iec61131: Struct definition does not allow Array Init

[Jan-Bulthuis]

Thank you for your work on this project! I hope to be able to use it to perform some codegen from the ST AST. However, I did run into an issue.

The generated parser parses a struct type as follows (leaving out some irrelevant bits):

while !self.check(&Token::EndStruct) {
    let name = self.expect_identifier()?;
    self.expect(Token::Colon)?;
    let field_type = self.parse_type_spec()?;
    
    let init_value = if self.match_token(&Token::Assign) {
        Some(self.parse_expression()?)
    } else {
        None
    };
}

when a field has an initializer, this is parsed using parse_expression. However this method diverges slightly from the grammar rules set forth by IEC61131-3 2013. Specifically, parse_expression does not allow array initialization as below

TYPE TEST_TYPE :
STRUCT
    ARR : ARRAY [1..2] OF REAL := [1.0, 2.0];
END_STRUCT;
END_TYPE

because [1.0, 2.0] is not a valid expression.

Similarly, arrays with initialization also do not seem to be allowed. The following also gives an error when parsing:

TYPE ARR_TYPE :
    ARRAY [1..16] OF REAL := [1.0, 2.0];
END_TYPE

If you would be willing to share how you generate the ast/parser/lexer files I would be more than happy to take a look myself and to submit a PR.