AST Validation in Rust Compiler

# AST validation _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 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: * no more than `u16::MAX` parameters; * c-variadic argument goes the last in the declaration; * documentation comments aren't applied to function parameters; * and other validations.

AST validation is a pass in the Rust compiler that checks the state of each item in the AST after macro expansion but before crate resolution. It uses the `AstValidator` type, which implements the `Visitor` trait, to perform simple checks on various AST items like functions, traits, and enums, emitting errors when language rules are violated. For example, when validating a function, it checks parameter limits, the position of c-variadic arguments, and the absence of documentation comments on parameters.