Improve slice indexing assertion

[Manishearth]

I'd like to have the message print out the index and length values like it does elsewhere, but I'm not sure how to do that without affecting perf here. Will assert!(cond, "index out of bounds got {} but len is ", idx, len) make things slower? It calls panic_fmt which is marked as cold but also calls format_args!, and I don't know if that allocates or does any heavy lifting.

cc @alexcrichton @gankro

[rust-highfive]

r? @alexcrichton

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[bluss]

str uses a separate function to put the formatting there

[alexcrichton]

Yeah I think the str strategy would be appropriate here for some good messages. There's no concern really about performance here, LLVM typically does the correct thing, but it could accidentally be easy to inflate code sizes with lots of fancy asserts.

If, however, a concrete function in libcore were provided (e.g. not generic), then it could be as fancy as it wants!

[Manishearth]

That should be marked cold, right?

[Gankra]

@Manishearth might as well.

[Manishearth]

Updated. Error message could be improved.

[alexcrichton]

@bors: r+ 5c873be

[bors]

⌛ Testing commit 5c873be with merge 4891c00...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt