Working directly with source code is very inconvenient and error-prone. Thus,
before we do anything else, we convert raw source code into an Abstract Syntax
Tree (AST). It turns out that doing even this involves a lot of work,
including lexing, parsing, macro expansion, name resolution, conditional
compilation, feature-gate checking, and validation of the AST. In this chapter,
we take a look at all of these steps.
Notably, there isn't always a clean ordering between these tasks. For example,
macro expansion relies on name resolution to resolve the names of macros and
imports. And parsing requires macro expansion, which in turn may require
parsing the output of the macro.