Update doc for unions

Addresses rust-lang#832

book/src/using-unions.md

@@ -1,10 +1,14 @@
 # Using the Union Types Generated by Bindgen
 
-**NOTE:** As of Rust version 1.17, Rust does not have a stable `union` type. Issue [#32836](https://github.com/rust-lang/rust/issues/32836) tracks the stabilization of a `union` type in Rust. 
+**NOTE**: Rust 1.19 stabilized the `union` type (see Rust issue [#32836](https://github.com/rust-lang/rust/issues/32836)).
 
-By using the flag `--unstable-rust`, bindgen will generate the preliminary unstable `union` type.
+You can pass the `--rust-target` option to tell `bindgen` to target a specific version of Rust.
+By default, `bindgen` will target the latest stable Rust.
+The `--rust-target` option accepts a specific stable version (such as "1.0" or "1.19") or "nightly".
 
-In general, most interactions with unions (either reading or writing) are unsafe.
+**NOTE**: The `--unstable-rust` option is deprecated; use `--rust-target nightly` instead.
+
+In general, most interactions with unions (either reading or writing) are unsafe, meaning you must surround union accesses in an `unsafe {}` block.
 
 For this discussion, we will use the following C type definitions:
 
@@ -31,46 +35,47 @@ typedef union {
 
 ### Library
 
-* [`bindgen::Builder::unstable_rust()`](https://docs.rs/bindgen/0.25.3/bindgen/struct.Builder.html#method.unstable_rust)
-* [`bindgen::Builder::derive_default()`](https://docs.rs/bindgen/0.25.3/bindgen/struct.Builder.html#method.derive_default)
+* [`bindgen::Builder::rust_target()`](https://docs.rs/bindgen/0.29.0/bindgen/struct.Builder.html#method.rust_target) <!-- Update when live -->
+* [`bindgen::Builder::derive_default()`](https://docs.rs/bindgen/0.29.0/bindgen/struct.Builder.html#method.derive_default)
 
 ### Command Line
 
-* `--unstable-rust`
+* `--rust-target`
 * `--with-derive-default`
 
-## Using the unstable `union` version
+## Which union type will Bindgen generate?
 
-With `struct`s generated by bindgen from C, it is possible to initialize fields in a "normal" rust way:
+Bindgen can emit one of two Rust types that correspond to C unions:
 
-```rust,ignore
-mod bindings;
+* Rust's `union` builtin (only available in Rust >= 1.19, including nightly)
+* Bindgen's `BindgenUnion` (available for all Rust targets)
 
-fn main() {
-    let x = bindings::alpha_t {
-        a: 1,
-        b: -1,
-    };
-}
-```
+Bindgen uses the following logic to determine which Rust union type to emit:
+
+* If the Rust target is >= 1.19 (including nightly) AND each field of the union can derive `Copy`, then generate a `union` builtin.
+* Otherwise, generate a `BindgenUnion`.
+
+## Using the `union` builtin
+
+When using the `union` builtin type, there are two choices for initialization:
 
-When using the unstable `union` type, there are two choices for initialization: Zeroed, and with a specific variant.
+1. Zero
+2. With a specific variant
 
 ```rust,ignore
-#![feature(untagged_unions)]
-mod bindings_unstable;
+mod bindings_builtin_union;
 
-fn unstable() {
+fn union_builtin() {
     // Initalize the union to zero
-    let x = bindings_unstable::greek_t::default();
+    let x = bindings_builtin_union::greek_t::default();
 
     // If `--with-derive-default` option is not used, the following may be used
     //   to initalize the union to zero:
-    let x = unsafe{ std::mem::zeroed::<bindings_unstable::greek_t>() };
+    let x = unsafe { std::mem::zeroed::<bindings_builtin_union::greek_t>() };
 
     // Or, it is possible to initialize exactly one variant of the enum:
-    let x = bindings_unstable::greek_t {
-        alfa: bindings_unstable::alpha_t {
+    let x = bindings_builtin_union::greek_t {
+        alfa: bindings_builtin_union::alpha_t {
             a: 1,
             b: -1,
         },
@@ -83,16 +88,17 @@ fn unstable() {
 }
 ```
 
-## Using the stable BindgenUnion types
+## Using the `BindgenUnion` type
 
-For versions of Rust that do not support the new `union` type, bindgen will generate types which provide union-like access to structure fields.
+If the target Rust version does not support the new `union` type or there is a field that cannot derive `Copy`, then bindgen will provide union-like access to a `struct`.
 
-Interacting with these unions is slightly different than the new `union` types. Whenever a variant of the union is accessed, it must be done through a reference.
+Interacting with these unions is slightly different than the new `union` types.
+You must access union variants through a reference.
 
 ```rust,ignore
 mod bindings;
 
-fn stable() {
+fn bindgenunion() {
     // `default()` or `zeroed()` may still be used with Bindgen's Union types
     let mut x = bindings::greek_t::default();
 
@@ -116,7 +122,7 @@ fn stable() {
     }
 ```
 
-If you attempt to access a BindgenUnion field directly, you will see errors like this:
+If you attempt to access a `BindgenUnion` field directly, you will see errors like this:
 
 ```text
 error[E0308]: mismatched types

src/options.rs

@@ -14,8 +14,10 @@ where
     I: Iterator<Item = String>,
 {
     let rust_target_help = format!(
-        "Version of the Rust compiler to target. Valid options are: {:?}.",
-        RUST_TARGET_STRINGS);
+        "Version of the Rust compiler to target. Valid options are: {:?}. Defaults to {:?}.",
+        RUST_TARGET_STRINGS,
+        String::from(RustTarget::default()),
+        );
 
     let matches = App::new("bindgen")
         .version(env!("CARGO_PKG_VERSION"))