Honor client's dynamic registration caps

cc https://github.com/rust-analyzer/rust-analyzer/pull/5516#issuecomment-757520828

crates/rust-analyzer/src/config.rs

@@ -334,6 +334,18 @@ impl Config {
         }
     }
 
+    pub fn did_save_text_document_dynamic_registration(&self) -> bool {
+        let caps =
+            try_or!(self.caps.text_document.as_ref()?.synchronization.clone()?, Default::default());
+        caps.did_save == Some(true) && caps.dynamic_registration == Some(true)
+    }
+    pub fn did_change_watched_files_dynamic_registration(&self) -> bool {
+        try_or!(
+            self.caps.workspace.as_ref()?.did_change_watched_files.as_ref()?.dynamic_registration?,
+            false
+        )
+    }
+
     pub fn location_link(&self) -> bool {
         try_or!(self.caps.text_document.as_ref()?.definition?.link_support?, false)
     }

crates/rust-analyzer/src/main_loop.rs

@@ -108,6 +108,7 @@ impl GlobalState {
             );
         };
 
+        if self.config.did_save_text_document_dynamic_registration() {
             let save_registration_options = lsp_types::TextDocumentSaveRegistrationOptions {
                 include_text: Some(false),
                 text_document_registration_options: lsp_types::TextDocumentRegistrationOptions {
@@ -140,6 +141,7 @@ impl GlobalState {
                 lsp_types::RegistrationParams { registrations: vec![registration] },
                 |_, _| (),
             );
+        }
 
         self.fetch_workspaces_request();
         self.fetch_workspaces_if_needed();

crates/rust-analyzer/src/reload.rs

@@ -182,6 +182,7 @@ impl GlobalState {
         }
 
         if let FilesWatcher::Client = self.config.files().watcher {
+            if self.config.did_change_watched_files_dynamic_registration() {
                 let registration_options = lsp_types::DidChangeWatchedFilesRegistrationOptions {
                     watchers: workspaces
                         .iter()
@@ -190,7 +191,10 @@ impl GlobalState {
                         .flat_map(|root| {
                             root.include.into_iter().map(|it| format!("{}/**/*.rs", it.display()))
                         })
-                    .map(|glob_pattern| lsp_types::FileSystemWatcher { glob_pattern, kind: None })
+                        .map(|glob_pattern| lsp_types::FileSystemWatcher {
+                            glob_pattern,
+                            kind: None,
+                        })
                         .collect(),
                 };
                 let registration = lsp_types::Registration {
@@ -203,6 +207,7 @@ impl GlobalState {
                     |_, _| (),
                 );
             }
+        }
 
         let mut change = Change::new();