Tracking Issue for `#[collapse_debuginfo]`

[davidtwco]

This is a tracking issue for the #[collapse_debuginfo] attribute (rust-lang/compiler-team#386).
The feature gate for the issue is #![feature(collapse_debuginfo)].

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Steps

• Implement the MCP (ssa: implement #[collapse_debuginfo] #99556)
• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• Ensure consistency of behaviour with #[track_caller] - Code in block passed to macro has line number of macro invocation #39153 (comment)

Implementation history

• Discussed in Code in block passed to macro has line number of macro invocation #39153..
  • ..then proposed in RFC RFC: Debuggable macro expansions rfcs#2117 where the language team recommended a compiler team MCP..
  • ..and then finally accepted in Debuggable Macro Expansions compiler-team#386.
• Initially implemented in ssa: implement #[collapse_debuginfo] #99556.

[petrochenkov]

Some conclusions from discussing #118903 with @azhogin:

• The final accumulated value of collapse_debuginfo: bool is determined at a specific call site, so we can use other properties than just #[collapse_debuginfo] on the macro definition. For example, whether the macro comes from the current crate or some other crate. (Value of -Z debug-macros for the current crate is already used like this, in particular).
• We could make an option or crate-level attribute -C collapse-debuginfo/#![collapse_debuginfo] setting the global default for unmarked macros, as a replacement for -Zdebug-macros.
• Both the option and the attributes could be configurable to make them cover more cases, for example #[collapse_debuginfo(always|never|local|external|something_else)] and -Ccollapse_debuginfo=always|never|local|external|something_else.
• #[collapse_debuginfo(external)] looks like a reasonable default for argument-less #[collapse_debuginfo], because you almost always want to go inside locally defined macros? (Standard library macros are not locally defined in almost all scenarios.)
• The global option should be able to override local annotations if it is "less collapsed", because it would be good to be able to step inside anything regardless of definition-site attributes if really necessary.
• Built-in macros can be made collapse_debuginfo(true) by default in the compiler to avoid library annotation noise, because for majority of them the compiler-generated code has no real location in source code besides call site. There are some exceptions though, like include!, which have real non-call-site locations in generated code.
• The use of feature(collapse_debuginfo) to change behavior instead of just gating the attributes is not idiomatic, features are not supposed to be used like this. The global default should be set in some other way.