error: the trait `core::fmt::String` is not implemented for the type

[GildedHonour]

$ cargo test
error: the trait `core::fmt::String` is not implemented for the type `core::option::Option<collections::string::String>` [E0277]
/Users/alex/.cargo/git/checkouts/gcc-rs-49038096d5e74406/master/src/lib.rs:28     println!("{} = {}", v, r);
                                                                                                         ^
note: in expansion of format_args!
<std macros>:2:42: 2:75 note: expansion site
<std macros>:1:1: 2:77 note: in expansion of println!
/Users/alex/.cargo/git/checkouts/gcc-rs-49038096d5e74406/master/src/lib.rs:28:5: 28:31 note: expansion site
/Users/alex/.cargo/registry/src/github.com-1ecc6299db9ec823/gcc-0.1.0/src/lib.rs:102:29: 102:32 error: the trait `core::fmt::Display` is not implemented for the type `&mut std::io::process::Command` [E0277]
/Users/alex/.cargo/registry/src/github.com-1ecc6299db9ec823/gcc-0.1.0/src/lib.rs:102     println!("running: {}", cmd);
                                                                                                                 ^~~
note: in expansion of format_args!
<std macros>:2:42: 2:75 note: expansion site
<std macros>:1:1: 2:77 note: in expansion of println!
/Users/alex/.cargo/registry/src/github.com-1ecc6299db9ec823/gcc-0.1.0/src/lib.rs:102:5: 102:34 note: expansion site
/Users/alex/.cargo/git/checkouts/gcc-rs-49038096d5e74406/master/src/lib.rs:106:29: 106:32 error: the trait `core::fmt::Display` is not implemented for the type `&mut std::io::process::Command` [E0277]
/Users/alex/.cargo/git/checkouts/gcc-rs-49038096d5e74406/master/src/lib.rs:106     println!("running: {}", cmd);
                                                                                                           ^~~
note: in expansion of format_args!
<std macros>:2:42: 2:75 note: expansion site
<std macros>:1:1: 2:77 note: in expansion of println!
/Users/alex/.cargo/git/checkouts/gcc-rs-49038096d5e74406/master/src/lib.rs:106:5: 106:34 note: expansion site
error: aborting due to previous error
Build failed, waiting for other jobs to finish...
error: aborting due to 2 previous errors
Could not compile `gcc`.

$ cargo -V
cargo 0.0.1-pre-nightly (bb28e71 2015-01-22 06:06:34 +0000)

$ rustc -V
rustc 1.0.0-nightly (4be79d6ac 2015-01-23 16:08:14 +0000)

[nagisa]

This behaviour is correct. You want to use {:?} formatter, which corresponds to Debug and is implemented for Option<T> where T:Debug.

[steveklabnik]

It's worth noting that last week, String and Show were changed to Display and Debug, but yes, this is the intended behavior here.

[GildedHonour]

This behaviour is correct. You want to use {:?} formatter, which corresponds to Debug and is implemented for Option where T:Debug.

Did you look the source of the error at all? I'm not the once who's code caused that error, in my code there's no println!("{} = {}", v, r); and println!("running: {}", cmd); or something similar.

I don't know where it's exactly. Isn't it in rust compiler or cargo?

[nagisa]

@GildedHonour the error happens in gcc crate on which you depend. You want to report an issue on gcc-rs repository.

EDIT: looking at gcc-rs the HEAD uses {:?}, so you possibly have dependency on an outdated version gcc-rs somewhere.