Remove validation for custom toolchains when reading rust-toolchain.toml

[wesleywiser]

This validation was introduced in e1306b3 However, it breaks a number of scenarios that previously worked without issue such as #4248. As requested by rustup maintainers in that thread, I'm taking out the extra validation added in that commit, leaving the rest of the code as-is.

Testing confirms this fixes #4248.

[djc]

Makes sense to me. Will wait for @rami3l to have a look.

I guess alternatively we could show a warning, and then store a setting that we've shown the warning, or something? (Or, store the date when we've last shown the warning and only show it again after x days?)

[rami3l]

@wesleywiser Thanks a lot for your comprehension and your quick patch!

I was not able to figure out a way to write an automated test for this scenario since the rustup CLI needs the custom toolchain to actually exist and unit testing looked infeasible due to use of the Cfg type.

Sorry for the back and forth, but I would still like to see an automated test for this scenario. Is it possible to trick rustup into believing the existence of that toolchain by installing e.g. stable with the components you want, and rename it directly under $RUSTUP_HOME/toolchains (written as cx.config.rustupdir.join("toolchains") in our test suites)?

[wesleywiser]

Thanks @rami3l for that suggestion! I've added a test that covers this 🙂

[rami3l]

@wesleywiser About the message that @djc has mentioned, I think it will probably be worthwhile to add a warning clarifying that we are not doing any checks, as demonstrated in the new test. That said, it will definitely not be a blocker anyway. Thanks again for your contribution!

@djc I'm afraid a "last emitted warning" check might not be enough now that we are considering some workstation with multiple repos/toolchain combinations...