document advance `extern crate`/`use` syntax

I took a stab at fixing #28064. Not sure if this all-features-in-one-example approach is the right one. Also I completely made up the terms "star globbing" and "brace expansion globbing" -- they are just called "glob-like syntax" in the reference.

diff --git a/src/doc/trpl/crates-and-modules.md b/src/doc/trpl/crates-and-modules.md
index 7e9025245636610ecc3f4670815487a57ef30b4d..0788f0adf88f9278d9b6cb1a7c57f8e895e88d10 100644 (file)
--- a/src/doc/trpl/crates-and-modules.md
+++ b/src/doc/trpl/crates-and-modules.md
@@ -532,3 +532,51 @@ Goodbye in English: Goodbye.
 Hello in Japanese: こんにちは
 Goodbye in Japanese: さようなら
 ```
+
+## Complex imports
+
+Rust offers several advanced options that can add compactness and
+convenience to your `extern crate` and `use` statements. Here is an example:
+
+```rust,ignore
+extern crate phrases as sayings;
+
+use sayings::japanese::greetings as ja_greetings;
+use sayings::japanese::farewells::*;
+use sayings::english::{self, greetings as en_greetings, farewells as en_farewells};
+
+fn main() {
+    println!("Hello in English; {}", en_greetings::hello());
+    println!("And in Japanese: {}", ja_greetings::hello());
+    println!("Goodbye in English: {}", english::farewells::goodbye());
+    println!("Again: {}", en_farewells::goodbye());
+    println!("And in Japanese: {}", goodbye());
+}
+```
+
+What's going on here?
+
+First, both `extern crate` and `use` allow renaming the thing that is being
+imported. So the crate is still called "phrases", but here we will refer
+to it as "sayings". Similarly, the first `use` statement pulls in the
+`japanese::farewells` module from the crate, but makes it available as
+`jp_farewells` as opposed to simply `farewells`. This can help to avoid
+ambiguity when importing similarly-named items from different places.
+
+The second `use` statement uses a star glob to bring in _all_ symbols from the
+`sayings::japanese::farewells` module. As you can see we can later refer to
+the Japanese `goodbye` function with no module qualifiers. This kind of glob
+should be used sparingly.
+
+The third `use` statement bears more explanation. It's using "brace expansion"
+globbing to compress three `use` statements into one (this sort of syntax
+may be familiar if you've written Linux shell scripts before). The
+uncompressed form of this statement would be:
+```rust,ignore
+use sayings::english;
+use sayings::english::greetings as en_greetings;
+use sayings::english::farewells as en_farewells;
+```
+As you can see, the curly brackets compress `use` statements for several items
+under the same path, and in this context `self` just refers back to that path.
+Note: The curly brackets cannot be nested or mixed with star globbing.