3 files changed, 66 insertions, 10 deletions

diff --git a/crates/rust-analyzer/src/config.rs b/crates/rust-analyzer/src/config.rs
index ed5e52871..68b2a2abd 100644
--- a/crates/rust-analyzer/src/config.rs
+++ b/crates/rust-analyzer/src/config.rs
@@ -127,6 +127,7 @@ pub struct ClientCapsConfig {
     pub resolve_code_action: bool,
     pub hover_actions: bool,
     pub status_notification: bool,
+    pub signature_help_label_offsets: bool,
 }
 
 impl Config {
@@ -302,6 +303,15 @@ impl Config {
             {
                 self.client_caps.code_action_literals = value;
             }
+            if let Some(value) = doc_caps
+                .signature_help
+                .as_ref()
+                .and_then(|it| it.signature_information.as_ref())
+                .and_then(|it| it.parameter_information.as_ref())
+                .and_then(|it| it.label_offset_support)
+            {
+                self.client_caps.signature_help_label_offsets = value;
+            }
 
             self.completion.allow_snippets(false);
             if let Some(completion) = &doc_caps.completion {
diff --git a/crates/rust-analyzer/src/handlers.rs b/crates/rust-analyzer/src/handlers.rs
index 447d73fd4..18d660f42 100644
--- a/crates/rust-analyzer/src/handlers.rs
+++ b/crates/rust-analyzer/src/handlers.rs
@@ -557,7 +557,11 @@ pub(crate) fn handle_signature_help(
         None => return Ok(None),
     };
     let concise = !snap.config.call_info_full;
-    let res = to_proto::signature_help(call_info, concise);
+    let res = to_proto::signature_help(
+        call_info,
+        concise,
+        snap.config.client_caps.signature_help_label_offsets,
+    );
     Ok(Some(res))
 }
 
diff --git a/crates/rust-analyzer/src/to_proto.rs b/crates/rust-analyzer/src/to_proto.rs
index 43fc52848..7fcb43a4f 100644
--- a/crates/rust-analyzer/src/to_proto.rs
+++ b/crates/rust-analyzer/src/to_proto.rs
@@ -219,16 +219,58 @@ pub(crate) fn completion_item(
     res
 }
 
-pub(crate) fn signature_help(call_info: CallInfo, concise: bool) -> lsp_types::SignatureHelp {
-    let parameters = call_info
-        .parameter_labels()
-        .map(|label| lsp_types::ParameterInformation {
-            label: lsp_types::ParameterLabel::Simple(label.to_string()),
-            documentation: None,
-        })
-        .collect();
+pub(crate) fn signature_help(
+    call_info: CallInfo,
+    concise: bool,
+    label_offsets: bool,
+) -> lsp_types::SignatureHelp {
+    let (label, parameters) = match (concise, label_offsets) {
+        (_, false) => {
+            let params = call_info
+                .parameter_labels()
+                .map(|label| lsp_types::ParameterInformation {
+                    label: lsp_types::ParameterLabel::Simple(label.to_string()),
+                    documentation: None,
+                })
+                .collect::<Vec<_>>();
+            let label =
+                if concise { call_info.parameter_labels().join(", ") } else { call_info.signature };
+            (label, params)
+        }
+        (false, true) => {
+            let params = call_info
+                .parameter_ranges()
+                .iter()
+                .map(|it| [u32::from(it.start()).into(), u32::from(it.end()).into()])
+                .map(|label_offsets| lsp_types::ParameterInformation {
+                    label: lsp_types::ParameterLabel::LabelOffsets(label_offsets),
+                    documentation: None,
+                })
+                .collect::<Vec<_>>();
+            (call_info.signature, params)
+        }
+        (true, true) => {
+            let mut params = Vec::new();
+            let mut label = String::new();
+            let mut first = true;
+            for param in call_info.parameter_labels() {
+                if !first {
+                    label.push_str(", ");
+                }
+                first = false;
+                let start = label.len() as u64;
+                label.push_str(param);
+                let end = label.len() as u64;
+                params.push(lsp_types::ParameterInformation {
+                    label: lsp_types::ParameterLabel::LabelOffsets([start, end]),
+                    documentation: None,
+                });
+            }
+
+            (label, params)
+        }
+    };
 
-    let label = if concise { call_info.parameter_labels().join(", ") } else { call_info.signature };
     let documentation = call_info.doc.map(|doc| {
         lsp_types::Documentation::MarkupContent(lsp_types::MarkupContent {
             kind: lsp_types::MarkupKind::Markdown,