Introduce textDocument/signatureHelp LSP method

internal/langserver/handlers/initialize.go

@@ -246,6 +246,9 @@ func initializeResult(ctx context.Context) lsp.InitializeResult {
 					ChangeNotifications: "workspace/didChangeWorkspaceFolders",
 				},
 			},
+			SignatureHelpProvider: lsp.SignatureHelpOptions{
+				TriggerCharacters: []string{"(", ","},
+			},
 		},
 	}
 

internal/langserver/handlers/service.go

@@ -273,6 +273,17 @@ func (svc *service) Assigner() (jrpc2.Assigner, error) {
 
 			return handle(ctx, req, svc.TextDocumentFormatting)
 		},
+		"textDocument/signatureHelp": func(ctx context.Context, req *jrpc2.Request) (interface{}, error) {
+			err := session.CheckInitializationIsConfirmed()
+			if err != nil {
+				return nil, err
+			}
+
+			ctx = ilsp.WithClientCapabilities(ctx, cc)
+			ctx = ilsp.ContextWithClientName(ctx, &clientName)
+
+			return handle(ctx, req, svc.SignatureHelp)
+		},
 		"textDocument/semanticTokens/full": func(ctx context.Context, req *jrpc2.Request) (interface{}, error) {
 			err := session.CheckInitializationIsConfirmed()
 			if err != nil {

internal/langserver/handlers/signature_help.go

@@ -0,0 +1,42 @@
+package handlers
+
+import (
+	"context"
+
+	ilsp "github.com/hashicorp/terraform-ls/internal/lsp"
+	lsp "github.com/hashicorp/terraform-ls/internal/protocol"
+)
+
+func (svc *service) SignatureHelp(ctx context.Context, params lsp.SignatureHelpParams) (*lsp.SignatureHelp, error) {
+	_, err := ilsp.ClientCapabilities(ctx)
+	if err != nil {
+		return nil, err
+	}
+
+	dh := ilsp.HandleFromDocumentURI(params.TextDocument.URI)
+	doc, err := svc.stateStore.DocumentStore.GetDocument(dh)
+	if err != nil {
+		return nil, err
+	}
+
+	d, err := svc.decoderForDocument(ctx, doc)
+	if err != nil {
+		return nil, err
+	}
+
+	pos, err := ilsp.HCLPositionFromLspPosition(params.Position, doc)
+	if err != nil {
+		return nil, err
+	}
+
+	_, err = d.SignatureAtPos(doc.Filename, pos)
+	if err != nil {
+		return nil, err
+	}
+
+	return &lsp.SignatureHelp{
+		Signatures:      []lsp.SignatureInformation{},
+		ActiveSignature: 0,
+		ActiveParameter: 0,
+	}, nil
+}