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fix: add wasmtime flags! macro support for WIT flags types#1327

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fix: add wasmtime flags! macro support for WIT flags types#1327
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rally/1318-wasm-guest-bindgen-macro-doesn-t-support-wit-flags

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@jsturtevant jsturtevant commented Mar 19, 2026 •
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When is_wasmtime_guest is true, emit wasmtime::component::flags! macro invocations instead of plain structs with bool fields. This enables WIT flags types (e.g. from wasi:filesystem) to satisfy the wasmtime::component::Lift and wasmtime::component::Lower trait bounds. See https://docs.rs/wasmtime/latest/wasmtime/component/macro.flags.html

The non-Wasmtime code path used by host_bindgen! and Hyperlight guest_bindgen! in this repo is unchanged. The Wasmtime path is used by downstream Wasmtime guest-bindgen consumers.

WIT flags input

The tests use the existing WIT flags in src/tests/rust_guests/witguest/guest.wit:

interface roundtrip {
    flags smallflags {
        flag-a,
        flag-b,
        flag-c,
    }

    roundtrip-flags-small: func(x: smallflags) -> smallflags;
}

Expected non-Wasmtime Rust output

host_bindgen! and Hyperlight guest_bindgen! keep generating a bool-field struct:

#[derive(Debug, Clone, PartialEq)]
pub struct Smallflags {
    pub flag_a: bool,
    pub flag_b: bool,
    pub flag_c: bool,
}

Expected Wasmtime guest Rust output

The Wasmtime guest generator emits the real Wasmtime flags macro and preserves the original WIT names on each constant:

::wasmtime::component::flags! {
    Smallflags {
        #[component(name = "flag-a")]
        const FLAG_A;

        #[component(name = "flag-b")]
        const FLAG_B;

        #[component(name = "flag-c")]
        const FLAG_C;
    }
}

Expected marshal/unmarshal output

Because the type produced by wasmtime::component::flags! is bitflag-style, the Hyperlight-generated Wasmtime guest marshal/unmarshal code cannot access fields like value.flag_a. Note: I did consider just porting over to useing bitflag-style everywhere but that's a much bigger change.

For non-Wasmtime bindings, Hyperlight still generates code that reads and writes bool fields:

// Unmarshal bytes into the bool-field struct.
(
    Smallflags {
        flag_a: (bytes[0] >> 0) & 0x1 == 1,
        flag_b: (bytes[0] >> 1) & 0x1 == 1,
        flag_c: (bytes[0] >> 2) & 0x1 == 1,
    },
    1,
)

// Marshal the bool-field struct into bytes.
let mut bytes = [0; 1];
bytes[0] |= (if value.flag_a { 1 } else { 0 }) << 0;
bytes[0] |= (if value.flag_b { 1 } else { 0 }) << 1;
bytes[0] |= (if value.flag_c { 1 } else { 0 }) << 2;
alloc::vec::Vec::from(bytes)

For Wasmtime guest bindings, Hyperlight now generates code that uses the Wasmtime flags API:

// Unmarshal bytes into the Wasmtime flags value.
{
    let mut value_flags = Smallflags::empty();
    if (bytes[0] >> 0) & 0x1 == 1 {
        value_flags |= Smallflags::FLAG_A;
    }
    if (bytes[0] >> 1) & 0x1 == 1 {
        value_flags |= Smallflags::FLAG_B;
    }
    if (bytes[0] >> 2) & 0x1 == 1 {
        value_flags |= Smallflags::FLAG_C;
    }
    (value_flags, 1)
}

// Marshal the Wasmtime flags value into bytes.
let mut bytes = [0; 1];
bytes[0] |= (if value.contains(Smallflags::FLAG_A) { 1 } else { 0 }) << 0;
bytes[0] |= (if value.contains(Smallflags::FLAG_B) { 1 } else { 0 }) << 1;
bytes[0] |= (if value.contains(Smallflags::FLAG_C) { 1 } else { 0 }) << 2;
alloc::vec::Vec::from(bytes)

