AST validation is a separate AST pass that visits each item in the tree and performs simple checks. This pass doesn't perform any complex analysis, type checking or name resolution.
Before performing any validation, the compiler first expands the macros. Then this pass performs validations to check that each AST item is in the correct state. And when this pass is done, the compiler runs the crate resolution pass.
Validations are defined in AstValidator type, which itself is located in rustc_ast_passes crate. This type implements various simple checks which emit errors when certain language rules are broken.
In addition, AstValidator implements Visitor trait that defines how to visit AST items (which can be functions, traits, enums, etc).
For each item, visitor performs specific checks. For example, when visiting a function declaration, AstValidator checks that the function has:
u16::MAX parameters;