Allow using opt-level="s"/"z" in profile overrides

[whitequark]

Initially, I've considered making a dedicated OptLevel enum, but this appeared to bring no practical benefit, only boilerplate, so I've used a String instead, which is also in line with the u32 that was there before, not even checked for being in range 0...3.

[rust-highfive]

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[jonas-schievink]

You're missing quotes around {level} (at least s and z require quotes)

[whitequark]

Oh, that simple... Works now, thanks!

[alexcrichton]

Thanks for the PR! As this is a nightly-only feature for now I think the test will need a gate at the top (similar to the plugin tests) to just run on nightly. Other than that though looks good to me!

[whitequark]

@alexcrichton Done

[alexcrichton]

Looks like tests are failing?

[matklad]

I think these s and z can be very cryptic for new users. We can use some longer form in configuration files. Not sure that it is a good idea though :)

[whitequark]

Tests passed now.

[alexcrichton]

Good question @matklad! I've opened up rust-lang/rust#35784 as a tracking issue for this unstable feature in general, and that seems appropriate to revisit when we move to stabilize the optimization levels.

Other than that, looks great to me, thanks @whitequark!

@bors: r+

[bors]

📌 Commit bc858b0 has been approved by alexcrichton

[bors]

⌛ Testing commit bc858b0 with merge 22fccc1...

[bors]

☀️ Test successful - cargo-cross-linux, cargo-linux-32, cargo-linux-64, cargo-mac-32, cargo-mac-64, cargo-win-gnu-32, cargo-win-gnu-64, cargo-win-msvc-32, cargo-win-msvc-64
Approved by: alexcrichton
Pushing 22fccc1 to master...

[MagaTailor]

Well, it's become a little inconsistent. The "s"/"z" suggestion is only printed after putting e.g. "3" in the Cargo.toml, while the most obvious 's' or 'z' on their own fail to parse with no hint.

And BTW, -C opt-level=z or s doesn't need quoting, just like the previous numeric levels.