clangd: Implement switch source/header extension (#14646)

Release Notes:

- Added switch source/header action for clangd language server (fixes
[#12801](https://github.com/zed-industries/zed/issues/12801)).

Note: I'm new to both rust and this codebase. I started my
implementation by copying how rust analyzer's "expand macro" LSP
extension is implemented. I don't yet understand some of the code I
copied (mostly the way to get the `server_to_query` in `clangd_ext.rs`
and the whole proto implementation).

---------

Co-authored-by: Kirill Bulatov <kirill@zed.dev>

crates/editor/src/clangd_ext.rs

@@ -0,0 +1,93 @@
+use std::path::PathBuf;
+
+use anyhow::Context as _;
+use gpui::{View, ViewContext, WindowContext};
+use language::Language;
+use url::Url;
+
+use crate::lsp_ext::find_specific_language_server_in_selection;
+
+use crate::{element::register_action, Editor, SwitchSourceHeader};
+
+static CLANGD_SERVER_NAME: &str = "clangd";
+
+fn is_c_language(language: &Language) -> bool {
+    return language.name().as_ref() == "C++" || language.name().as_ref() == "C";
+}
+
+pub fn switch_source_header(
+    editor: &mut Editor,
+    _: &SwitchSourceHeader,
+    cx: &mut ViewContext<'_, Editor>,
+) {
+    let Some(project) = &editor.project else {
+        return;
+    };
+    let Some(workspace) = editor.workspace() else {
+        return;
+    };
+
+    let Some((_, _, server_to_query, buffer)) =
+        find_specific_language_server_in_selection(&editor, cx, &is_c_language, CLANGD_SERVER_NAME)
+    else {
+        return;
+    };
+
+    let project = project.clone();
+    let buffer_snapshot = buffer.read(cx).snapshot();
+    let source_file = buffer_snapshot
+        .file()
+        .unwrap()
+        .file_name(cx)
+        .to_str()
+        .unwrap()
+        .to_owned();
+
+    let switch_source_header_task = project.update(cx, |project, cx| {
+        project.request_lsp(
+            buffer,
+            project::LanguageServerToQuery::Other(server_to_query),
+            project::lsp_ext_command::SwitchSourceHeader,
+            cx,
+        )
+    });
+    cx.spawn(|_editor, mut cx| async move {
+        let switch_source_header = switch_source_header_task
+            .await
+            .with_context(|| format!("Switch source/header LSP request for path \"{}\" failed", source_file))?;
+        if switch_source_header.0.is_empty() {
+            log::info!("Clangd returned an empty string when requesting to switch source/header from \"{}\"", source_file);
+            return Ok(());
+        }
+
+        let goto = Url::parse(&switch_source_header.0).with_context(|| {
+            format!(
+                "Parsing URL \"{}\" returned from switch source/header failed",
+                switch_source_header.0
+            )
+        })?;
+
+        workspace
+            .update(&mut cx, |workspace, view_cx| {
+                workspace.open_abs_path(PathBuf::from(goto.path()), false, view_cx)
+            })
+            .with_context(|| {
+                format!(
+                    "Switch source/header could not open \"{}\" in workspace",
+                    goto.path()
+                )
+            })?
+            .await
+            .map(|_| ())
+    })
+    .detach_and_log_err(cx);
+}
+
+pub fn apply_related_actions(editor: &View<Editor>, cx: &mut WindowContext) {
+    if editor.update(cx, |e, cx| {
+        find_specific_language_server_in_selection(e, cx, &is_c_language, CLANGD_SERVER_NAME)
+            .is_some()
+    }) {
+        register_action(editor, cx, switch_source_header);
+    }
+}