Can we allow opportunistic in-place argument passing?

@nikic

@nikic suggested to me at the all-hands that sometimes it would be nice to allow function arguments to "opportunistically" be passed in-place: even if the argument is copy, which usually guarantees the caller will see the value at a different address than the callee, it could sometimes be useful to avoid the copy and just pass the value in-place.

@nikic do you have a concrete example of that?
@Amanieu do you think your MIR move semantics proposal could help with this?

We already have this which turns copy into move in MIR call terminators: rust-lang/rust#113758

And how is that justified opsem-wise? Does it check that the address is not observed, or something like that?

It only applies to locals whose address is never taken.

We already have this which turns copy into move in MIR call terminators: rust-lang/rust#113758

This transformation is not quite correct as far as current semantics is concerned. It ignores the fact that address of a local might be captured when it is passed in place (as an argument or a destination, although I am not sure if the latter is an issue). For example:

pub fn f(a: [u8; 1024]) {
    g(a);
}

unsafe extern "Rust" {
    safe fn g(a: [u8; 1024]);
}

--- a.f.3-2-029.DeadStoreElimination-final.before.mir	2026-06-07 00:19:24.265982702 +0200
+++ a.f.3-2-029.DeadStoreElimination-final.after.mir	2026-06-07 00:19:24.266036594 +0200
@@ -1,15 +1,15 @@
-// MIR for `f` before DeadStoreElimination-final
+// MIR for `f` after DeadStoreElimination-final
 
 fn f(_1: [u8; 1024]) -> () {
     debug a => _1;
     let mut _0: ();
     let _2: ();
 
     bb0: {
-        _2 = g(copy _1) -> [return: bb1, unwind continue];
+        _2 = g(move _1) -> [return: bb1, unwind continue];
     }
 
     bb1: {
         return;
     }
 }

The change from copy to move potentially introduces behavior which wasn't possible before, since address _1 might be observed before a call to f and inside g.

It probably makes more sense under semantics from "MIR move elimination".

This is actually completely independent from the "MIR move elimination" semantics. See minirust/minirust#65 (comment) for the semantics that I have in mind for move operands to calls. Effectively, passing a place by move "donates" that place to an allocation pool for the duration of the call, and locals (including argument/return place locals) in callees may be allocated from this pool. This gives us the needed flexibility to do all of the optimizations we want.

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Can we allow opportunistic in-place argument passing? #614

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Can we allow opportunistic in-place argument passing? #614

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Amanieu commented on Jun 6, 2026

RalfJung commented on Jun 6, 2026

Amanieu commented on Jun 6, 2026

tmiasko commented on Jun 6, 2026

Amanieu commented on Jun 7, 2026

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