add -C overflow-checks option

[froydnj]

In addition to defining and handling the new option, we also add a method on librustc::Session for determining the necessity of overflow checks. This method provides a single point to sort out the three (!) different ways for turning on overflow checks: -C debug-assertions, -C overflow-checks, and -Z force-overflow-checks.

I was seeing a run-pass/issue-28950.rs failure on my machine with these patches, but I was also seeing the failure without the changes to the core compiler. We'll see what travis says.

Fixes#33134. r? @alexcrichton

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[durka]

• insta-stable?
• Does this option only allow turning on overflow checks (e.g., in release mode)? Should it instead be -C overflow-checks={on,off} so you could turn them off in debug mode?

[froydnj]

insta-stable?

Sorry, I'm not sure what this means. Can you elaborate?

Should it instead be -C overflow-checks={on,off} so you could turn them off in debug mode?

The option specification permits -C overflow-checks={on,off}. Perhaps the commit message should be rewritten, or the documentation for the option, or both?

[durka]

I mean does this need an unstable period? I'm not sure how that works with options. I guess this is kind of a stabilization of -Z force-overflow-checks, in which case, shouldn't that be removed?

[froydnj]

I'm not sure how it works with options either! Discussions in #33134 suggested that -Z force-overflow-checks will be removed soonish after this lands, but I suspect waiting a release cycle or maybe even two would be recommended.

[alexcrichton]

Looks great to me, thanks @froydnj! (additionally for the drive-by cleanup)

@durka this feature has already been unstable with -Z for quite some time now and has additionally gone through an RFC (albeit an old one), so we shouldn't need a stabilization period for this.

@bors: r+

[bors]

📌 Commit ffc6ddd has been approved by alexcrichton