rustc 1.18 beta type inference regression

[vbarrielle]

Hello, it looks like type inference has a regression on the latest beta.

With rustc at the version rustc 1.18.0-beta.2 (f4e8e81c3 2017-05-11), the code at https://github.com/vbarrielle/sprs/blob/master/src/sparse/triplet.rs#L464-L483 fails with:

error[E0282]: type annotations needed
   --> src/sparse/triplet.rs:477:13
    |
477 |         let csc = triplet_mat.to_csc();
    |             ^^^
    |             |
    |             consider giving `csc` a type
    |             cannot infer type for `sparse::CsMat<f64, _, std::vec::Vec<usize>, std::vec::Vec<usize>, std::vec::Vec<f64>>`

error: aborting due to previous error

error: Could not compile `sprs`.

However, this compiles fine on stable. It also compiles fine on the latest nightly, so it may have been fixed while fixing another type inference bug eg #41796.

[vbarrielle]

This now compiles on latest beta rustc 1.18.0-beta.4 (0308c9865 2017-05-27).