feat(hints): support minimum optimization levels by 0xPoe · Pull Request #17368 · rust-lang/cargo

0xPoe · 2026-08-16T09:10:26Z

What does this PR try to resolve?

This PR would allow Rust library to provide a simple hint about the minimum opt-level to build them with via the hint.min-opt-level setting in the manifest.

How to test and review this PR?

Check the unit tests and review it commit by commit.

r?@ghost

0xPoe

🔢 Self-check (PR reviewed by myself and ready for feedback)

Code compiles successfully
Unit tests added
No AI-generated elegant nonsense in PR.
Comments added where necessary
PR title and description updated
Documentation updated
PR size is reasonable

r?@joshtriplett

View changes since this review

0xPoe · 2026-08-21T06:42:22Z

 [package]
 name = "cargo-util-schemas"
-version = "0.14.3"
+version = "0.15.0"


Because we added a new field to a public struct without marking it as non_exhaustive, I bumped the major version.

0xPoe · 2026-08-21T06:43:42Z

    },
    "TomlDebugInfo": {
-      "type": ["string", "integer", "boolean"],
+      "type": [


This one was also generated automatically.

0xPoe · 2026-08-21T06:50:28Z

        &self,
        pkg_id: PackageId,
+        pkg_hints: Option<&Hints>,
+        hint_min_opt_level: bool,


Not sure if this is the best way to determine whether users want to enable this hint.

epage · 2026-08-21T20:27:16Z

 via `profile`, which takes precedence, and which can only be specified in the
 top-level crate being built.

+## Package `min-opt-level` hint


Can you have a subsection that is written as-if it was the end-user documentation?

View changes since the review

epage · 2026-08-21T20:27:47Z

+The hint accepts the numeric optimization levels 0, 1, 2, and 3. Other values
+produce a warning and are ignored. If the selected [profile](profiles.md) has a


Why do they produce a warning instead of error?

View changes since the review

If we proceed with errors, does that mean that introducing a new optimization level in the future, which is accepted by the new Cargo, would cause an error with older versions of Cargo? Would this be considered a backward compatibility issue? But I guess introducing a new optimization level might be impossible, especially a numeric optimization level?

epage · 2026-08-21T20:37:59Z

+    let pkg_hint_min_opt_level = match parse_min_opt_level_hint(hints.min_opt_level.as_ref()) {
+        Ok(level) => level,
+        Err(MinOptLevelHintError::OutOfRange(level)) => {
+            unit_capped_warn(&format!(
+                "ignoring unsupported value ({level}) for 'hints.min-opt-level', which only supports integers from 0 to 3"
+            ))?;
+            None
+        }
+        Err(MinOptLevelHintError::WrongType(value_type)) => {
+            unit_capped_warn(&format!(
+                "ignoring unsupported value type ({value_type}) for 'hints.min-opt-level', which expects an integer"
+            ))?;
+            None
+        }
+    };
+    if matches!(pkg_hint_min_opt_level, Some(1..=3)) && !bcx.gctx.cli_unstable().hint_min_opt_level
+    {
+        unit_capped_warn(
+            "ignoring 'hints.min-opt-level', pass `-Zhint-min-opt-level` to enable it",
+        )?;
+    }


Why is this check placed here?

View changes since the review

I simply followed the mostly-unused placement and tried to group them together. However, after reconsidering, this may not make sense because this case differs from it: this hint does not directly change the cmd here. The per-compilation-unit check also does not apply to the min-opt-level hint. I will try to find the right place to perform this check.

epage · 2026-08-21T20:39:59Z

+        let mut profile = maker.get_profile(
+            Some(pkg_id),
+            is_member,
+            unit_for.is_for_host(),
+            min_opt_level,
+        );


Why do we pass it into get_profile, rather than overriding it here?

