impl Hash for !

[clarfonthey]

This was missing in some generic code I was writing and I figured that it would be worthwhile to add this. Blanket-requiring these traits to allow usage of errors in HashSets and HashMaps isn't too unreasonable of a use case, and a prerequisite for allowing ! as an error in such situations is this impl.

[rust-highfive]

r? @KodrAus

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[rust-highfive]

The job x86_64-gnu-llvm-3.9 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

[00:03:25]    Compiling rustc_tsan v0.0.0 (file:///checkout/src/librustc_tsan)
[00:03:26] warning: unused variable: `state`
[00:03:26]    --> libcore/hash/mod.rs:608:35
[00:03:26]     |
[00:03:26] 608 |         fn hash<H: Hasher>(&self, state: &mut H) {
[00:03:26]     |                                   ^^^^^ help: consider using `_state` instead
[00:03:26]     = note: #[warn(unused_variables)] on by default
[00:03:26] 
[00:03:27]    Compiling rustc_asan v0.0.0 (file:///checkout/src/librustc_asan)
[00:03:39]    Compiling libc v0.0.0 (file:///checkout/src/rustc/libc_shim)

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[varkor]

It doesn't seem that important, but using *self instead of unreachable!() is more consistent with how other impls for ! are defined.

[clarfonthey]

@varkor I was under the impression that impls for ! which return a value of another type generally do *self, but ones which return unit do unreachable!(). Although this seems pretty arbitrary and I may have just misremembered what I saw of existing impls.

I can change it to *self if that seems more reasonable.

[varkor]

All the implementations I can see in libstd use *self, though there are a couple in rustc that use other methods. I don't think it's hugely important though — it's fairly obvious what's going on with either method.

[scottmcm]

I like *self just so that it's trivial to translate -- no need to emit a bunch of panic call HIR+MIR+IR only for LLVM to figure out that it's unneeded again and remove it all.

[clarfonthey]

Rebased and replaced unreachable!() with *self.

[KodrAus]

Sorry for taking a while to get to this! It looks good to me.

@bors r+

[bors]

📌 Commit 570590f has been approved by KodrAus

[bors]

⌛ Testing commit 570590f with merge 4f29363...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: KodrAus
Pushing 4f29363 to master...