Add lsp signature help (#1755)

* Add lsp signature help

* Do not move signature help popup on multiple triggers

* Highlight current parameter in signature help

* Auto close signature help

* Position signature help above to not block completion

* Update signature help on backspace/insert mode delete

* Add lsp.auto-signature-help config option

* Add serde default annotation for LspConfig

* Show LSP inactive message only if signature help is invoked manually

* Do not assume valid signature help response from LSP

Malformed LSP responses are common, and these should not crash the
editor.

* Check signature help capability before sending request

* Reuse Open enum for PositionBias in popup

* Close signature popup and exit insert mode on escape

* Add config to control signature help docs display

* Use new Margin api in signature help

* Invoke signature help on changing to insert mode

1 files changed, 17 insertions, 2 deletions

diff --git a/helix-lsp/src/client.rs b/helix-lsp/src/client.rs
index 9187a61e..f6cec6aa 100644
--- a/helix-lsp/src/client.rs
+++ b/helix-lsp/src/client.rs
@@ -322,6 +322,16 @@ impl Client {
                         content_format: Some(vec![lsp::MarkupKind::Markdown]),
                         ..Default::default()
                     }),
+                    signature_help: Some(lsp::SignatureHelpClientCapabilities {
+                        signature_information: Some(lsp::SignatureInformationSettings {
+                            documentation_format: Some(vec![lsp::MarkupKind::Markdown]),
+                            parameter_information: Some(lsp::ParameterInformationSettings {
+                                label_offset_support: Some(true),
+                            }),
+                            active_parameter_support: Some(true),
+                        }),
+                        ..Default::default()
+                    }),
                     rename: Some(lsp::RenameClientCapabilities {
                         dynamic_registration: Some(false),
                         prepare_support: Some(false),
@@ -646,7 +656,12 @@ impl Client {
         text_document: lsp::TextDocumentIdentifier,
         position: lsp::Position,
         work_done_token: Option<lsp::ProgressToken>,
-    ) -> impl Future<Output = Result<Value>> {
+    ) -> Option<impl Future<Output = Result<Value>>> {
+        let capabilities = self.capabilities.get().unwrap();
+
+        // Return early if signature help is not supported
+        capabilities.signature_help_provider.as_ref()?;
+
         let params = lsp::SignatureHelpParams {
             text_document_position_params: lsp::TextDocumentPositionParams {
                 text_document,
@@ -657,7 +672,7 @@ impl Client {
             // lsp::SignatureHelpContext
         };
 
-        self.call::<lsp::request::SignatureHelpRequest>(params)
+        Some(self.call::<lsp::request::SignatureHelpRequest>(params))
     }
 
     pub fn text_document_hover(