Permit impl item to end with semicolon

[dtolnay]

Writing signatures of a trait impl with a trailing semicolon is ubiquitous when discussing library APIs in Rust.

impl<'a, K: Debug + Ord, V: Debug> Error for OccupiedError<'a, K, V>;
                                                                    ^

Despite being not compiler-accepted syntax, the notation is easily understandable to human Rust programmers.

Some examples in the wild:

• Tracking Issue for tcplistener_into_incoming rust-lang/rust#88373 (comment)
• Tracking Issue for Iterator::map_windows (feature iter_map_windows) rust-lang/rust#87155 (comment)
• Tracking Issue for map_try_insert rust-lang/rust#82766 (comment)

In fact the notation serves an important role in API design discussions: talking about impl Trait for SomeType; makes no claim about how the methods of Trait are implemented, just that Trait is implemented. How it's implemented is unspecified. This is in contrast to someone having written impl Trait for SomeType {} which is legal syntax for an impl that inherits all default method implementations from the trait definition, which is most often not what the participant intended to communicate.

// I propose adding this API to the standard library!

trait SomeTrait {
    fn method(&self) -> Option<&T> { /* default impl returns None */ }
}

// we'll provide the obvious SomeTrait impl for Thing
impl SomeTrait for std::thing::Thing;

// confusing: am I saying we will *not* provide a Thing-specific implementation of `method`?
impl SomeTrait for std::thing::Thing {}

Sadly, today the notation throws off tree-sitter. You can tell from GitHub's syntax highlighting in the links above. Notice how impl keywords after a semicolon aren't getting colored as keywords.

Running tree-sitter on the trait impl signatures added in this PR to corpus/declarations.txt confirms that tree-sitter doesn't really have that much idea what is going on. Before:

(source_file
  (ERROR
    (type_parameters
      (constrained_type_parameter
        (type_identifier)
        (trait_bounds (type_identifier) (type_identifier))))
    (identifier)
    (ERROR (generic_type (type_identifier) (type_arguments (type_identifier))))
    (abstract_type
      (ERROR
        (bracketed_type
          (ERROR (type_identifier))
          (bounded_type (type_identifier) (type_identifier)))
        (identifier))
      (generic_type (type_identifier) (type_arguments (type_identifier)))))
  (empty_statement))

This PR makes the body field of impl_item optional, and will parse impl signatures with a trailing semicolon as having no body.

(source_file
  (impl_item
    (type_parameters
      (constrained_type_parameter
        (type_identifier)
          (trait_bounds (type_identifier) (type_identifier))))
    (type_identifier)
    (generic_type (type_identifier) (type_arguments (type_identifier))))
  (impl_item
    (type_parameters
      (constrained_type_parameter
        (type_identifier)
        (trait_bounds (type_identifier) (type_identifier))))
    (type_identifier)
    (generic_type (type_identifier) (type_arguments (type_identifier)))))

[maxbrunsfeld]

Sorry for the delay on this. Thanks for the great explanation.