lsp: Add support for clangd's inactiveRegions extension (#26146)

Closes #13089 

Here we use `experimental` to advertise our support for
`inactiveRegions`. Note that clangd does not currently have a stable
release that reads the `experimental` object (PR
https://github.com/llvm/llvm-project/pull/116531), this can be tested
with one of clangd's recent "unstable snapshots" in their
[releases](https://github.com/clangd/clangd/releases).

Release Notes:

- Added support for clangd's `inactiveRegions` extension.

![Screen Recording 2025-03-05 at 22 39
58](https://github.com/user-attachments/assets/ceade8bd-4d8e-43c3-9520-ad44efa50d2f)

---------

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

crates/project/src/lsp_store/clangd_ext.rs

@@ -0,0 +1,75 @@
+use std::sync::Arc;
+
+use ::serde::{Deserialize, Serialize};
+use gpui::WeakEntity;
+use language::CachedLspAdapter;
+use lsp::LanguageServer;
+use util::ResultExt as _;
+
+use crate::LspStore;
+
+pub const CLANGD_SERVER_NAME: &str = "clangd";
+
+#[derive(Debug, Eq, PartialEq, Clone, Deserialize, Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct InactiveRegionsParams {
+    pub text_document: lsp::OptionalVersionedTextDocumentIdentifier,
+    pub regions: Vec<lsp::Range>,
+}
+
+/// InactiveRegions is a clangd extension that marks regions of inactive code.
+pub struct InactiveRegions;
+
+impl lsp::notification::Notification for InactiveRegions {
+    type Params = InactiveRegionsParams;
+    const METHOD: &'static str = "textDocument/inactiveRegions";
+}
+
+pub fn register_notifications(
+    lsp_store: WeakEntity<LspStore>,
+    language_server: &LanguageServer,
+    adapter: Arc<CachedLspAdapter>,
+) {
+    if language_server.name().0 != CLANGD_SERVER_NAME {
+        return;
+    }
+    let server_id = language_server.server_id();
+
+    language_server
+        .on_notification::<InactiveRegions, _>({
+            let adapter = adapter.clone();
+            let this = lsp_store;
+
+            move |params: InactiveRegionsParams, mut cx| {
+                let adapter = adapter.clone();
+                this.update(&mut cx, |this, cx| {
+                    let diagnostics = params
+                        .regions
+                        .into_iter()
+                        .map(|range| lsp::Diagnostic {
+                            range,
+                            severity: Some(lsp::DiagnosticSeverity::INFORMATION),
+                            source: Some(CLANGD_SERVER_NAME.to_string()),
+                            message: "inactive region".to_string(),
+                            tags: Some(vec![lsp::DiagnosticTag::UNNECESSARY]),
+                            ..Default::default()
+                        })
+                        .collect();
+                    let mapped_diagnostics = lsp::PublishDiagnosticsParams {
+                        uri: params.text_document.uri,
+                        version: params.text_document.version,
+                        diagnostics,
+                    };
+                    this.update_diagnostics(
+                        server_id,
+                        mapped_diagnostics,
+                        &adapter.disk_based_diagnostic_sources,
+                        cx,
+                    )
+                    .log_err();
+                })
+                .ok();
+            }
+        })
+        .detach();
+}