Validate target source paths before compilation with clearer errors

[TanmayArya-1p]

What does this PR try to resolve?

Closes #10173

This PR adds early validation of target source paths with clearer error messages.

For context, I tried to solve this issue before ( #16329 ) but ended up making an oversight, I would recommend reading this to understand the issues faced in resolving this.

So what changed in this PR?

• Root Target Units are validated early on in the compilation pipeline when the BuildContext is generated here. This is right after command line arguments/manifest targets are parsed and just before the build tasks are spawned.
• Cases that are covered by this PR:
  • If the target path is invalid and points to nothing.
  • If the target path is valid, but is a directory (not a file)
  • If the target path is a valid directory with a main.rs/lib.rs depending on the Target kind: In this case a help: message is emitted.

Doing this also means that Cargo validates each root target's path that is requested to be compiled before spawning any build tasks.
This PR enforces the invariant: No compilation occurs before validating every required target's source path.
I'm not sure if this should be expected behaviour and I'd love to hear from others on this.

[rustbot]

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[epage]

Feel free to edit your commits for how they should be merged.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[TanmayArya-1p]

Thanks for the review!
I fixed the issues you pointed out, Let me know if it needs to be changed.

Also could you rerun the CI workflow because it failed with this error:

https://github.com/rust-lang/cargo/actions/runs/19952419092/job/57215065207#annotation:2:61

Error: fatal: unable to access 'https://github.com/rust-lang/cargo/': Failed to connect to github.com port 443 after 21029 ms: Could not connect to server
Error: The process 'C:\Program Files\Git\bin\git.exe' failed with exit code 128

[epage]

I fixed the issues you pointed out, Let me know if it needs to be changed.

FYI it would be a big help for reviewing if the commits were cleaned up, especially if passing tests are added in their own commit (showing the old behavior) and then the feature work happens in the follow up that updates the tests so they still pass.

[TanmayArya-1p]

Sure, i'll clean up the commit history

[epage]

Can we explicitly enumerate all TargetKinds so the compiler can remind us to re-evaluate this if a new variant is added?

[epage]

Thanks, this is looking great! Just adjusting the match for maintainability and this is good to go!