change String::from_str() to .to_string()

[cburgdorf]

I recently learned from @huonw that "foo".to_string() should be preferred over String::from_str("foo").

This commit updates tutorial.md, rc.rs and string.rs according to the convention.

There are more files to be patched but I thought it might be fine to patch them in batches.

[cburgdorf]

Travis seemed to timed out hence the error. Not sure how to trigger a new run.

[lilyball]

Seems like a poor time to do this migration, when "foo".to_string() is temporarily going to be much more expensive than String::from_str("foo").

[brson]

As of #15493 it's not obvious that to_string should be preferred over from_str because there's a perf difference, but this is a small patch that mostly touches test cases.

[lilyball]

This may mostly touch test cases, but it sets a precedent and suggests more PRs should be submitted that do the same transformation elsewhere.

My recommendation is to avoid doing any of this work until we have a better idea about how to make "foo".to_string() become cheap again.

[cburgdorf]

Ah, well, never mind then. I was referring to iron/iron#86 (comment) and thought it might be a patch small enough to get me started with some non-documentation patches. I'll find something different to hack on ;-)

[lilyball]

@cburgdorf That's a reasonable reaction to that comment, TBH. It just happens to come at an inopportune time. For new code, certainly, use "foo".to_string(), that is the idiomatically correct approach. But there's no convincing win to modifying existing uses of String::from_str() at this time.

Thank you for your contribution, and I'm looking forward to whatever else you submit. I'm closing this PR at this time for the already-discussed reasons. Once "foo".to_string() becomes fast again, I would encourage you to consider re-submitting.

[cburgdorf]

👍