This is a tracking issue for toggling xray/mcount function instrumentation via an attribute.
The feature gate for the issue is #![feature(instrument_fn)].

This attribute (in some form) has been proposed by the ongoing rust-lang/rfcs#3917, and earlier via mcp rust-lang/compiler-team#561

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.
Discussion comments will get marked as off-topic or deleted.
Repeated discussions on the tracking issue may lead to the tracking issue getting locked.

Steps

• Implementation
• Adjust documentation ([see instructions on rustc-dev-guide][doc-guide])
• Style updates for any new syntax ([nightly-style-procedure])
  • Style team decision on new formatting
  • Formatting for new syntax has been added to the [Style Guide]
  • (non-blocking) Formatting has been implemented in rustfmt
• Stabilization PR ([see instructions on rustc-dev-guide][stabilization-guide])

Unresolved Questions

• how much configurability should this option expose? There are multiple frameworks for instrumenting functions (mcount, xray, maybe fentry).

Implementation history

This is included in #153742, but could be spun out for better discussion.