View changes since the review

Because the hint takes precedence within ProfileMaker::get_profile, applying it afterward would override explicit settings such as:

[profile.dev.package.foo] opt-level = 0

epage · 2026-08-21T20:42:01Z

-    with_opt_level(&mut cargo, "dep", "0");
+    let mut cargo = p.cargo("check -v -Zhint-min-opt-level");
+    cargo.masquerade_as_nightly_cargo(&["hint-min-opt-level"]);
+    with_opt_level(&mut cargo, "dep", "2");


If you move -Zhint-min-opt-level into a separate .arg() along with masquerade_as_nightly_cargo, we have one line to delete and don't need to worry accidentally deleting part of the important test command.

I just did this in freshness_checksum.rs and am tempted to roll it out further.

View changes since the review

epage · 2026-08-21T20:44:44Z

+        ("positive", "positive", "2", "2"),
+        ("wrong-type", "wrong_type", r#""s""#, "0"),
+        ("out-of-range", "out_of_range", "4", "0"),
    ] {


Would make the feature commit cleaner to have expected_opt_level in the previous commit

View changes since the review

epage · 2026-08-21T20:45:11Z

 }
+
+#[cargo_test]
+fn min_opt_level_without_feature_gate() {


Why are tests being added in this commit?

View changes since the review

As I thought, testing a nonexistent -Z flag doesn’t make sense. However, I can move it to the previous commit.

rustbot · 2026-08-26T07:53:19Z

This PR was rebased onto a different master 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.

Record the current ignored-hint behavior across the RFC precedence, scope, validation, and boundary cases. Signed-off-by: 0xPoe <poe.liu@pm.me>

RFC 3924 lets a package request a numeric optimization floor without overriding an application's profile choices. Apply the hint after profile and built-in host defaults but before explicit package and build overrides, while leaving the unordered size levels unchanged. Signed-off-by: 0xPoe <poe.liu@pm.me>

Document the unstable gate, accepted range, profile precedence, non-recursive scope, and the narrow cases where packages should request optimization. Include an explicit debugging override so top-level users retain control. Signed-off-by: 0xPoe <poe.liu@pm.me>

0xPoe changed the title ~~feat(hints): support minimum optimization levels~~ WIP: feat(hints): support minimum optimization levels Aug 16, 2026

0xPoe changed the title ~~WIP: feat(hints): support minimum optimization levels~~ WIP(Not Ready For Review): feat(hints): support minimum optimization levels Aug 16, 2026

0xPoe force-pushed the hint-min-opt-level branch 7 times, most recently from b532a3f to 2b19c48 Compare August 21, 2026 07:18

0xPoe commented Aug 21, 2026

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rustbot assigned joshtriplett Aug 21, 2026

0xPoe changed the title ~~WIP(Not Ready For Review): feat(hints): support minimum optimization levels~~ feat(hints): support minimum optimization levels Aug 21, 2026

0xPoe marked this pull request as ready for review August 21, 2026 07:20

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Aug 21, 2026

epage reviewed Aug 21, 2026

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0xPoe force-pushed the hint-min-opt-level branch 2 times, most recently from 14ab0ce to 415812c Compare August 26, 2026 07:53

0xPoe force-pushed the hint-min-opt-level branch from 415812c to aaf6533 Compare August 26, 2026 07:56

0xPoe marked this pull request as draft August 26, 2026 07:59

rustbot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Aug 26, 2026

0xPoe added the S-waiting-on-author Status: The marked PR is awaiting some action (such as code changes) from the PR author. label Aug 26, 2026

0xPoe force-pushed the hint-min-opt-level branch from aaf6533 to 2c86de0 Compare August 26, 2026 08:00

0xPoe added 3 commits August 26, 2026 10:03

test(hints): add min-opt-level behavior tests

25bf5df

Record the current ignored-hint behavior across the RFC precedence, scope, validation, and boundary cases. Signed-off-by: 0xPoe <poe.liu@pm.me>

0xPoe force-pushed the hint-min-opt-level branch from 2c86de0 to 18f313e Compare August 26, 2026 08:04

		The hint accepts the numeric optimization levels 0, 1, 2, and 3. Other values
		produce a warning and are ignored. If the selected [profile](profiles.md) has a

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0xPoe commented Aug 16, 2026

What does this PR try to resolve?

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