Installing multiple versions doesn't work well

[catamorphism]

If you do make install with previous installed versions, multiple copies of all the libraries get left in (for me) /usr/local/lib/rustc/x86_64-apple-darwin/lib. Then the next time you try building something with the installed rustc, it'll promptly fail with error: multiple matching crates for core

[graydon]

This is by design, actually. It'll get fixed, to the extent that we intend to fix it, when we remove version numbers from filenames (they are already mangled on to symbols) and add version numbers to metadata and source files. I'll bump the priority on this one.

[graydon]

Considering the hash change and duplicate-metadata-hash change, there's actually no simple way for us to dodge this experience in the 0.5 timeframe; and it'll realistically be possible in all cases when someone fails to specify a particular version or tighter metadata match-set they're interested in. IOW the problem is more in the ubiquity of statements like extern mod std; with no further qualification, than in the ability to do side-by-side installation of multiple things named std.

Not saying we should avoid trying to mitigate, just that I think as a "bug" it's neither fixable in this timeframe nor necessarily ever fully fixable. It's an ambiguity that's forced on us by the loose coupling implied by underspecified linkage directives. We can only mitigate at most.

[catamorphism]

Long-term issue; de-milestoning