More consistent version naming

[mcandre]

rust, rustup, and the download URLs all have a lot of different ways of referring to rust version numbers. rust v1.27.0-nightly was once a thing, installable via rustup install nightly, but since rust v1.28.0-nightly released, now 1.27.0 has rotated from nightly to beta, breaking packages like clippy that expect rust v1.27.0-nightly to be named specifically 1.27.0-nightly, rather than 1.27.0-beta, even though the content is the same.

rustup gets confused and cannot do rustup update 1.27.0-nightly, nor rustup update nightly-1.27.0, nor rustup update 1.27.0-beta nor rustup update beta-1.27.0, but simply beta, until rust v1.29.0-nightly releases and then v1.28.0-nightly becomes the new v1.28.0-beta and v1.27.0-beta becomes 1.27.0, hopefully.

The only thing that seems to work for consistently pinning unstable rust version numbers for rustup is using the release date, so for 1.27.0[formerly -nightly], that would be rustup update nightly-2018-05-16. This is probably not what most users would expect to have to type, manually looking up the date information in Internet logs and performing the mapping themselves. This is something that rustup could do a great job at, for a more automated, flexible command line interface. What do you think?

Update

In fact, even using dates are insufficient due to an apparent bug in rustup activating the date strings, see #1417

[Diggsey]

I'm not sure where you're getting these version numbers from? The identifiers supported by rustup (and before that, multirust) have always been the same:

channel = nightly | beta | stable
date = yyyy-mm-dd
stable-version = x.y.z

<channel>
<channel>-<date>
<stable-version>

When an identifier consists of solely a channel, like nightly, it will track the latest version of that channel.

When an identifier includes a date or a stable version number, then it will be locked to that date/version.

You can always refer to any toolchain by date, including the latest one.

eg.
rustup toolchain install stable-2018-05-10