Closes #1318

Signed-off-by: James Sturtevant jsturtevant@gmail.com

@jsturtevant jsturtevant added the kind/bugfix For PRs that fix bugs label Mar 19, 2026
@jsturtevant jsturtevant marked this pull request as draft March 20, 2026 20:42
@jsturtevant

jsturtevant commented Mar 20, 2026

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moved this to draft, I was integrating into hyperlight-wasm and found the usage of flags needs to be slightly different. than this. will push an update shortly

@jsturtevant jsturtevant force-pushed the rally/1318-wasm-guest-bindgen-macro-doesn-t-support-wit-flags branch 6 times, most recently from f0c8e64 to b68803b Compare June 19, 2026 15:48
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jsturtevant commented Jun 19, 2026
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Comment thread src/tests/rust_guests/witguest/bindgen-test-cases/world.wit Outdated
@jsturtevant jsturtevant force-pushed the rally/1318-wasm-guest-bindgen-macro-doesn-t-support-wit-flags branch 9 times, most recently from b319154 to 564c59e Compare June 19, 2026 16:49
@jsturtevant jsturtevant marked this pull request as ready for review June 19, 2026 16:49
Copilot AI review requested due to automatic review settings June 19, 2026 16:49
@jsturtevant jsturtevant force-pushed the rally/1318-wasm-guest-bindgen-macro-doesn-t-support-wit-flags branch from 564c59e to 6e6617d Compare June 19, 2026 16:51
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Pull request overview

This PR updates the Hyperlight component Rust code generator to correctly support WIT flags types when generating Wasmtime guest bindings, by emitting wasmtime::component::flags! (and updating marshal/unmarshal codegen accordingly) so the generated types satisfy Wasmtime’s Lift/Lower trait bounds. Non-Wasmtime generation paths remain unchanged.

Changes:

  • Emit ::wasmtime::component::flags! { ... } for WIT flags types when is_wasmtime_guest is enabled (instead of a pub struct with bool fields).
  • Update Hyperlight marshal/unmarshal codegen for Wasmtime guests to use the Wasmtime flags API (empty(), |=, .contains(...)) rather than field access.
  • Add an integration test validating the Wasmtime flags macro emission and wire it into just test-integration.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file
File Description
src/hyperlight_component_util/tests/wasmtime_guest_codegen.rs Adds an integration test to validate Wasmtime guest codegen emits wasmtime::component::flags! for WIT flags.
src/hyperlight_component_util/src/rtypes.rs Switches Wasmtime-guest flags type emission from bool-field structs to the Wasmtime flags! macro, preserving original WIT names.
src/hyperlight_component_util/src/hl.rs Updates marshal/unmarshal codegen for flags in the Wasmtime-guest path to use flags operations (empty, `
src/hyperlight_component_util/src/emit.rs Introduces kebab_to_flags_const helper for generating SCREAMING_SNAKE_CASE flag constant identifiers.
Justfile Ensures test-integration runs after generating WIT-derived .wasm inputs and runs the new component-util integration test.

Comment thread src/hyperlight_component_util/tests/wasmtime_guest_codegen.rs
ludfjig
ludfjig previously approved these changes Jun 19, 2026
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LGTM!

Comment thread src/hyperlight_component_util/src/hl.rs Outdated
Comment thread src/hyperlight_component_util/src/hl.rs Outdated
@jsturtevant
fix: add wasmtime flags macro support
26d09f5 
Signed-off-by: James Sturtevant <jsturtevant@gmail.com>
@jsturtevant jsturtevant force-pushed the rally/1318-wasm-guest-bindgen-macro-doesn-t-support-wit-flags branch from e3d190c to 26d09f5 Compare June 19, 2026 23:16
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wasm_guest_bindgen! macro doesn't support WIT flags types (needed for wasi:filesystem)

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