Compile UI tests as libraries by default

[WaffleLapkin]

Proposal

I propose to compile UI tests as libraries by default. I.e. compiletest will add --crate-type=lib by default, unless it's a run-pass/run-fail test (or other test type that requires a binary).

We could also add //@ crate-type [...] annotation to override this if need be.

The upside of compiling as libraries by default is that we will no longer need to workaround "missing main function" by adding fn main() {} in every test.

While it's not much, it's still useless noise that we also have to write every time.

Mentors or Reviewers

@jieyouxu would you like to review the compiletest changes?

For ui tests themselves literally anyone can review it, since it will just be fn main() {} deletion.

Process

The main points of the Major Change Process are as follows:

• File an issue describing the proposal.
• A compiler team member or contributor who is knowledgeable in the area can second by writing @rustbot second.
  • Finding a "second" suffices for internal changes. If however, you are proposing a new public-facing feature, such as a -C flag, then full team check-off is required.
  • Compiler team members can initiate a check-off via @rfcbot fcp merge on either the MCP or the PR.
• Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

[rustbot]

Important

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

Concerns or objections to the proposal should be discussed on Zulip and formally registered here by adding a comment with the following syntax:

@rfcbot concern reason-for-concern 
<description of the concern> 

Concerns can be lifted with:

@rfcbot resolve reason-for-concern 

See documentation at https://forge.rust-lang.org

cc @rust-lang/compiler