Rust error E0534: #### Note: this error code is no longer emitted by the compiler

The compiler team's explanation

Note: this error code is no longer emitted by the compiler

This is because it was too specific to the inline attribute. Similar diagnostics occur for other attributes too. The example here will now emit E0805

The inline attribute was malformed.

Erroneous code example:

#[inline()] // error: expected one argument
pub fn something() {}

fn main() {}

The parenthesized inline attribute requires the parameter to be specified:

#[inline(always)]
fn something() {}

or:

#[inline(never)]
fn something() {}

Alternatively, a paren-less version of the attribute may be used to hint the compiler about inlining opportunity:

#[inline]
fn something() {}

For more information see the inline attribute section of the Reference.

From rustc --explain E0534 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.