Defining the parser: debug impls and testing

As the final part of the parser, we need to write some tests. To do so, we will create a database, set the input source text, run the parser, and check the result. Before we can do that, though, we have to address one question: how do we inspect the value of an interned type like Expression?

The DebugWithDb trait

Because an interned type like Expression just stores an integer, the traditional Debug trait is not very useful. To properly print a Expression, you need to access the Salsa database to find out what its value is. To solve this, salsa provides a DebugWithDb trait that acts like the regular Debug, but takes a database as argument. For types that implement this trait, you can invoke the debug method. This returns a temporary that implements the ordinary Debug trait, allowing you to write something like

#![allow(unused)]
fn main() {
eprintln!("Expression = {:?}", expr.debug(db));
}

and get back the output you expect.

The DebugWithDb trait is automatically derived for all #[input], #[interned], and #[tracked] structs.

Forwarding to the ordinary Debug trait

For consistency, it is sometimes useful to have a DebugWithDb implementation even for types, like Op, that are just ordinary enums. You can do that like so:

#![allow(unused)]

fn main() {
}

Writing the unit test

Now that we have our DebugWithDb impls in place, we can write a simple unit test harness. The parse_string function below creates a database, sets the source text, and then invokes the parser:

#![allow(unused)]
fn main() {
/// Create a new database with the given source text and parse the result.
/// Returns the statements and the diagnostics generated.
#[cfg(test)]
fn parse_string(source_text: &str) -> String {
    use salsa::Database as _;

    crate::db::Database::default().attach(|db| {
        // Create the source program
        let source_program = SourceProgram::new(db, source_text.to_string());

        // Invoke the parser
        let statements = parse_statements(db, source_program);

        // Read out any diagnostics
        let accumulated = parse_statements::accumulated::<Diagnostic>(db, source_program);

        // Format the result as a string and return it
        format!("{:#?}", (statements, accumulated))
    })
}
}

Combined with the expect-test crate, we can then write unit tests like this one:

#![allow(unused)]
fn main() {
#[test]
fn parse_print() {
    let actual = parse_string("print 1 + 2");
    let expected = expect_test::expect![[r#"
        (
            Program {
                [salsa id]: Id(0),
                statements: [
                    Statement {
                        span: Span {
                            [salsa id]: Id(4),
                            start: 0,
                            end: 11,
                        },
                        data: Print(
                            Expression {
                                span: Span {
                                    [salsa id]: Id(3),
                                    start: 6,
                                    end: 11,
                                },
                                data: Op(
                                    Expression {
                                        span: Span {
                                            [salsa id]: Id(0),
                                            start: 6,
                                            end: 7,
                                        },
                                        data: Number(
                                            OrderedFloat(
                                                1.0,
                                            ),
                                        ),
                                    },
                                    Add,
                                    Expression {
                                        span: Span {
                                            [salsa id]: Id(2),
                                            start: 10,
                                            end: 11,
                                        },
                                        data: Number(
                                            OrderedFloat(
                                                2.0,
                                            ),
                                        ),
                                    },
                                ),
                            },
                        ),
                    },
                ],
            },
            [],
        )"#]];
    expected.assert_eq(&actual);
}
}