Add instrument_fn attribute by pmur · Pull Request #157681 · rust-lang/rust

pmur · 2026-06-09T20:12:40Z

This attribute enables or disables function instrumentation when using -Zinstrument-xray or -Zinstrument-mcount.

It supports the following usage:

#[instrument_fn = "on|off"]

For XRay, "on" is equivalent to always instrument, and "off" is equivalent to never instrumenting.

For mcount, "on" has no effect. "off" disables instrumentation of the function.

This is tracked by #157081, and related to the pending rfc rust-lang/rfcs#3917, and earlier as part mcp rust-lang/compiler-team#561

rustbot · 2026-06-09T20:12:44Z

Some changes occurred in compiler/rustc_passes/src/check_attr.rs

cc @jdonszelmann, @JonathanBrouwer

Some changes occurred in compiler/rustc_hir/src/attrs

cc @jdonszelmann, @JonathanBrouwer

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann, @JonathanBrouwer

rustbot · 2026-06-09T20:12:46Z

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rustbot · 2026-06-09T20:39:01Z

This PR was rebased onto a different main 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.

This attribute enables or disables function instrumentation when using `-Zinstrument-xray` or `-Zinstrument-mcount`. It supports the following usage: `#[instrument_fn = "on|off"]` For XRay, "on" is equivalent to always instrument, and "off" is equivalent to never instrumenting. For mcount, "on" has no effect. "off" disables instrumentation of the function.

rust-log-analyzer · 2026-06-09T21:59:10Z

The job aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

To only update this specific test, also pass `--test-args attributes/instrument_fn.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/attributes/instrument_fn.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/attributes/instrument_fn" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers"
stdout: none
--- stderr -------------------------------
error[E0658]: attributes on expressions are experimental
##[error]  --> /checkout/tests/ui/attributes/instrument_fn.rs:13:9
   |
LL |         #[instrument_fn = "off"] //~ ERROR attribute cannot be used on
   |         ^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: see issue #15701 <https://github.com/rust-lang/rust/issues/15701> for more information
   = help: add `#![feature(stmt_expr_attributes)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date

error: `#[instrument_fn]` attribute cannot be used on structs
##[error]  --> /checkout/tests/ui/attributes/instrument_fn.rs:3:1
   |
LL | #[instrument_fn = "on"] //~ ERROR attribute cannot be used on
   | ^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: `#[instrument_fn]` can only be applied to functions

error: `#[instrument_fn]` attribute cannot be used on modules
##[error]  --> /checkout/tests/ui/attributes/instrument_fn.rs:6:1
   |
LL | #[instrument_fn = "on"] //~ ERROR attribute cannot be used on
   | ^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: `#[instrument_fn]` can only be applied to functions

error: `#[instrument_fn]` attribute cannot be used on inherent impl blocks
##[error]  --> /checkout/tests/ui/attributes/instrument_fn.rs:9:1
   |
LL | #[instrument_fn = "on"] //~ ERROR attribute cannot be used on
   | ^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: `#[instrument_fn]` can only be applied to functions

error: `#[instrument_fn]` attribute cannot be used on expressions
##[error]  --> /checkout/tests/ui/attributes/instrument_fn.rs:13:9
   |
LL |         #[instrument_fn = "off"] //~ ERROR attribute cannot be used on
   |         ^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: `#[instrument_fn]` can only be applied to functions

error: `#[instrument_fn]` attribute cannot be used on traits
##[error]  --> /checkout/tests/ui/attributes/instrument_fn.rs:19:1
   |
LL | #[instrument_fn = "off"] //~ ERROR attribute cannot be used on
   | ^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: `#[instrument_fn]` can only be applied to functions

error: `#[instrument_fn]` attribute cannot be used on required trait methods
##[error]  --> /checkout/tests/ui/attributes/instrument_fn.rs:21:5
   |
LL |     #[instrument_fn = "off"] //~ ERROR attribute cannot be used on
   |     ^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: `#[instrument_fn]` can only be applied to functions with a body

error[E0805]: malformed `instrument_fn` attribute input
##[error]  --> /checkout/tests/ui/attributes/instrument_fn.rs:36:1
   |
LL | #[instrument_fn(entry = "on")] //~ ERROR malformed
   | ^^^^^^^^^^^^^^^--------------^
   |                |
   |                expected a single argument here
   |
help: must be of the form
   |
LL - #[instrument_fn(entry = "on")] //~ ERROR malformed
LL + #[instrument_fn = "on|off"] //~ ERROR malformed
   |

error[E0539]: malformed `instrument_fn` attribute input
##[error]  --> /checkout/tests/ui/attributes/instrument_fn.rs:39:1
   |
LL | #[instrument_fn] //~ ERROR malformed
   | ^^^^^^^^^^^^^^^^ valid arguments are "on" or "off"
   |
help: must be of the form
   |
LL | #[instrument_fn = "on|off"] //~ ERROR malformed
   |                 ++++++++++

error[E0539]: malformed `instrument_fn` attribute input
##[error]  --> /checkout/tests/ui/attributes/instrument_fn.rs:42:1
   |
LL | #[instrument_fn = 1] //~ ERROR malformed
   | ^^^^^^^^^^^^^^^^^^-^
   |                   |
   |                   valid arguments are "on" or "off"
   |
help: must be of the form
   |
LL - #[instrument_fn = 1] //~ ERROR malformed
LL + #[instrument_fn = "on|off"] //~ ERROR malformed
   |

error: aborting due to 10 previous errors

Some errors have detailed explanations: E0539, E0658, E0805.

mejrs

I can't review the codegen side of this, but I can review the attribute stuff.

