Rust package using unstable library feature 'iter_nth_back'

Question

I have a Rust application that I don't know much about, and I'm calling it from a Python program that I'm working on. I haven't changed anything in the Rust source code, but the build process for these two projects just stopped working with an error like this:

[... several similar errors]
error[E0658]: use of unstable library feature 'iter_nth_back'
   --> /usr/local/cargo/registry/src/github.com-1ecc6299db9ec823/ndarray-0.13.0/src/iterators/mod.rs:585:46
    |
585 |         either_mut!(self.inner, iter => iter.nth_back(n))
    |                                              ^^^^^^^^
    |
    = note: for more information, see https://github.com/rust-lang/rust/issues/56995

error: aborting due to 5 previous errors

For more information about this error, try `rustc --explain E0658`.
error: Could not compile `ndarray`.

I looked at the list of ndarray releases, and release 0.13.0 came out a few days ago, so I suspect that it's incompatible with my build environment.

How can I get my build working again?

Here are the full steps to reproduce the problem:

$ sudo docker run -it --rm rust:1.36.0
# USER=foo cargo new hello_world --bin
     Created binary (application) `hello_world` package
# cd hello_world/
# echo 'bio = "^0"' >> Cargo.toml
# cargo build
    Updating crates.io index
  Downloaded bio v0.29.0
  Downloaded [... many more ...]
  Downloaded ndarray v0.13.0
  Downloaded [... many more ...]
   Compiling proc-macro2 v1.0.4
   Compiling [... many more ...]
   Compiling ndarray v0.13.0
   Compiling [... many more ...]
   Compiling statrs v0.11.0
error[E0658]: use of unstable library feature 'iter_nth_back'
   --> /usr/local/cargo/registry/src/github.com-1ecc6299db9ec823/ndarray-0.13.0/src/iterators/mod.rs:134:5
    |
134 | /     fn nth_back(&mut self, n: usize) -> Option<*mut A> {
135 | |         let index = self.index?;
136 | |         let len = self.dim[0] - index[0];
137 | |         if n < len {
...   |
147 | |         }
148 | |     }
    | |_____^
    |
    = note: for more information, see https://github.com/rust-lang/rust/issues/56995
[... other, similar errors ...]
   Compiling serde_derive v1.0.101
   Compiling strum_macros v0.16.0
   Compiling snafu-derive v0.5.0
error: aborting due to 5 previous errors

For more information about this error, try `rustc --explain E0658`.
error: Could not compile `ndarray`.
warning: build failed, waiting for other jobs to finish...
error: build failed
#

Interestingly, if I run the latest docker image for rust, 1.38, I don't see the error. However, my build process is installing Rust from Ubuntu Xenial's package manager, so I probably can't upgrade it easily.

Answer 1

New Answer: Upgrade Rust

It turned out that upgrading rust wasn't as hard as I thought, and then I don't have to change the Rust source code.

In my .travis.yml file, I replaced this:

- sudo apt-get install -y cargo

with this:

- sudo curl https://sh.rustup.rs -sSf | sh -s -- -y
- source ~/.cargo/env

Original Answer: Pin Dependencies

I tried adding an explicit ndarray dependency and pinning it at 0.12, but that didn't work. Then I noticed that bio also had a new release in the last few days. Pinning that dependency worked.

$ sudo docker run -it --rm rust:1.36.0
# USER=foo cargo new hello_world --bin
     Created binary (application) `hello_world` package
# cd hello_world/
# echo 'bio = "~0.28.2"' >> Cargo.toml
# cargo build
    Updating crates.io index
  Downloaded bio v0.28.2
  Downloaded [... many more ...]
  Downloaded ndarray v0.12.1
  Downloaded [... many more ...]
   Compiling proc-macro2 v1.0.4
   Compiling [... many more ...]
   Compiling ndarray v0.12.1
   Compiling [... many more ...]
   Compiling csv v1.1.1
   Compiling hello_world v0.1.0 (/hello_world)
    Finished dev [unoptimized + debuginfo] target(s) in 14m 15s
#