Rust Error E0535: Unknown Argument in Inline Attribute

Understanding Rust Error E0535

Rust Error E0535 occurs when an unknown argument is provided to the inline attribute in a Rust program. The inline attribute is used to provide hints to the compiler about when a function should be inlined, and only two arguments are accepted: always and never.

Example leading to Error E0535:

```rust
#[inline(unknown)] // error: invalid argument
pub fn something() {}

fn main() {}
```

In the example above, the inline attribute is given an unknown argument 'unknown', which causes Rust Error E0535.

Fixing Rust Error E0535

To fix this error, you need to replace the unknown argument with one of the accepted arguments: always or never.

Using the inline(never) attribute:

```rust
#[inline(never)] // ok!
pub fn something() {}

fn main() {}
```

Using the inline(always) attribute:

```rust
#[inline(always)] // ok!
pub fn something() {}

fn main() {}
```

In both corrected examples, the inline attribute is given a valid argument, either 'never' or 'always', which prevents Rust Error E0535.

Understanding the Inline Attribute

The inline attribute provides a hint to the compiler about how a function should be optimized. The 'always' option advises the compiler to inline the function at any opportunity, while the 'never' option advises the compiler never to inline the function. Note, however, that these are suggestions and not absolute requirements. The compiler may choose not to follow the hint in certain situations.

Choosing between always and never