Function preprocess 

pub fn preprocess(
    tokens: &[Token],
    ctx: &Context,
    options: &Options,
    diags: &mut Diagnostics,
) -> Preprocessed

Runs the preprocessor over a lexed translation unit.

The returned list always ends with TokenKind::Eof. Problems the lexer found are reported here: what is wrong with a token itself only when that token survives, and what is wrong with the text — its spelling, and the comments before it — as soon as the token is read. Neither is reported for a group skipped by #if 0, whose text is never read at all and may hold anything.