mir-transform: Treat optimize(none) the same as opt-level=0

[clubby789]

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cc @RalfJung

This is a quick initial sketch of a solution and the API will need some refining. The idea is that optimisations have a general condition under which they're enabled (i.e. some compiler option is enabled, we're not on some incompatible target) and a minimum MIR opt level. When we then decide whether or not to run a pass, we calculate an MIR opt level from, in order:

• #[optimize(none)] (1 if present)
• -Zmir-opt-level
• -Copt-level (2 if opt-level >= 2, 1 otherwise)

[clubby789]

Typo error caused an ICE in drop elaboration

- optimizations.0 >= min_level
+ optimizations.0 < min_level

which seems a bit surprising

EDIT: Seems like some other opts rely on SimplifyCfg removing dead BBs. Not sure if we need to factor that into the pass handling? Probably fine to ignore in this PR

[clubby789]

Note that this now fails because

#[optimize(none)]
pub fn const_branch() -> i32 {
    if true { 1 } else { 0 }
}

produces return 1

[RalfJung]

EDIT: Seems like some other opts rely on SimplifyCfg removing dead BBs. Not sure if we need to factor that into the pass handling? Probably fine to ignore in this PR

What do you mean by "rely" here and why does this become more of a problem with this PR?

[clubby789]

What do you mean by "rely" here

I didn't look into it deeply, but a few passes seemed to ICE if they ran with dead BBs.

and why does this become more of a problem with this PR?

It doesn't; I just made a typo during implementation which exposed this and I was unsure if that was intentional/known.

[RalfJung]

It doesn't; I just made a typo during implementation which exposed this and I was unsure if that was intentional/known.

We'd have to ask the people that wrote the passes, so it'd help if you had a concrete example and backtrace. I presume this can be reproduced with -Zmir-enable-passes already on nightly?

[clubby789]

The ICE is from a debug assertion; with the typo reverted, and debug-assertions enabled, the ICE is as follows (when building std) rustc-ice-2026-08-10T06_58_41-132370.txt

Minimised to a baseline rustc with debug assertions:

// rustc ice.rs -Zmir-enable-passes=-SimplifyCfg-initial,-SimplifyCfg-promote-consts,-SimplifyCfg-post-analysis
fn mir_drop<T>(_place: T) {
    panic!()
}

fn main() {
    mir_drop(());
}

Not sure if this is a problem at all in practice? If you think it is I can split out a new issue since it's not too relevant to this PR specifically

[RalfJung]

Thanks! Could you file an issue? Then we can ping some folks to see what the expected contract for these passes is.

[clubby789]

Refactored around this API:

impl PassPolicy {
    /// Create a [`PassPolicy::Optional`] that is not an optimization,
    /// enabled by default under the given condition.
    pub(crate) fn optional_non_optimization(enabled_by_default: bool) -> Self;

    /// Create a [`PassPolicy::Optional`] optimization enabled at the given MIR optimization level.
    pub(crate) fn optimization(ctx: &PassCtx<'_>, min_mir_opt_level: usize) -> Self;

    /// Add another condition to an optional pass's default enablement.
    pub(crate) fn and_enabled(self, enabled: bool) -> Self;

    /// Add a minimum mir-opt-level to an optional pass.
    /// Will panic if used on a required pass.
    fn with_min_mir_opt_level(self, ctx: &PassCtx<'_>, min_mir_opt_level: usize) -> Self ;

[RalfJung]

I am not sure that PassPolicy::optimization(ctx, 2) is really more readable than PassPolicy::optimization(ctx.mir_opt_level() >= 2). The meaning of the "2" becomes rather opaque.

[rustbot]

Some changes occurred in coverage instrumentation.

cc @Zalathar

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[rustbot]

r? @JohnTitor

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[clubby789]

I dropped the optimization method, since it's generally easier to just have an optional constructor which can take a ctx.mir_opt_level() comparison. I also updated the opt-none test to something that is no longer optimised at level 0.