Use associated function syntax for `Debug` and `Display` backtrace impl

[azriel91]

Rust 1.31 reports a compilation error saying it is ambiguous which
fmt method to call.

---

Not sure if this happened prior to Rust 1.31, but when compiling failure using Rust 1.31.0 (stable) this error shows up:

[07:33:30] failure master
$ cargo +stable test --all
...
error[E0034]: multiple applicable items in scope
   --> src\backtrace\mod.rs:132:20
    |
132 |                 bt.fmt(f)
    |                    ^^^ multiple `fmt` found
    |
    = note: candidate #1 is defined in an impl of the trait `std::fmt::Debug` for the type `backtrace::backtrace::Backtrace`
    = note: candidate #2 is defined in an impl of the trait `std::fmt::Display` for the type `backtrace::backtrace::Backtrace`

error[E0034]: multiple applicable items in scope
   --> src\backtrace\mod.rs:140:20
    |
140 |                 bt.fmt(f)
    |                    ^^^ multiple `fmt` found
    |
    = note: candidate #1 is defined in an impl of the trait `std::fmt::Debug` for the type `backtrace::backtrace::Backtrace`
    = note: candidate #2 is defined in an impl of the trait `std::fmt::Display` for the type `backtrace::backtrace::Backtrace`

error: aborting due to 2 previous errors

[BurntSushi]

I don't think this is caused by Rust 1.31. failure on master doesn't compile on Rust 1.30 either. My guess is that this PR in backtrace which was just recently merged is the instigator, which I guess made which impl to select ambiguous? Interesting.

[azriel91]

Ah, I did recently run cargo update in the other project that used this. A very subtle breaking change 👻.

[BurntSushi]

Indeed. My suspicion is that adding an impl like backtrace did is within the realm of "acceptable" breakage, but yeah, I'm not sure how one anticipates something like this.

cc @alexcrichton

[mpapierski]

That was fast. Thanks! Looking forward for failure v0.1.4.

[alexcrichton]

I just boarded a plane and can't deal with this for an hour, I will yank the current version and we can figure how how to add the relevant impl to the backtrace crate later

[mpapierski]

@alexcrichton That sounds great. Do you mean yank backtrace 0.3.10?

For anyone still puzzled there is temporary workaround:

cargo update -p backtrace --precise 0.3.9 

[alexcrichton]

Apologies for the breakage here!

For posterity, the root cause was that backtrace 0.3.10 was just published and added Display for Backtrace. This in turn causes the method resolution error in this crate which wanted the Debug implementation.

I deleted Display for Backtrace, published 0.3.11, and yanked 0.3.10. I'll hold off on re-adding Display until a later date, but landing this in the meantime would likely be good regardless!

[RalfJung]

What's up with CI for this PR? In my PR doing the same thing, CI failed because backtrace seems to be no longer compatible with Rust 1.18. Now, this PR seems to have no CI at all.

[azriel91]

@BurntSushi, @alexcrichton, @RalfJung: Since backtrace requires a higher Rust version, what should happen next?

• Raise the minimum Rust version for failure
• Cap the version of backtrace in failure
• Make backtrace Rust 1.18 compatible

First two options affect failure, last option affects backtrace crate (somewhat wider audience)

---

Edit, just saw alexcrichton/backtrace-rs#135, shall claim it.

[azriel91]

Option 3: alexcrichton/backtrace-rs#137

[azriel91]

Pinned backtrace to 0.3.9 temporarily, pending alexcrichton/backtrace-rs#137 (and release) before it is Rust 1.18.0 compatible.

Feel free to push over the branch if there's something urgent; I'm off for the night.

[azriel91]

backtrace 0.3.13 hasn't yet been released, but its minimum supported Rust version is increased to 1.25.0, so I did that here as well.