Add the markdown preview for the output of `rustc --explain E...`

[XVilka]

Currently it outputs the raw markdown, but it makes sense to provide an embedded preview. Luckily, there is an amazing Rust library for that - termimad.
And it is scrollable out of the box.

[Mark-Simulacrum]

I think there's a fairly high cost to this for relatively little gain. Rendering the markdown means that the compiler itself gains fairly hefty dependencies in a markdown parser and terminal printing (currently limited to rustdoc, for the most part). It also makes it much harder to copy/paste the code as it's now indented and potentially colored, as well as it looks like asking the compiler to implement TUI-like scrolling and such. Today's solution of "just dump some text to the command line" is considerably easier, IMO.

[XVilka]

Maybe some simple integration then? With configuration option that it can pipe to something like that automatically? Basically executing the external binary. Should be very easy to implement and no deps required.

[Mark-Simulacrum]

What does that gain over rustc --explain ... | <tool> or an alias on the command line that does that? I guess to an extent the reason why I'm pushing back might be that I personally almost never look at explain on the command line (or anywhere, really) so it's hard for me to judge the frequency of use. Configuration doesn't really work either because rustc doesn't have any configuration not passed to it via command line flags...

Maybe a reasonable solution here is to write a crate, e.g. rustc-explain that could be called cargo installd that would invoke rustc internally to get the output? That way iteration and such would also be faster.

[XVilka]

Yes, creating such a crate might be a good idea, agree.

[Mark-Simulacrum]

I'm going to close this -- the external crate idea seems reasonable and if it gains traction we can re-explore adding it to the compiler in some fashion.

[pickfire]

@XVilka Did you found any alternatives? Or even cargo explain? Oh, I just realized it is a thing https://crates.io/crates/cargo-explain

[tgross35]

@Mark-Simulacrum would a PR possibly be accepted that does very simple markdown parsing? Nothing complex with external dependencies, just use termcolor (already a dep) to add a background color to backticked blocks, and bold/italic/underlines # sections

[tgross35]

The parser could even be regex, I quickly tested and the following work:

const CODE_BLOCK: &str = r#"```((?s).*?)```"#;
const CODE_INLINE: &str = r#"`((?s).*?)`"#;
const HEADING1: &str = r#"^#{1}\s+(.*)"#;
const HEADING2: &str = r#"^#{2}\s+(.*)"#;
const HEADING3: &str = r#"^#{3}\s+(.*)"#;
const HEADING4: &str = r#"^#{4}\s+(.*)"#;
const BOLD: &str = r#"\*\*((?s).+?)\*\*"#;
const ITALIC: &str = r#"_((?s).+?)_"#;
const LIST: &str = r#"^[-*]\s(.*?)$"#;

First step would just be to replace these with asymmetric HTML-style tags e.g. <codeblock>...</codeblock>, <bold>...</bold>. Then, iterate through and write to the buffer, change the termcolor output whenever one of those tags is hit.

It wouldn't work for anything super complex, but this would still provide much nicer output than the current raw markdown.