Can you add (compile error) tests for:

// crate level
#![instrument = "on"]

and

let _x = #[instrument = "on"] || {};

View changes since this review

mejrs · 2026-06-09T22:15:20Z

 fn instrument_function_attr<'ll>(
    cx: &SimpleCx<'ll>,
    sess: &Session,
+    instrument_fn: &Option<bool>,


Suggested change

instrument_fn: &Option<bool>,

instrument_fn: Option<bool>,

mejrs · 2026-06-09T22:16:11Z

+    const ON_DUPLICATE: OnDuplicate = OnDuplicate::Error;
+


Suggested change

const ON_DUPLICATE: OnDuplicate = OnDuplicate::Error;

Error is the default value.

mejrs · 2026-06-09T22:18:40Z

            "xray-instruction-threshold",
            &threshold.to_string(),
        ));
+


😠

Suggested change

mejrs · 2026-06-09T22:19:50Z

    (unstable, import_trait_associated_functions, "1.86.0", Some(134691)),
    /// Allows associated types in inherent impls.
    (incomplete, inherent_associated_types, "1.52.0", Some(8995)),
+    /// Enable #[instrument_fn] on function (todo: tracking issue)


Suggested change

/// Enable #[instrument_fn] on function (todo: tracking issue)

/// Enable #[instrument_fn] on functions

Old todo?

mejrs · 2026-06-09T22:23:01Z

-            Some(llvm_mcount_intrinsic) => llvm_mcount_intrinsic.as_ref(),
-            None => sess.target.mcount.as_ref(),
-        };
+        // #[instrument_fn], the default is on.


Isn't it an error to use #[instrument_fn]? There is no default, the value must always be specified.

mejrs · 2026-06-09T22:30:16Z

+        let mut instrument = None;
+        match args {
+            ArgParser::NameValue(nv) => {
+                if let Some(option) = nv.value_as_str()


this is just stylistic, feel free to ignore.

Suggested change

if let Some(option) = nv.value_as_str()

if let Some(option @ (sym::on | sym::off)) = nv.value_as_str()

or my preference:

Suggested change

if let Some(option) = nv.value_as_str()

match nv.value_as_str(){

Some(sym::on) => Some(true),

Some(sym::off) => Some(false),

None => error,

}

mejrs · 2026-06-09T22:34:53Z

+        let mut never = options.never;
+        let mut always = options.always;
+
+        // Apply optional #[instrument_fn] override.
+        match instrument_fn {
+            Some(true) => {
+                always = true;
+            }
+            Some(false) => {
+                never = true;
+            }
+            None => {}
        }
-        if options.never {
+
+        if never {
            attrs.push(llvm::CreateAttrStringValue(cx.llcx, "function-instrument", "xray-never"));
        }
+        if always {
+            attrs.push(llvm::CreateAttrStringValue(cx.llcx, "function-instrument", "xray-always"));
+        }
+


Suggested change

let mut never = options.never;

let mut always = options.always;

// Apply optional #[instrument_fn] override.

match instrument_fn {

Some(true) => {

always = true;

}

Some(false) => {

never = true;

}

None => {}

}

if options.never {

if never {

attrs.push(llvm::CreateAttrStringValue(cx.llcx, "function-instrument", "xray-never"));

}

if always {

attrs.push(llvm::CreateAttrStringValue(cx.llcx, "function-instrument", "xray-always"));

}

// Apply optional #[instrument_fn] override.

match instrument_fn {

Some(true) => {

attrs.push(llvm::CreateAttrStringValue(cx.llcx, "function-instrument", "xray-always"));

}

Some(false) => {

attrs.push(llvm::CreateAttrStringValue(cx.llcx, "function-instrument", "xray-never"));

}

None => { /* invalid attribute input */ }

}

rustbot · 2026-06-09T22:35:16Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

mejrs · 2026-06-09T22:40:12Z

    InstructionSet(InstructionSetAttr),

+    /// Represents `#[instrument_fn]`
+    InstrumentFn(Option<bool>),


Also a preference thing; I would use an enum for this (perhaps with a Err(guar) variant as well)

enum Instrument { On, Off, }

View changes since the review

rust-bors · 2026-06-10T08:47:05Z

☔ The latest upstream changes (presumably #157683) made this pull request unmergeable. Please resolve the merge conflicts.

rustbot assigned oli-obk Jun 9, 2026

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pmur force-pushed the murp/add-instrument_fn-attr branch from 8c64d48 to 8d7140c Compare June 9, 2026 20:38

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pmur force-pushed the murp/add-instrument_fn-attr branch from 8d7140c to 42c43f0 Compare June 9, 2026 21:02

mejrs requested changes Jun 9, 2026

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rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jun 9, 2026

mejrs reviewed Jun 9, 2026

View reviewed changes

	/// Enable #[instrument_fn] on function (todo: tracking issue)
	/// Enable #[instrument_fn] on functions

	if let Some(option) = nv.value_as_str()
	if let Some(option @ (sym::on \| sym::off)) = nv.value_as_str()

-                if let Some(option) = nv.value_as_str()
+match nv.value_as_str(){
+     Some(sym::on) => Some(true),
+     Some(sym::off) => Some(false),
+     None => error,
+}

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