internal/lsp: add snippet completion for function type parameters

Fixes golang/go#51544

Change-Id: I29cf2a0fe878eed263b406ff3ebf1eaeffe10e4b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/390854
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Peter Weinberger <pjw@google.com>

Author: findleyr

internal/lsp/source/completion/format.go

@@ -130,7 +130,7 @@ Suffixes:
 		case invoke:
 			if sig, ok := funcType.Underlying().(*types.Signature); ok {
 				s := source.NewSignature(ctx, c.snapshot, c.pkg, sig, nil, c.qf)
-				c.functionCallSnippet("", s.Params(), &snip)
+				c.functionCallSnippet("", s.TypeParams(), s.Params(), &snip)
 				if sig.Results().Len() == 1 {
 					funcType = sig.Results().At(0).Type()
 				}
@@ -307,7 +307,7 @@ func (c *completer) formatBuiltin(ctx context.Context, cand candidate) (Completi
 		}
 		item.Detail = "func" + sig.Format()
 		item.snippet = &snippet.Builder{}
-		c.functionCallSnippet(obj.Name(), sig.Params(), item.snippet)
+		c.functionCallSnippet(obj.Name(), sig.TypeParams(), sig.Params(), item.snippet)
 	case *types.TypeName:
 		if types.IsInterface(obj.Type()) {
 			item.Kind = protocol.InterfaceCompletion

internal/lsp/source/completion/snippet.go

@@ -49,7 +49,7 @@ func (c *completer) structFieldSnippet(cand candidate, detail string, snip *snip
 }
 
 // functionCallSnippets calculates the snippet for function calls.
-func (c *completer) functionCallSnippet(name string, params []string, snip *snippet.Builder) {
+func (c *completer) functionCallSnippet(name string, tparams, params []string, snip *snippet.Builder) {
 	// If there is no suffix then we need to reuse existing call parens
 	// "()" if present. If there is an identifier suffix then we always
 	// need to include "()" since we don't overwrite the suffix.
@@ -73,7 +73,26 @@ func (c *completer) functionCallSnippet(name string, params []string, snip *snip
 		}
 	}
 
-	snip.WriteText(name + "(")
+	snip.WriteText(name)
+
+	if len(tparams) > 0 {
+		snip.WriteText("[")
+		if c.opts.placeholders {
+			for i, tp := range tparams {
+				if i > 0 {
+					snip.WriteText(", ")
+				}
+				snip.WritePlaceholder(func(b *snippet.Builder) {
+					b.WriteText(tp)
+				})
+			}
+		} else {
+			snip.WritePlaceholder(nil)
+		}
+		snip.WriteText("]")
+	}
+
+	snip.WriteText("(")
 
 	if c.opts.placeholders {
 		// A placeholder snippet turns "someFun<>" into "someFunc(<*i int*>, *s string*)".

internal/lsp/source/types_format.go

@@ -39,10 +39,10 @@ func FormatType(typ types.Type, qf types.Qualifier) (detail string, kind protoco
 }
 
 type signature struct {
-	name, doc        string
-	params, results  []string
-	variadic         bool
-	needResultParens bool
+	name, doc                   string
+	typeParams, params, results []string
+	variadic                    bool
+	needResultParens            bool
 }
 
 func (s *signature) Format() string {
@@ -75,6 +75,10 @@ func (s *signature) Format() string {
 	return b.String()
 }
 
+func (s *signature) TypeParams() []string {
+	return s.typeParams
+}
+
 func (s *signature) Params() []string {
 	return s.params
 }
@@ -171,24 +175,33 @@ func formatFieldList(ctx context.Context, snapshot Snapshot, list *ast.FieldList
 // FormatTypeParams turns TypeParamList into its Go representation, such as:
 // [T, Y]. Note that it does not print constraints as this is mainly used for
 // formatting type params in method receivers.
-func FormatTypeParams(tp *typeparams.TypeParamList) string {
-	if tp == nil || tp.Len() == 0 {
+func FormatTypeParams(tparams *typeparams.TypeParamList) string {
+	if tparams == nil || tparams.Len() == 0 {
 		return ""
 	}
 	var buf bytes.Buffer
 	buf.WriteByte('[')
-	for i := 0; i < tp.Len(); i++ {
+	for i := 0; i < tparams.Len(); i++ {
 		if i > 0 {
 			buf.WriteString(", ")
 		}
-		buf.WriteString(tp.At(i).Obj().Name())
+		buf.WriteString(tparams.At(i).Obj().Name())
 	}
 	buf.WriteByte(']')
 	return buf.String()
 }
 
 // NewSignature returns formatted signature for a types.Signature struct.
 func NewSignature(ctx context.Context, s Snapshot, pkg Package, sig *types.Signature, comment *ast.CommentGroup, qf types.Qualifier) *signature {
+	var tparams []string
+	tpList := typeparams.ForSignature(sig)
+	for i := 0; i < tpList.Len(); i++ {
+		tparam := tpList.At(i)
+		// TODO: is it possible to reuse the logic from FormatVarType here?
+		s := tparam.Obj().Name() + " " + tparam.Constraint().String()
+		tparams = append(tparams, s)
+	}
+
 	params := make([]string, 0, sig.Params().Len())
 	for i := 0; i < sig.Params().Len(); i++ {
 		el := sig.Params().At(i)
@@ -199,6 +212,7 @@ func NewSignature(ctx context.Context, s Snapshot, pkg Package, sig *types.Signa
 		}
 		params = append(params, p)
 	}
+
 	var needResultParens bool
 	results := make([]string, 0, sig.Results().Len())
 	for i := 0; i < sig.Results().Len(); i++ {
@@ -228,6 +242,7 @@ func NewSignature(ctx context.Context, s Snapshot, pkg Package, sig *types.Signa
 	}
 	return &signature{
 		doc:              d,
+		typeParams:       tparams,
 		params:           params,
 		results:          results,
 		variadic:         sig.Variadic(),

internal/lsp/testdata/snippets/func_snippets118.go.in

@@ -0,0 +1,19 @@
+// +build go1.18
+//go:build go1.18
+
+package snippets
+
+type SyncMap[K comparable, V any] struct{}
+
+func NewSyncMap[K comparable, V any]() (result *SyncMap[K, V]) { //@item(NewSyncMap, "NewSyncMap", "", "")
+	return
+}
+
+func Identity[P ~int](p P) P { //@item(Identity, "Identity", "", "")
+	return p
+}
+
+func _() {
+	_ = NewSyncM //@snippet(" //", NewSyncMap, "NewSyncMap[${1:}]()", "NewSyncMap[${1:K comparable}, ${2:V any}]()")
+	_ = Identi //@snippet(" //", Identity, "Identity[${1:}](${2:})", "Identity[${1:P ~int}](${2:p P})")
+}

internal/lsp/testdata/summary_go1.18.txt.golden

@@ -2,7 +2,7 @@
 CallHierarchyCount = 2
 CodeLensCount = 5
 CompletionsCount = 266
-CompletionSnippetCount = 107
+CompletionSnippetCount = 109
 UnimportedCompletionsCount = 5
 DeepCompletionsCount = 5
 FuzzyCompletionsCount = 8