What is this lint about

Previously inputs for many built-in attributes weren't validated and nonsensical attributes like #[no_std(arbitraty, string = "foo")] were accepted.
#57321 introduced a check for top-level shape (#[attr] vs #[attr(...)] vs #[attr = ...]) for built-in attributes that produces an error by default, but produces a warning for incorrect attribute forms available on stable that were actually encountered in practice (during crater run).
Currently the list is

#[doc]
#[ignore(...)]
#[inline = "..."]
#[link]
#[link = "..."]

, but it can be extended if more regressions are reported.

How to fix this warning/error

Use one of the correct attribute forms suggested by the compiler.

Current status

• Implement basic input validation for built-in attributes #57321 introduces the ill_formed_attribute_input lint as warn-by-default
• Transition future compat lints to {ERROR, DENY} - Take 2 #65785 makes the ill_formed_attribute_input lint deny-by-default
• Start reporting future breakage for ILL_FORMED_ATTRIBUTE_INPUT in dependencies #144544 warns in dependencies
• PR ? makes the ill_formed_attribute_input lint a hard error