Support build templates

[liufengyun]

Support build templates

What has changed

Checklist

• Added / updated tests under tests/pos/ or tests/warn/
• Docs updated if the change affects user-visible behavior
• All commits are signed off (why?)

How to sign off commits

Use git commit -s to add the Signed-off-by line automatically:

To add a sign-off to the last commit retroactively:

git commit --amend -s --no-edit

To add sign-off to the last 3 commits:

git rebase --signoff HEAD~3

Security impact

No

Compatibility impact

No

• Source compatibility: existing code continue to compile
• SAST compatibility
  • forward compatibility: new libraries can be used by old compiler
  • backward comopatibility: old libraries can be used by new compiler
• Standard Library compatibility:
  • forward compatibility: new code can work with old stdlib
  • backward compatibility: old code work with the new stdlib
• Runtime Library compatibility:
  • forward compatibility: new code can work with old runtime
  • backward compatibility: old code work with the new runtime
• Build tool
  • Build spec compatibility: old projects continue to build
  • Joy package compatibility: old .joy can be consumed by new build tool