cmd/doc: print Go syntax when printing struct.field docs

Fixes #20928

Change-Id: I7f7aafb8ff4b5deb50c286a9ae81c34ee85e56a9
Reviewed-on: https://go-review.googlesource.com/47730Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/doc/doc_test.go

@@ -89,7 +89,7 @@ var tests = []test{
 			`VarFive = 5`,                       // From block starting with unexported variable.
 			`type unexportedType`,               // No unexported type.
 			`unexportedTypedConstant`,           // No unexported typed constant.
-			`Field`,                             // No fields.
+			`\bField`,                           // No fields.
 			`Method`,                            // No methods.
 			`someArgument[5-8]`,                 // No truncated arguments.
 			`type T1 T2`,                        // Type alias does not display as type declaration.
@@ -395,9 +395,11 @@ var tests = []test{
 		"field",
 		[]string{p, `ExportedType.ExportedField`},
 		[]string{
+			`type ExportedType struct`,
 			`ExportedField int`,
 			`Comment before exported field.`,
 			`Comment on line with exported field.`,
+			`other fields elided`,
 		},
 		nil,
 	},
@@ -413,6 +415,14 @@ var tests = []test{
 		nil,
 	},
 
+	// Field of struct with only one field.
+	{
+		"single-field struct",
+		[]string{p, `ExportedStructOneField.OnlyField`},
+		[]string{`the only field`},
+		[]string{`other fields elided`},
+	},
+
 	// Case matching off.
 	{
 		"case matching off",

src/cmd/doc/pkg.go

@@ -833,6 +833,7 @@ func (pkg *Package) printFieldDoc(symbol, fieldName string) bool {
 		pkg.Fatalf("symbol %s is not a type in package %s installed in %q", symbol, pkg.name, pkg.build.ImportPath)
 	}
 	found := false
+	numUnmatched := 0
 	for _, typ := range types {
 		// Type must be a struct.
 		spec := pkg.findTypeSpec(typ.Decl, typ.Name)
@@ -844,9 +845,12 @@ func (pkg *Package) printFieldDoc(symbol, fieldName string) bool {
 		for _, field := range structType.Fields.List {
 			// TODO: Anonymous fields.
 			for _, name := range field.Names {
-				if match(fieldName, name.Name) {
+				if !match(fieldName, name.Name) {
+					numUnmatched++
+					continue
+				}
 				if !found {
-						pkg.Printf("struct %s {\n", typ.Name)
+					pkg.Printf("type %s struct {\n", typ.Name)
 				}
 				if field.Doc != nil {
 					for _, comment := range field.Doc.List {
@@ -863,8 +867,10 @@ func (pkg *Package) printFieldDoc(symbol, fieldName string) bool {
 			}
 		}
 	}
-	}
 	if found {
+		if numUnmatched > 0 {
+			pkg.Printf("\n    // ... other fields elided ...\n")
+		}
 		pkg.Printf("}\n")
 	}
 	return found

src/cmd/doc/testdata/pkg.go

@@ -80,6 +80,10 @@ func (ExportedType) unexportedMethod(a int) bool {
 	return true
 }
 
+type ExportedStructOneField struct {
+	OnlyField int // the only field
+}
+
 // Constants tied to ExportedType. (The type is a struct so this isn't valid Go,
 // but it parses and that's all we need.)
 const (