lsp/completions: Support snippets based function signatures for built in suggestions

internal/lsp/completions/providers/builtins.go

@@ -1,8 +1,10 @@
 package providers
 
 import (
+	"fmt"
 	"strings"
 
+	"github.com/open-policy-agent/opa/ast"
 	"github.com/styrainc/regal/internal/lsp/cache"
 	"github.com/styrainc/regal/internal/lsp/hover"
 	"github.com/styrainc/regal/internal/lsp/rego"
@@ -56,6 +58,8 @@ func (*BuiltIns) Run(c *cache.Cache, params types.CompletionParams, _ *Options)
 			continue
 		}
 
+		insertTextFormat := uint(2) // snippet
+
 		items = append(items, types.CompletionItem{
 			Label:  key,
 			Kind:   completion.Function,
@@ -64,6 +68,7 @@ func (*BuiltIns) Run(c *cache.Cache, params types.CompletionParams, _ *Options)
 				Kind:  "markdown",
 				Value: hover.CreateHoverContent(builtIn),
 			},
+			InsertTextFormat: &insertTextFormat,
 			TextEdit: &types.TextEdit{
 				Range: types.Range{
 					Start: types.Position{
@@ -75,10 +80,34 @@ func (*BuiltIns) Run(c *cache.Cache, params types.CompletionParams, _ *Options)
 						Character: params.Position.Character,
 					},
 				},
-				NewText: key,
+				NewText: newTextForBuiltIn(builtIn),
 			},
 		})
 	}
 
 	return items, nil
 }
+
+func newTextForBuiltIn(bi *ast.Builtin) string {
+	args := make([]string, len(bi.Decl.Args()))
+
+	for i, arg := range bi.Decl.NamedFuncArgs().Args {
+		args[i] = strings.Split(arg.String(), ":")[0]
+	}
+
+	if len(args) == 0 {
+		return bi.Name + "($0)"
+	}
+
+	argString := ""
+
+	for i, arg := range args {
+		if i > 0 {
+			argString += ", "
+		}
+
+		argString += fmt.Sprintf("${%d:%s}", i+1, arg)
+	}
+
+	return fmt.Sprintf("%s(%s)", bi.Name, argString)
+}

internal/lsp/completions/providers/builtins_test.go

@@ -6,9 +6,50 @@ import (
 	"testing"
 
 	"github.com/styrainc/regal/internal/lsp/cache"
+	"github.com/styrainc/regal/internal/lsp/rego"
 	"github.com/styrainc/regal/internal/lsp/types"
 )
 
+func TestNewTextForBuiltIn(t *testing.T) {
+	t.Parallel()
+
+	testCases := map[string]struct {
+		BuiltIn string
+		Output  string
+	}{
+		"print": {
+			BuiltIn: "print",
+			Output:  "print($0)",
+		},
+		"strings.count": {
+			BuiltIn: "strings.count",
+			Output:  "strings.count(${1:search}, ${2:substring})",
+		},
+		"json.patch": {
+			BuiltIn: "json.patch",
+			Output:  "json.patch(${1:object}, ${2:patches)",
+		},
+	}
+
+	bis := rego.GetBuiltins()
+
+	for name, tc := range testCases {
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			bi, ok := bis[tc.BuiltIn]
+			if !ok {
+				t.Fatalf("BuiltIn %s not found", tc.BuiltIn)
+			}
+
+			output := newTextForBuiltIn(bi)
+			if output != tc.Output {
+				t.Fatalf("Expected\n%s\ngot\n%s", tc.Output, output)
+			}
+		})
+	}
+}
+
 func TestBuiltIns_if(t *testing.T) {
 	t.Parallel()
 

internal/lsp/types/types.go

@@ -137,12 +137,12 @@ type CompletionItem struct {
 	Label        string                      `json:"label"`
 	LabelDetails *CompletionItemLabelDetails `json:"labelDetails,omitempty"`
 	// https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#completionItemKind
-	Kind            completion.ItemKind `json:"kind"`
-	Detail          string              `json:"detail"`
-	Documentation   *MarkupContent      `json:"documentation,omitempty"`
-	Preselect       bool                `json:"preselect"`
-	TextEdit        *TextEdit           `json:"textEdit,omitempty"`
-	InserTextFormat *uint               `json:"insertTextFormat,omitempty"`
+	Kind             completion.ItemKind `json:"kind"`
+	Detail           string              `json:"detail"`
+	Documentation    *MarkupContent      `json:"documentation,omitempty"`
+	Preselect        bool                `json:"preselect"`
+	TextEdit         *TextEdit           `json:"textEdit,omitempty"`
+	InsertTextFormat *uint               `json:"insertTextFormat,omitempty"`
 
 	// Mandatory is used to indicate that the completion item is mandatory and should be offered
 	// as an exclusive completion. This is not part of the LSP spec, but used in regal providers