Making Cargo --offline Optional

[cwshugg]

This PR implements an environment variable that can be used to toggle the usage of the --offline option when trybuild invokes cargo. The default behavior has not been modified; --offline is still used by default, but there is now the option to disable the usage --offline by setting this environment variable:

TRYBUILD_NO_OFFLINE_MODE=1

Always invoking cargo with --offline was discussed in #50 and implemented in this commit). As discussed in #296, in some situations, trybuild is unable to access certain resources outside of the target directory. I've noticed a similar problem when setting CARGO_TARGET_DIR to somewhere else on the system (I have a constraint that requires this), and found that removing --offline allows dependencies to be pulled when trybuild sets the target directory here in the code.

I figured, for this reason, and for others in the future, it may be handy to have the option to choose whether or not --offline is used by trybuild. Would love to hear any feedback and make suggested changes - thanks!

[cwshugg]

Hi @dtolnay - could I request a review for this PR if you have the time? Thanks!

[dtolnay]

Thanks for the PR!

I would prefer not to add this to this crate. But there is an opportunity for someone to make a more fully featured compile error testing library that provides this.

I would accept some other fix that better supports different target directory arrangements as long as it works automatically.

[cwshugg]

Hey @dtolnay - thanks for the review, and for explaining. A few ideas for automatic solutions - if you would accept either of these, I'd be happy to implement one:

• Disable --offline when it's detected that CARGO_TARGET_DIR is outside of the cargo crate under test
• Do not override CARGO_TARGET_DIR when invoking cargo if it has already been set by the user

Could I point you to #296? Others seem to be having similar issues, moreso related to file paths specified within their Rust code. The root of their issues, and the issue I have been seeing, both seem to be at the fact that CARGO_TARGET_DIR is overridden. Is it an absolute must that CARGO_TARGET_DIR be set to CURRENT_TARGET_DIRECTORY/tests/trybuild?