Enabling target_feature gated language features when using wasm32 simd intrinsics

So I am trying to use core::arch::wasm32::i32x4_* functions in order to compile SIMD intrinsics into WebAssembly.

The intrinsic functions I am trying to use are implemented here and documented here. Most of these functions are not stable and so I am using rustc 1.45.0-nightly (2020-05-30). In addition, the compiler required that I opt into these options:

// https://github.com/rust-lang/rust/issues/44839
#![feature(wasm_target_feature)]

// https://github.com/rust-lang/rust/issues/69098
#![feature(target_feature_11)]

I see in mod.rs of wasm32 that the functions I am trying to use are feature gated with #[cfg(any(target_feature = "simd128", dox))]

The function I am implementing, looks like this:

#[cfg(target_arch = "wasm32")]
#[target_feature(enable = "simd128")]
unsafe fn _dot_simd(&self) -> i32 {
    // use core::arch::wasm32::*;
    core::arch::wasm32::memory_size(3);  /*<-- this line works since it is stable */
    core::arch::wasm32::i32x4_splat(32); /*<-- this line errors */
    42
}

If I understand #[target_feature(enable = "simd128")] correctly, the code above would work since it forces "simd128" feature be enabled regardless of the hardware. I also tried enabling that feature through -C target_feature=+simd128 but that also did not change anything. The error I am getting is

i32x4_splat(32);
   ^^^^^^^^^^^ not found in core::arch::wasm32

And the command I run to compile is wasm-pack build which under the hood runs:

cargo build --lib --release --target wasm32-unknown-unknown

Could you help me see what I am missing?