Compiler switch to list bound checks

[leonardo-m]

This is my first Rust enhancement request.

With the Go language compiler version 1.7, you can build the code using -gcflags="-d=ssa/check_bce/debug=1" to show which code lines still need checking bounds:

http://www.tapirgames.com/blog/golang-1.7-bce

Finding where the compiler is not able to optimize away run-time bound checks is important for some kinds of code where performance matters:

#30917

Currently to do that in Rust I have to generate the asm with --emit asm, search the function(s) (this sometimes is not so easy because of inlining) and then read the asm to understand if and where it performs bound checks. This is a bit laborious and not handy. So is it possible to add to Rustc a compilation switch similar to Go that outputs all the lines and column where LLVM isn't able to remove array bound tests?

In-bound test(s):
16 | foo[i] -= 1;
         ^

32 | foo[i] += bar[j];
         ^         ^

(Having this compiler switch is only about one third of this performance battle).

[est31]

Awesome idea! I'm not really fluent in asm, let alone fluent enough to know which instruction takes how much time, but I still want to improve performance, and this would make it easier for me.

[arthurprs]

Definitely useful but I'm unsure how hard this would be. Most (if not all) of the elimination are done by the llvm optimizer.

[steveklabnik]

Triage; not aware of any changes

[matthiaskrgr]

Perhaps the index_slicing clippy lint partially covers this?