Auto merge of #37911 - liigo:rustdoc-playground, r=alexcrichton

rustdoc: get back missing crate-name when --playground-url is used

follow up PR #37763
r? @alexcrichton (since you r+ed to #37763 )

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Edit: When `#![doc(html_playground_url="")]` is used, the current crate name is saved to `PLAYGROUND`, so rustdoc may generate `extern crate NAME;` into code snips automatically. But when `--playground-url` was introduced in PR #37763, I forgot saving crate name to `PLAYGROUND`. This PR fix that.

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Update:
- add test
- unstable `--playground-url`

src/librustdoc/html/render.rs

@@ -428,6 +428,7 @@ pub fn derive_id(candidate: String) -> String {
 /// Generates the documentation for `crate` into the directory `dst`
 pub fn run(mut krate: clean::Crate,
            external_html: &ExternalHtml,
+           playground_url: Option<String>,
            dst: PathBuf,
            passes: FxHashSet<String>,
            css_file_extension: Option<PathBuf>,
@@ -451,6 +452,13 @@ pub fn run(mut krate: clean::Crate,
         css_file_extension: css_file_extension.clone(),
     };
 
+    // If user passed in `--playground-url` arg, we fill in crate name here
+    if let Some(url) = playground_url {
+        markdown::PLAYGROUND.with(|slot| {
+            *slot.borrow_mut() = Some((Some(krate.name.clone()), url));
+        });
+    }
+
     // Crawl the crate attributes looking for attributes which control how we're
     // going to emit HTML
     if let Some(attrs) = krate.module.as_ref().map(|m| &m.attrs) {