Add parse function to Rust RBS bindings (#53)

The `parse` function enables parsing RBS code from Rust.
This provides a safe Rust interface to the C parser, handling memory
management and encoding setup.

Author: alexcrocha

rust/ruby-rbs/src/lib.rs

@@ -1,5 +1,44 @@
 include!(concat!(env!("OUT_DIR"), "/bindings.rs"));
+use rbs_encoding_type_t::RBS_ENCODING_UTF_8;
 use ruby_rbs_sys::bindings::*;
+use std::sync::Once;
+
+static INIT: Once = Once::new();
+
+/// Parse RBS code into an AST.
+///
+/// ```rust
+/// use ruby_rbs::parse;
+/// let rbs_code = r#"type foo = "hello""#;
+/// let signature = parse(rbs_code.as_bytes());
+/// assert!(signature.is_ok(), "Failed to parse RBS signature");
+/// ```
+pub fn parse(rbs_code: &[u8]) -> Result<*mut rbs_signature_t, String> {
+    unsafe {
+        INIT.call_once(|| {
+            rbs_constant_pool_init(RBS_GLOBAL_CONSTANT_POOL, 26);
+        });
+
+        let start_ptr = rbs_code.as_ptr() as *const i8;
+        let end_ptr = start_ptr.add(rbs_code.len());
+
+        let raw_rbs_string_value = rbs_string_new(start_ptr, end_ptr);
+
+        let encoding_ptr = &rbs_encodings[RBS_ENCODING_UTF_8 as usize] as *const rbs_encoding_t;
+        let parser = rbs_parser_new(raw_rbs_string_value, encoding_ptr, 0, rbs_code.len() as i32);
+
+        let mut signature: *mut rbs_signature_t = std::ptr::null_mut();
+        let result = rbs_parse_signature(parser, &mut signature);
+
+        rbs_parser_free(parser);
+
+        if result {
+            Ok(signature)
+        } else {
+            Err(String::from("Failed to parse RBS signature"))
+        }
+    }
+}
 
 pub struct RBSString {
     pointer: *const rbs_string_t,
@@ -17,3 +56,19 @@ impl RBSString {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_parse() {
+        let rbs_code = r#"type foo = "hello""#;
+        let signature = parse(rbs_code.as_bytes());
+        assert!(signature.is_ok(), "Failed to parse RBS signature");
+
+        let rbs_code2 = r#"class Foo end"#;
+        let signature2 = parse(rbs_code2.as_bytes());
+        assert!(signature2.is_ok(), "Failed to parse RBS signature");
+    }
+}