$ cargo -V
cargo 1.41.0

EDIT: This is resolved when extern crate proc_macro; is prepended to the offending file rustyline-derive/src/lib.rs.

EDIT 2: It seems this is intended?

• https://users.rust-lang.org/t/how-to-use-proc-macro-on-rust-2018/20833/3
• use proc_macro regressed from nightly-2018-09-14 to nightly 2018-09-15 in edition 2018 rust-lang/rust#54418

[gwenn]

rust-lang/cargo#7700
=> cargo 1.42