bug - comprehension over a Rust interop iterator emits .iter() and fails to compile

[dannymeijer]

Area

Compiler (frontend/backend/codegen)

Summary

A list comprehension over a Rust-interop iterator typechecks, but emits .iter() on a value that consumes itself, so the generated Rust does not compile.

std::env::Args is an Iterator by value. It has no iter() method. The comprehension lowers as though the source were a collection.

Reproduction steps

from rust::std::env import args


pub def main() -> None:
    arguments: list[str] = [argument for argument in args()]
    println(f"{len(arguments)}")

incan check src/main.incn     # passes
incan oven bake --project .   # generated Rust fails to compile

A plain for loop over the same value lowers correctly, which is the workaround:

    mut arguments: list[str] = []
    for argument in args():
        arguments.append(argument)

Output / logs

error[E0599]: no method named `iter` found for struct `Args` in the current scope
   --> src/main.rs:91:10
    |
 90 |       let arguments: Vec<String> = (args())
    |  __________________________________-
 91 | |         .iter()
    | |_________-^^^^
    |
    = help: items from traits can only be used if the trait is implemented and in scope
note: `Iterable` defines an item `iter`, perhaps you need to implement it

Environment

• Compiler built from 0.6.0-dev.4 at 0a8395835
• Stage: emission (comprehension lowering over a rust:: interop iterator)
• Impact: blocks incan oven bake for any component whose entrypoint reads process arguments this way, and therefore blocks incan test for that component. The for-loop form above is a semantically identical workaround and is what the affected source now does, marked with a TODO.
• Note the asymmetry: for x in args() lowers correctly while [x for x in args()] does not, so the two forms disagree on what an interop iterator is.
• Found in the Incan-authored Oven control plane under workspaces/oven while making its test suite runnable (feature - RFC 119 native Rust facets and direct-rustc planning #1037, PR chore - retain the Incan-authored Oven control plane (#1037) #1487).