Constant EXPECT 

pub const EXPECT: HeaderName;

Available on crate feature http only.

Indicates expectations that need to be fulfilled by the server in order to properly handle the request.

The only expectation defined in the specification is Expect: 100-continue, to which the server shall respond with:

• 100 if the information contained in the header is sufficient to cause an immediate success,

• 417 (Expectation Failed) if it cannot meet the expectation; or any other 4xx status otherwise.

For example, the server may reject a request if its Content-Length is too large.

No common browsers send the Expect header, but some other clients such as cURL do so by default.