MySQL "ALTER ... DROP PRIMARY KEY" is not parsed correctly

[MichaelScofield]

The following alter statement to drop primary key directly is allowed in MySQL:

ALTER TABLE table_name DROP PRIMARY KEY;

However, it's not parsed correctly in sqlparser, for example:

use sqlparser::dialect::MySqlDialect;
use sqlparser::parser::Parser;
use sqlparser::tokenizer::Token;
use sqlparser::tokenizer::Tokenizer;

#[test]
fn test() {
    let dialect = &MySqlDialect {};
    let sql = r"ALTER TABLE my_table DROP PRIMARY KEY;";
    let mut tokenizer = Tokenizer::new(dialect, sql);
    let tokens: Vec<Token> = tokenizer.tokenize().unwrap();
    let mut parser = Parser::new(tokens, dialect);
    parser.next_token();
    let statement = parser.parse_alter();
    println!("{:?}", statement);
}

Which prints

Ok(AlterTable { name: ObjectName([Ident { value: "my_table", quote_style: None }]), operation: DropColumn { column_name: Ident { value: "PRIMARY", quote_style: None }, if_exists: false, cascade: false } })

Looks like the "drop primary key" is interpreted as "drop column 'PRIMARY'".