The code in this directory defines the AST that represents Carbon code in the
rest of explorer.
The AST is not quite immutable, because some node properties are set during some
phase of static analysis, rather than during parsing. However, AST mutations are
monotonic: once set, a node property cannot be changed. Furthermore, if a
property is set after parsing, its documentation specifies what phase is
responsible for setting it. Certain properties have has_foo() members for
querying whether they are set, but those are for internal use within the phase
that sets them. As a result, you can think of the AST as if it were immutable,
but with certain parts that you can't yet observe, depending on what phase of
compilation you're in.
All node types in the AST are derived from AstNode, and use
LLVM-style RTTI to support
safe down-casting and similar operations. Each abstract class Foo in the
hierarchy has a kind method which returns a enum FooKind that identifies the
concrete type of the object, and a FooKind value can be safely static_casted
to BarKind if that value represents a type that's derived from both Foo and
Bar.
These FooKind enums are generated from a description of the class hierarchy
that is provided by X macros defined in
ast_kinds.h that specify the classes derived from each AST base
class. Those macros must be kept up to date as the class hierarchy changes.
The AST class hierarchy is structured in a fairly unsurprising way, with
abstract classes such as Statement and Expression, and concrete classes
representing individual syntactic constructs, such as If for if-statements.
Sometimes it is useful to work with a subset of node types that "cuts across"
the primary class hierarchy. Rather than deal with the pitfalls of multiple
inheritance, we handle these cases using a form of type erasure: we specify a
notional interface that those types conform to, and then define a "view" class
that behaves like a pointer to an instance of that interface. Types declare that
they model an interface Foo by defining a public static member named
ImplementsCarbonFoo. See ValueNodeView for an example of this
pattern.