some missing files

Author: mark-i-m

src/SUMMARY.md

@@ -113,7 +113,6 @@
     - [Constant evaluation](./const-eval.md)
         - [miri const evaluator](./miri.md)
     - [Monomorphization](./backend/monomorph.md)
-    - [Compiler Backend](./backend/backend.md)
     - [Lowering MIR](./backend/lowering-mir.md)
     - [Code Generation](./backend/codegen.md)
         - [Updating LLVM](./backend/updating-llvm.md)

src/ast-validation.md

@@ -0,0 +1,6 @@
+# AST Validation
+
+AST validation is the process of checking various correctness properties about
+the AST after macro expansion.
+
+**TODO**: write this chapter.

src/pat-exhaustive-checking.md

@@ -0,0 +1,7 @@
+# Pattern and Exhaustiveness Checking
+
+In Rust, pattern matching and bindings have a few very helpful properties. The
+compiler will check that bindings are irrefutable when made and that match arms
+are exhaustive.
+
+**TODO**: write this chapter.