change the output of such query, and trigger previous memoized values to be
potentially invalidated.

### Query Groups

A query group is a set of queries which have been defined together as a unit.
The database is formed by combining query groups. Query groups are akin to
"Salsa modules".

A set of queries in a query group are just a set of methods in a trait.

To create a query group a trait annotated with a specific attribute
(`#[salsa::query_group(...)]`) has to be created.

An argument must also be provided to said attribute as it will be used by Salsa
to create a `struct` to be used later when the database is created.

Example input query group:

```rust,ignore
/// This attribute will process this tree, produce this tree as output, and produce
/// a bunch of intermediate stuff that Salsa also uses. One of these things is a
/// "StorageStruct", whose name we have specified in the attribute.
///
/// This query group is a bunch of **input** queries, that do not rely on any
/// derived input.
#[salsa::query_group(InputsStorage)]
pub trait Inputs {
    /// This attribute (`#[salsa::input]`) indicates that this query is a base
    /// input, therefore `set_manifest` is going to be auto-generated
    #[salsa::input]
    fn manifest(&self) -> Manifest;

    #[salsa::input]
    fn source_text(&self, name: String) -> String;
}
```

To create a **derived** query group, one must specify which other query groups
this one depends on by specifying them as supertraits, as seen in the following
example:

```rust,ignore
/// This query group is going to contain queries that depend on derived values.
/// A query group can access another query group's queries by specifying the
/// dependency as a supertrait. Query groups can be stacked as much as needed using
/// that pattern.
#[salsa::query_group(ParserStorage)]
pub trait Parser: Inputs {
    /// This query `ast` is not an input query, it's a derived query this means
    /// that a definition is necessary.
    fn ast(&self, name: String) -> String;
}
```

When creating a derived query the implementation of said query must be defined
outside the trait.  The definition must take a database parameter as an `impl
Trait` (or `dyn Trait`), where trait is the query group that the definition
belongs to, in addition to the other keys.

```rust,ignore
/// This is going to be the definition of the `ast` query in the `Parser` trait.
/// So, when the query `ast` is invoked, and it needs to be recomputed, Salsa is
/// going to call this function and it's going to give it the database as `impl
/// Parser`. The function doesn't need to be aware of all the queries of all the
/// query groups
fn ast(db: &impl Parser, name: String) -> String {
    //! Note, `impl Parser` is used here but `dyn Parser` works just as well
    /* code */
    ///By passing an `impl Parser`, this is allowed
    let source_text = db.input_file(name);
    /* do the actual parsing */
    return ast;
}
```

Eventually, after all the query groups have been defined, the database can be
created by declaring a `struct`.

To specify which query groups are going to be part of the database an `attribute`
(`#[salsa::database(...)]`) must be added. The argument of said `attribute` is a
list of `identifiers`, specifying the query groups **storages**.

```rust,ignore
///This attribute specifies which query groups are going to be in the database
#[salsa::database(InputsStorage, ParserStorage)]
#[derive(Default)] //optional!
struct MyDatabase {
    ///You also need this one field
    runtime : salsa::Runtime<MyDatabase>,
}
///And this trait has to be implemented
impl salsa::Database for MyDatabase {
    fn salsa_runtime(&self) -> &salsa::Runtime<MyDatabase> {
        &self.runtime
    }
}
```

Example usage:

```rust,ignore
fn main() {
    let db = MyDatabase::default();
    db.set_manifest(...);
    db.set_source_text(...);
    loop {
        db.ast(...); //will reuse results
        db.set_source_text(...);
    }
}
```