Output the previous version of a toolchain when it is updated

[RobbieClarken]

This change modifies the cli output of rustup update to display the previous version of rustc for toolchains that are updated.

Before:

$ rustup update

info: syncing channel updates for 'stable-x86_64-apple-darwin'
info: syncing channel updates for 'nightly-x86_64-apple-darwin'
--snip--
  stable-x86_64-apple-darwin unchanged - rustc 1.39.0 (4560ea788 2019-11-04)
   nightly-x86_64-apple-darwin updated - rustc 1.41.0-nightly (7afe6d9d1 2019-12-03)

After:

$ rustup update

info: syncing channel updates for 'stable-x86_64-apple-darwin'
info: syncing channel updates for 'nightly-x86_64-apple-darwin'
--snip--
  stable-x86_64-apple-darwin unchanged - rustc 1.39.0 (4560ea788 2019-11-04)
   nightly-x86_64-apple-darwin updated - rustc 1.41.0-nightly (7afe6d9d1 2019-12-03) (from rustc 1.40.0-nightly (22bc9e1d9 2019-09-30))

Fixes #2110.

[kinnison]

I feel like the work you did in fn install() in src/toolchain.rs could be simplified -- the exists boolean and the previous_version variable could be simplified into a single previous_version where instead of exists you have previous_version.is_some() -- However this isn't necessary right now.

If you want to clean up a little, then let me know, if you'd rather not attempt that now then that's fine too -- let me know that and I'll move on.

[RobbieClarken]

I feel like the work you did in fn install() in src/toolchain.rs could be simplified -- the exists boolean and the previous_version variable could be simplified into a single previous_version where instead of exists you have previous_version.is_some()

Good point! I've made that change. Let me know if it looks good and I'll squash the commits.

[RobbieClarken]

The build failure looks to be a filesystem error related to rustup set default-host. I don't think it has anything to do with the changes in this PR:

failures:

---- env_override_beats_file_override stdout ----
Copying from /checkout/target/x86_64-unknown-linux-gnu/release/rustup-init to /checkout/target/x86_64-unknown-linux-gnu/tests/running-test-9VnOmv/rustup-exexL7Uwu/rustup
running "/checkout/target/x86_64-unknown-linux-gnu/tests/running-test-9VnOmv/rustup-exexL7Uwu/rustup" "set" "default-host" "x86_64-unknown-linux-gnu"
thread 'env_override_beats_file_override' panicked at 'failed to run test command: Os { code: 26, kind: Other, message: "Text file busy" }', src/libcore/result.rs:1165:5

failures:
    env_override_beats_file_override

[kinnison]

The build failure looks to be a filesystem error related to rustup set default-host. I don't think it has anything to do with the changes in this PR:

Yep, that's a transient failure which only ever seems to manifest in CI, making it super-hard for me to track down.

[kinnison]

Assuming the CI passes, please rebase/squash ready for merge. Ensure your commit message is clean too, rebase sometimes splats them all together nastily. 👍

[RobbieClarken]

Assuming the CI passes, please rebase/squash ready for merge. Ensure your commit message is clean too, rebase sometimes splats them all together nastily. 👍

Done and done.

[kinnison]

👍