Removed wrong usage of default from the Option API

[Kimundi]

This implements parts of the changes to Option I proposed and discussed in this thread: https://mail.mozilla.org/pipermail/rust-dev/2013-November/006254.html, and on IRC.

In short, the string "default" should not be used in any context that has nothing to do with the std::default::Default trait.

This PR consists of this change:

• Renamed map_default -> map_or and mutate_default -> mutate_or_set.

[huonw]

I feel like the arg-swap is a negative. The closure can be a large block of code, which leaves the default value hidden behind it. shrugs

[bill-myers]

Shouldn't there be a map_or_else that takes a closure for the default value?

Typical use is when the default value needs memory allocations to be constructed, or needs to be retrieved with a system call or a communication mechanism.

[alexcrichton]

This is a change to some core types which needs to be discussed.

[Kimundi]

@bill-myers It's hard to draw the line there - if we just added everything that might be useful we might end up with a lot of maintenance burden for functions no one ever uses - in this case map_or_else does not exists, and is thus not used by any part of the codebase, while map_or is used.

In any case, just like map_or(f, a) is shortform for map(f).unwrap_or(a), map_or_else(f, g) would be shortform for map(f).or_else(|| Some(g())).unwrap(), which, while ugly, is at least expressible with the existing methods.

---

EDIT: With this I don't mean to discourage adding such a function though, there are a few API holes like this, for example an expect that accepts a closure for computing the failure message.

[alexcrichton]

This is on the next meeting agenda

[alexcrichton]

We decided in today's meeting that the renaming is a good idea, but the reordering of parameters is not as favorable. Would you mind removing the commit with the reordering of the parameters?

[alexcrichton]

Ah, I should link to the notes as well: https://github.com/mozilla/rust/wiki/Meeting-weekly-2014-01-07