Fix E0739: #[target_feature] Attribute Not Allowed on Safe Functions

Thu, 23 Jul 2026 07:56:04 +0800

1. Symptoms

When attempting to compile Rust code that uses the #[target_feature] attribute on a safe function, the compiler produces error E0739 with a clear diagnostic message. The error indicates that the attribute cannot be applied to non-unsafe functions because using CPU-specific features requires unsafe guarantees.

The typical compiler output looks like this:

error[E0739]: the `#[target_feature(...)]` attribute is not yet allowed on safe functions
  --> src/main.rs:10:1
   |
10 | #[target_feature(sse, sse2)]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^ attribute was applied to safe function
   |
   = note: require the function to be marked `unsafe` because using the target_feature attribute is unsafe

In this example, the compiler is telling you that the function at line 10 in src/main.rs uses the #[target_feature] attribute but is not marked as unsafe. The error also appears when using multiple features like #[target_feature(avx, avx2)] or #[target_feature(simd128)] on WebAssembly targets.

Target_feature on ErrorVault — Developer Error Code Dictionary

Fix E0739: #[target_feature] Attribute Not Allowed on Safe Functions

1. Symptoms