Attributes

The Sway compiler supports a list of attributes that perform various operations that are useful for building, testing and documenting Sway programs. Below is a list of all available attributes:

Allow

The #[allow(...)] attribute overrides checks so that violations will go unreported. The following checks can be disabled:

• #[allow(dead_code)] disable checks for dead code;
• #[allow(deprecated)] disables checks for usage of deprecated structs, functions and other items.

Doc

The #[doc(..)] attribute specifies documentation.

Line doc comments beginning with exactly three slashes ///, are interpreted as a special syntax for doc attributes. That is, they are equivalent to writing #[doc("...")] around the body of the comment, i.e., /// Foo turns into #[doc("Foo")]

Line comments beginning with //! are doc comments that apply to the module of the source file they are in. That is, they are equivalent to writing #![doc("...")] around the body of the comment. //! module level doc comments should be at the top of Sway files.

Documentation can be generated from doc attributes using forc doc.

Inline

The inline attribute suggests that a copy of the attributed function should be placed in the caller, rather than generating code to call the function where it is defined.

Note: The Sway compiler automatically inlines functions based on internal heuristics. Incorrectly inlining functions can make the program slower, so this attribute should be used with care.

The #[inline(never)] attribute suggests that an inline expansion should never be performed.

The #[inline(always)] attribute suggests that an inline expansion should always be performed.

Note: #[inline(..)] in every form is a hint, with no requirements on the language to place a copy of the attributed function in the caller.

Payable

The lack of #[payable] implies the method is non-payable. When calling an ABI method that is non-payable, the compiler emits an error if the amount of coins forwarded with the call is not guaranteed to be zero. Note that this is strictly a compile-time check and does not incur any runtime cost.

Storage

In Sway, functions are pure by default but can be opted into impurity via the storage function attribute. The storage attribute may take read and/or write arguments indicating which type of access the function requires.

The #[storage(read)] attribute indicates that a function requires read access to the storage.

The #[storage(write)] attribute indicates that a function requires write access to the storage.

More details in Purity.

Test

The #[test] attribute marks a function to be executed as a test.

The #[test(should_revert)] attribute marks a function to be executed as a test that should revert.

More details in Unit Testing.

Deprecated

The #[deprecated] attribute marks an item as deprecated and makes the compiler emit a warning for every usage of the deprecated item. This warning can be disabled using #[allow(deprecated)].

It is possible to improve the warning message with #[deprecated(note = "your message")]

Fallback

The #[fallback] attribute makes the compiler use the marked function as the contract call fallback function, which means that, when a contract is called, and the contract selection fails, the fallback function will be called instead.