Allow rustup doc books open subsections and paragraphs

[YegorStolyarov]

Reason: additional param to doc command, topic, is only supported for rust reference. I tried to implement minimalistic changes to make it work with books as well.
based on this issue #3345

What does it do: exact links to offline docs in books.
for example, rustup doc --rustc platform-support/armv6k-nintendo-3ds.html#building-the-target now opens exact chapter and paragraph in rustc book

Code changes:

• added logic case for condition (book flag) AND (topic param provided) . now topic param is ignored in case of opening book
• have to add "file:///" to browser url because opener crate which used in rustup have problems to recognize url as file when it contains "#" symbol.

my first PR, thank you for the feedback.

[Rustin170506]

Thanks for your contribution.

@rbtcollins Do you have any thoughts about adding this feature?

[Rustin170506]

Why do we need to define this one here? Maybe we can put it into the if statement.

[YegorStolyarov]

This is the way to resolve &str borrowing issue. If we put this definition into if statement we will get error:
error[E0597]: cached_path does not live long enough. because source String must live as long as let doc_url lives.
TBH I just used the same workaround that is already used with let topical_path: PathBuf; one line above.
maybe there is a nicer way to solve this.

[Rustin170506]

I think we can put it into open_docs. We don't need to leak it here. And the overall logic is very similar with the open_docs.

[YegorStolyarov]

I have tried to use open_docs(), but there was a problem with differentiating web links from file links when we use urls with anchors. Problem is caused by "#" symbol in url and located in the opener crate

So I had to use utils::open_browser() to prepend "file:///" to the url arbitrary, after generating url with toolchain.doc_path()

Or maybe we should go further and prepend "file:///" everywhere?

[rbtcollins]

I like the concept of the feature.

I think we should see how it interacts:

• using fragments with rustup doc ... on
  • mac
  • windows
  • linux
  • wsl

If the fragment doesn't get consistently forwarded to the browser then it may just cause more confusion.

I'd also love to see more testing around doc TBH. We have a very good test coverage in most of rustup, but some bits are not as strong ... I think here an appropriate testing strategy would be to do a unit test in the same file, checking for the emitted url for a few combinations of parameters. It might help to break out a helper function that takes the Option<>'s from clap and returns the url.

[YegorStolyarov]

Currently I'm stuck with writing tests for my feature.
It's difficult for me to reproduce needed context using existing test helpers, or I am doing it wrong.

I tried to move url generating logic into separate function:

fn get_doc_url(m: &ArgMatches, t: &Toolchain<'_>) -> Result<(String, bool)> {
    if let Some((short, _, path)) = DOCS_DATA.iter().find(|(name, _, _)| m.get_flag(name)) {
        if let Some(topic) = m.get_one::<String>("topic") {
            let cached_path = format!("{}/{}", short, topic);
            Ok((cached_path, true))
        } else {
            Ok((path.to_string(), false))
        }
    } else if let Some(topic) = m.get_one::<String>("topic") {
        let topical_path = topical_doc::local_path(&t.doc_path("").unwrap(), topic)?;
        Ok((topical_path.to_str().unwrap().to_string(), false))
    } else {
        Ok(("index.html".to_string(), false))
    }
}

So it needs a mock toolchain for test.

Need some clues or an advice.

[Rustin170506]

I tried to move url generating logic into separate function:

If you moved it into a separate function, then you can write some unit tests to cover it.

[rbtcollins]

Toolchain takes a Cfg, a LocalToolchainName and a path (to the toolchain root dir).

Cfg has a pretty shallow needs for testing. See this example. We could of course make it better.

LocalToolchainName is a very narrow object with no exterior state, you can just create the one you want. E.g. LocalToolchainName::from(ToolchainDesc::from_str("stable").unwrap()).unwrap();

The path - since this is a unit test and meant to be in a read-only code path, you could just supply any arbitrary string you want quite sensibly I think.

test::mock::topical_doc_data might be useful, or at least relevant.