Auto merge of #84176 - GuillaumeGomez:src-to-definition, r=jyn514

Generate links to definition in rustdoc source code pages

## Description

This PR adds an option (disabled by default) to add links in the source code page on ident. So for for example:

```rust
mod other_module;
struct Foo;
fn bar() {}

fn x<T: other_module::Trait>(f: Foo, g: other_module::Whatever, t: &T) {
    let f: Foo = Foo;
    bar();
    f.some_method();
}
```

In the example (mostly in the `x` function), `other_module::Trait`, `Foo`, `other_module::Whatever`, `bar` and `some_method` are now links (and `other_module` at the top too).

In case there is a type coming from another crate, it'll link to its documentation page and not its definition (but you can then click on `[src]` so I guess it's fine).

Another important detail: I voluntarily didn't add links for primitive types. I think we can discuss about adding links on them or not in a later PR (adding the support for them would require only a few lines).

Here is a video summing up everything I wrote above:

https://user-images.githubusercontent.com/3050060/114622354-21307b00-9cae-11eb-834d-f6d8178a37bd.mp4

## Performance impact

So, on my computer, the performance remains more or less the same (which is quite surprising but that's a nice surprise). Here are the numbers:

Without the option:
 * core:  1m 21s
 * alloc: 26.78s
 * std: 27.30s
 * proc_macro: 4.50s

With source to definition links generation (I enabled by default the option):
 * core: 1m 25s
 * alloc: 25.76s
 * std: 27.07s
 * proc_macro: 4.66s

So no real change here (again, I'm very surprised by this fact).

For the size of the generated source files (only taking into account the `src` folder here since it's the only one impacted) by running `du -shc .` (when I am in the source folder).

Without the option: 11.939 MB
With the option: 12.611 MB

So not a big change here either. In all those docs, I ran `grep -nR '<a class=' . | wc -l` and got 43917. So there are quite a lot of links added. :)

cc `@rust-lang/rustdoc`
r? `@jyn514`

0 files changed, 0 insertions, 0 deletions