I'm trying to understand something in https://github.com/SergioBenitez/Rocket/blob/master/examples/raw_sqlite/src/main.rs -- specifically:
conn.execute("INSERT INTO entries (id, name) VALUES ($1, $2)", &[&0 as &dyn ToSql, &"Rocketeer"]) .expect("insert single entry into entries table");
I think I understand the underlying problem being solved here: A slice can only contain a single type, and essentially what's being passed is an reference to a slice of references. In terms of C what's being constructed is an "array of pointers", but Rust needs needs more strict typing; it needs references to the same type.
- If I remove the explicit type cast
as &dyn ToSqlthe compiler complains about mixed types ("expected integer, found &str"). This makes sense to me.
- If I remove the first entry and keep the second (i.e.
&[&"Rocketeer"]), it compiles fine.
- If I remove the second entry and the explicit type cast
as &dyn ToSqlfrom the first entry, (i.e.
&[&0]), it compiles fine.
I'm a little confused by this. I understand that the explicit type cast can be used to tell the compiler "this is the type I want for the elements of this slice", and it will coerce the following elements if possible.
But why does it still work if I just have one element/type? I assume it's getting the ToSql coercion from somewhere else, but why couldn't it do the same if there are multiple elements?