add fn ptrs to reference

Author: RalfJung

reference/src/SUMMARY.md

@@ -13,5 +13,6 @@
   - [Packed SIMD vectors](./layout/packed-simd-vectors.md)
 - [Validity](./validity.md)
   - [Unions](./validity/unions.md)
+  - [Function Pointers](./validity/function-pointers.md)
 - [Optimizations](./optimizations.md)
   - [Return value optimization](./optimizations/return_value_optimization.md)

reference/src/validity/function-pointers.md

@@ -0,0 +1,25 @@
+# Validity of function pointers
+
+**Disclaimer**: This chapter is a work-in-progress. What's contained here
+represents the consensus from issue [#72]. The statements in here are not (yet)
+"guaranteed" not to change until an RFC ratifies them.
+
+A function pointer is "valid" (in the sense that it can be produced without causing immediate UB) if and only if it is non-null.
+
+That makes this code UB:
+
+```rust
+fn bad() {
+    let x: fn() = unsafe { mem::transmute(0usize) } // This is UB!
+}
+```
+
+However, any integer value other than NULL is allowed for function pointers:
+
+```rust
+fn good() {
+    let x: fn() = unsafe { mem::transmute(1usize) } // This is not UB.
+}
+```
+
+[#72]: https://github.com/rust-lang/unsafe-code-guidelines/issues/72