Avoid iteration for singleton selections

[charliermarsh]

Summary

If we have a singleton Range, we don't need to iterate over the map of available ranges; instead, we can just get the singleton directly.

Closes #6131.

[charliermarsh]

Most of the test failures are just aesthetic, but there is one real failure due to this:

// If we have at least one stable release, we shouldn't allow the "if-necessary"
// pre-release strategy, regardless of whether that stable release satisfies the
// current range.
prerelease = Some(PrereleaseCandidate::NotNecessary);

So the selector actually does rely on seeing versions outside the range, hmm.

[charliermarsh]

A little bit of a penalty here but should be somewhat rare.

[charliermarsh]

We only need to do this slow path when we see a pre-release singleton; and the check will short-circuit as soon as we see any non-pre-release.

[charliermarsh]

It'd be nice to avoid this though.

[konstin]

You could store whether all versions are prereleases in the version map, probably helpful for debugging anyway

[charliermarsh]

Good idea.

[konstin]

You could store whether all versions are prereleases in the version map, probably helpful for debugging anyway

[konstin]

Suggested change

	if let Some(version) = range.as_singleton() {
	// Performance optimization: If we only have a single version, return that version directly.
	if let Some(version) = range.as_singleton() {

Without a comment, i can already see myself looking at this code later and wondering whether the branch is load-bearing or not.