"cannot call a generic method through an object"

[oblitum]

I've just built rust HEAD and I'm getting this error for the try macro at src/cargo/lib.rs when building cargo.

src/cargo/lib.rs:51:1: 59:2 note: in expansion of try!
src/cargo/util/toml.rs:192:23: 194:13 note: expansion site
src/cargo/lib.rs:54:35: 54:49 error: cannot call a generic method through an object
src/cargo/lib.rs:54         match $expr.map_err(|err| err.to_error()) {
                                                      ^~~~~~~~~~~~~~

[steveklabnik]

What's odd is that HEAD did build on bors: 75ca63d#commitcomment-6850056

[alexcrichton]

Cargo currently tracks the latest nightly, and it appears that this change happened after the latest nightly.

[steveklabnik]

The commit that introduced this line is 11 days old, though. Or you mean, a change in rustc? IRC seemed to imply this would have never worked.

[alexcrichton]

I expect that rustc changed overnight. This is definitely working as we've landed many things, and I'm building/testing it locally on the current nightly and it's working.

[simonask]

Seeing this too, as many others. It's unfortunate, since Cargo got a lot of attention from today's TWiR.

Building cargo with a fresh install of Rust from this commit:
rust-lang/rust@efebeec

[huonw]

I guess this is due to rust-lang/rust#15242 changing which method is being called. The two relevant things are

trait CargoError {
// ...
    fn to_error<E: FromError<Self>>(self) -> E {
// ...
}

impl CargoError for Box<CargoError> {
// ...

previously (Box<CargoError>).to_error() would be calling the method given via the impl there (i.e. self: Box<CargoError>), but after #15242 it's now trying to call the method from the vtable (i.e. self: erased type).

See this comment too.