SQL Server: Increase support for multi statement object definitions (without semicolons)

[aharpervc]

It would be extremely useful if this library was able to properly parse multiple statements, particularly in a object definition body, without requiring semi-colons which are optional for SQL Server.

---

The following otherwise valid SQL fails parsing (MSSQL dialect):

create or alter procedure test()
as
begin
    declare @x bit = 1

    if @x = 1
    begin
        select 1
    end
end

The error is at the if @x = 1 line:

end of statement, found: if

The problem appears to be related to parsing multiple statements in the object definition body. The parsing concludes successfully if the code is changed to declare @x bit = 1; with a semi-colon.

There's also the similar case of two top level statements:

declare @x bit = 1

if @x = 1
begin
    select 1
end

The API I was using was parse_sql:

static DIALECT: sqlparser::dialect::MsSqlDialect = sqlparser::dialect::MsSqlDialect {};
let sql_text = ...

let mut statements = Parser::parse_sql(&DIALECT, sql_text.as_str())?; // error: end of statement, found: if

There's a workaround for the multiple top level statements scenario, but that won't work for the stored procedure example:

fn parse_all_statements(sql_text: &str) -> Result<Vec<Statement>, Box<dyn std::error::Error>> {
    let mut parser = Parser::new(&DIALECT)
        .try_with_sql(&sql_text)?;

    let mut statements = Vec::new();

    loop {
        if let Token::EOF = parser.peek_token_ref().token {
            break;
        }

        statements.push(parser.parse_statement()?);

        while let Token::SemiColon = parser.peek_token_ref().token {
            parser.advance_token();
        }
    }

    return Ok(statements);
}

---

follow up from: #1791 (comment)

[aharpervc]

The related PR in this repo went stale, but the branch on my fork is still functional with all tests passing, if anyone else is interested: https://github.com/aharpervc/datafusion-sqlparser-rs/tree/expand-parse-without-semicolons