Update ast-validation.md

Author: dbelik

src/ast-validation.md

@@ -1,6 +1,37 @@
 # AST Validation
 
-AST validation is the process of checking various correctness properties about
-the AST after macro expansion.
+* [About](#about)
+* [Validation](#validation)
 
-**TODO**: write this chapter. [#656](https://github.com/rust-lang/rustc-dev-guide/issues/656)
+## About
+
+_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` class, which 
+itself is located in `rustc_ast_passes` crate. This
+class 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 functions are declared with at least one named argument;
+* c-variadic argument goes the last in the declaration;
+* documentation comments aren't applied to function parameters;
+* and other validations.