Stack vs. Heap documentation could use more colours

[huonw]

https://doc.rust-lang.org/stable/book/the-stack-and-the-heap.html includes many large tables with some connection to code, but they and the transitions between them are somewhat complicated. So one (somewhat) low-hanging fruit for improving the stack v. heap documentation would be using more colours to connect stack variables with the function they come from.

E.g. "Let’s try a three-deep example:" could look like

fn red() {
    let i = 6;
}

fn blue() {
    let a = 5;
    let b = 100;
    let c = 1;

    red();
}

fn main() {
    let x = 42;

    blue();
}

(There's a mistake in the text: "And then red() calls blue()" should be "And then blue() calls red()".)

It would be important to just use these as an extra piece, not core to the explanation, for (colour-)blind people.

Inspired/suggested by @velartrill on twitter, who also suggests using greys in a slightly different way.

[brson]

Neat idea.

[Manishearth]

Relevant code: https://github.com/rust-lang/rust/blob/master/src/doc/book/the-stack-and-the-heap.md

[steveklabnik]

Update: there has been some work done here, but there could be more.

[steveklabnik]

I am going to give this one a close, as I'm not going to be working on this chapter of the book anymore.

If someone wants to make this look more colorful, please be my guest!