Add `-Z embed-source` (DWARFv5 source code embedding extension)

[Mrmaxmeier]

Proposal

DWARFv5 has an extension for embedding program source code directly into the debug info sections: https://dwarfstd.org/issues/180201.1.html

LLVM implements this extension, though it currently uses a custom pre-standardization attribute opcode.

Clang supports it through a -gembed-source flag but Rust does not expose this feature at the moment.

Implementing this seems straight-forward (https://github.com/rust-lang/rust/pull/126985) but I'm not entirely sure how to deal with unsupported platforms.

• Should -Z embed-source=yes fail the build on non-DWARF or non-LLVM platforms?
• Should -Z embed-source=yes automatically bump the DWARF version to v5 or should it require an explicit -Z dwarf-version=5? Clang currently defaults to v4 and requires setting the DWARF version explicitly.
• Should -Z embed-source=yes fail the build if -g was not provided?

Process

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

• [x] 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]

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:

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

Concerns can be lifted with:

 @rustbot resolve reason-for-concern 

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

cc @rust-lang/compiler @rust-lang/compiler-contributors