Rust error E0539: malformed `deprecated` attribute input

The compiler team's explanation

An invalid meta-item was used inside an attribute.

Erroneous code example:

#![feature(staged_api)]
#![allow(internal_features)]
#![stable(since = "1.0.0", feature = "test")]

#[deprecated(note)] // error!
#[unstable(feature = "deprecated_fn", issue = "123")]
fn deprecated() {}

#[unstable(feature = "unstable_struct", issue)] // error!
struct Unstable;

#[rustc_const_unstable(feature)] // error!
const fn unstable_fn() {}

#[stable(feature = "stable_struct", since)] // error!
struct Stable;

#[rustc_const_stable(feature)] // error!
const fn stable_fn() {}

To fix the above example, you can write the following:

#![feature(staged_api)]
#![allow(internal_features)]
#![stable(since = "1.0.0", feature = "test")]

#[deprecated(since = "1.39.0", note = "reason")] // ok!
#[unstable(feature = "deprecated_fn", issue = "123")]
fn deprecated() {}

#[unstable(feature = "unstable_struct", issue = "123")] // ok!
struct Unstable;

#[rustc_const_unstable(feature = "unstable_fn", issue = "124")] // ok!
const fn unstable_fn() {}

#[stable(feature = "stable_struct", since = "1.39.0")] // ok!
struct Stable;

#[stable(feature = "stable_fn", since = "1.39.0")]
#[rustc_const_stable(feature = "stable_fn", since = "1.39.0")] // ok!
const fn stable_fn() {}

Several causes of this are, an attribute may have expected you to give a list but you gave a name = value pair:

// wrong, should be `#[repr(C)]`
#[repr = "C"]
struct Foo {}

Or a name = value pair, but you gave a list:

// wrong, should be `note = "reason"`
#[deprecated(since = "1.0.0", note("reason"))]
struct Foo {}

Or it expected some specific word but you gave an unexpected one:

// should be `always` or `never`
#[inline(maybe_if_you_feel_like_it)]
fn foo() {}

From rustc --explain E0539 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.