Add parser-"preview" to all code examples in docs

[lukaslueg]

Coming from dynamically typed languages like Python, the docs' code examples tend to provide a repeating dive in the learning curve for me: Rust's syntax is quite complex if you haven't seen much of C++ or similar languages before and most code examples (have to) use syntax features a Rust-beginner has not been introduced to before. Even then, my personal experience was a mix of reading the docs and reading the std's source code to get a hold on how syntax features are used. As a small, probably embarrassing, example, it took me ages for figure out what the 'a in

impl<'a, T: ?Sized, U: ?Sized> AsRef<U> for &'a T where T: AsRef<U> {

stands for...

To remedy the problem, all code examples on docs.rust-lang.org could feature in-place annotations that are automatically generated by passing the code through rust's parser to extract language constructs. Hovering the mouse over blocks, lines, words and characters would cause a javascript-driven tooltip to appear that states what kind of syntax element the mouse cursor is currently hovering over, briefly states what it does and where to find the full documentation on that particular construct.
This would greatly help the already taken assumption that docs are read out of order. It would allow readers to take a brief look or branch off right when they feel comfortable to do so.

As a somewhat more ambitious goal, the annotations could translate the selected code into natural language. As a simple example:

let my_string = String::from("hello");

roughly translates to A [immutable] [variable binding] to 'my_string', taking the return value from the [associated function] 'from' on type ['String']; the implements the [trait] [convert::From]. The parameter "hello" is a [string literal]."

[steveklabnik]

This seems, very, very, very hard.

[steveklabnik]

Triage: this is kind of a cool idea, but agreeing with my past self, it seems very difficult. If someone wants to write a patch for this, please do, but I don't think I want to keep an issue open tracking it. Thanks!

[durka]

explain-rs seems relevant here.