mac-0.1.0 fails test suite due to type inference regression in Rust 1.15

[brson]

   Doc-tests mac

running 12 tests
test format_if_0 ... FAILED
test do_while_1 ... ok
test addrs_of_0 ... ok
test do_while_0 ... ok
test inspect_1 ... ignored
test if_cfg_0 ... ok
test if_cfg_1 ... ok
test match_cfg_0 ... ok
test inspect_0 ... ok
test matches_0 ... ok
test unwrap_or_return_0 ... ok
test test_eq_0 ... ok

failures:

---- format_if_0 stdout ----
        error[E0283]: type annotations required: cannot resolve `_: std::borrow::ToOwned`
  --> <anon>:11:21
   |
11 |   let not_formatted = format_if!(false, "Vague error", "Error code {:?}", {
   |  _____________________^ starting here...
12 | |     // Note that the argument is not evaluated.
13 | |     panic!("oops");
14 | | });
   | |__^ ...ending here
   |
   = note: required by `std::borrow::Cow::Owned`
   = note: this error originates in a macro outside of the current crate

error: aborting due to previous error(s)

thread 'format_if_0' panicked at 'Box<Any>', /buildslave/rust-buildbot/slave/stable-dist-rustc-linux/build/src/librustc/session/mod.rs:201
note: Run with `RUST_BACKTRACE=1` for a backtrace.
https://github.com/reem/rust-mac dcc9155e9d23a81e92e325899d4b6bed098bcded

failures:
    format_if_0

test result: FAILED. 10 passed; 1 failed; 1 ignored; 0 measured

error: test failed

This is on 1.15. Not 1.14.

cc @reem

[nagisa]

Likely #39440? This is not considered a breakage.

Actionable items: send PR to the crate or inform the author of the issue.

[durka]

Merged 10 days ago and it's in stable already?

[nagisa]

Oh well, then whatever other new impl for Cow?

[durka]

Yeah. Why wasn't this caught during the beta period though?

[durka]

It's actually because of the never-type stuff. The type error is complaining about a block that says { panic!(); }. Annotating the type as Cow<str> fixes it.

[durka]

Minimized:

use std::borrow::Cow;

fn main() {
    let _ = if false {
        Cow::Owned(format!("{:?}", panic!())) /* as Cow<str> */ // uncomment to fix
    } else {
        Cow::Borrowed("")
    };
}

[nagisa]

Oh wow, this sounds nasty. Especially since typeck shouldn’t even bother figuring out the type of the branch with panic!() in it.

[nikomatsakis]

This is because of #39485, where we updated our handling of diverging types to be more consistent.

[nikomatsakis]

Oh, well, it says stable-to-stable, so I guess not. But yes, related to the handling of diverging types.

[nikomatsakis]

cc @eddyb @canndrew