Fix rust generator guest import error in `no_std` build

[mambisi]

error snapshot

    |
100 |     pub fn generate_native_address(seed: &[u8]) -> Vec<u8> {
    |                                                    ^^^ not found in this scope
    |
help: consider importing one of these items
    |
3   |     use alloc::vec::Vec;
    |
3   |     use crate::Vec;
    |
3   |     use rune_std::vec::Vec;
    |
3   |     use wit_bindgen_guest_rust::rt::vec::Vec;
    |

adds

#[allow(unused_imports)]
use wit_bindgen_guest_rust::rt::{{alloc, vec::Vec, string::String}};

in the beginning of the mod generation, remove it from ending

[alexcrichton]

Thanks! I think this breaks some existing tests, however, so that'll need fixing as well.

Can you additionally add tests for this to the tests/codegen.rs file?

[mambisi]

Yeah Sure

[alexcrichton]

Sorry but for fixing the tests I don't mean adding imports to the test. The generated code shouldn't require the code around the macro to import anything as the generated code should be entirely standalone.

[mambisi]

Got it

[sunfishcode]

Thanks for the PR! This ended up getting fixed in #488. For no_std use cases, wit-bindgen now has a --std-feature flag in which it emits code that uses #[feature = "std"] guarding things that need std.