Invalid expected alignment for fixed-size array

[kvark]

This is from miri testing naga, aka:

git clone https://github.com/gfx-rs/naga
cd naga
cargo +nightly miri test

Have miri complaining about this code:

/// Additional information about the glsl shader
///
/// Stores additional information about the glsl shader which might not be
/// stored in the shader [`Module`](Module).
#[derive(Debug)]
pub struct ShaderMetadata {
    /// The glsl version specified in the shader trought the use of the
    /// `#version` preprocessor directive.
    pub version: u16,
    /// The glsl profile specified in the shader trought the use of the
    /// `#version` preprocessor directive.
    pub profile: Profile,
    /// The shader stage in the pipeline, passed to the [`parse`](Parser::parse)
    /// method via the [`Options`](Options) struct.
    pub stage: ShaderStage,

    /// The workgroup size for compute shaders, defaults to `[1; 3]` for
    /// compute shaders and `[0; 3]` for non compute shaders.
    pub workgroup_size: [u32; 3],
    /// Wether or not early fragment tests where requested by the shader,
    /// defaults to `false`.
    pub early_fragment_tests: bool,

    /// The shader can request extensions via the
    /// `#extension` preprocessor directive, in the directive a behavior
    /// parameter is used to control whether the extension should be disabled,
    /// warn on usage, enabled if possible or required.
    ///
    /// This field only stores extensions which were required or requested to
    /// be enabled if possible and they are supported.
    pub extensions: FastHashSet<String>,
}

impl ShaderMetadata {
    fn reset(&mut self, stage: ShaderStage) {
        self.version = 0;
        self.profile = Profile::Core;
        self.stage = stage;
        self.workgroup_size = [if stage == ShaderStage::Compute { 1 } else { 0 }; 3];
        self.early_fragment_tests = false;
        self.extensions.clear();
    }
}

test front::glsl::parser_tests::constants ... error: Undefined Behavior: accessing memory with alignment 4, but alignment 8 is required
   --> src/front/glsl/mod.rs:102:9
    |
102 |         self.workgroup_size = [if stage == ShaderStage::Compute { 1 } else { 0 }; 3];
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ accessing memory with alignment 4, but alignment 8 is required
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

First problem - this is just safe Rust code, there is no room for UB.
Second problem - it says the required alignment of 8 is required, while I'd expect 4 to be required.

[RalfJung]

We recently fixed a bug wrt alignment checks during array initialization, see #1925, #1919. This looks like a duplicate. Are you using the very latest Miri? The bug should be fixed there.

[kvark]

I used the latest as of today - from cargo 1.58.0-nightly (2e2a16e98 2021-11-08)

[RalfJung]

That's almost 2 months old -- that's definitely not the latest version.

[kvark]

Damn you are right, sorry about the noise!
Tested on 1.59.0-nightly (f1ce0e6a0 2022-01-05),and it's all good.

[RalfJung]

No worries, I'm glad it's working now :)