go/types: change {Type,Object,Selection}String to accept a Qualifier function

The optional Qualifier function determines what prefix to attach to
package-level names, enabling clients to qualify packages in different
ways, for example, using only the package name instead of its complete
path, or using the locally appropriate name for package given a set of
(possibly renaming) imports.

Prior to this change, clients wanting this behavior had to copy
hundreds of lines of complex printing logic.

Fun fact: (*types.Package).Path and (*types.Package).Name are valid
Qualifier functions.

We provide the RelativeTo helper function to create Qualifiers so that
the old behavior remains a one-liner.

Fixes golang/go#11133

This CL is a copy of https://go-review.googlesource.com/#/c/11692/
to the golang.org/x/tools repository.

Change-Id: I26d0f3644d077a26bfe350989f9c545f018eefbf
Reviewed-on: https://go-review.googlesource.com/11790
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>

10 files changed, 143 insertions, 92 deletions

diff --git a/src/go/internal/gccgoimporter/importer_test.go b/src/go/internal/gccgoimporter/importer_test.go
index 4fa01c762c..f3bcadbaf7 100644
--- a/src/go/internal/gccgoimporter/importer_test.go
+++ b/src/go/internal/gccgoimporter/importer_test.go
@@ -34,7 +34,7 @@ func runImporterTest(t *testing.T, imp Importer, initmap map[*types.Package]Init
 			return
 		}
 
-		got := types.ObjectString(pkg, obj)
+		got := types.ObjectString(obj, types.RelativeTo(pkg))
 		if got != test.want {
 			t.Errorf("%s: got %q; want %q", test.name, got, test.want)
 		}
diff --git a/src/go/internal/gcimporter/gcimporter_test.go b/src/go/internal/gcimporter/gcimporter_test.go
index fe4a758cd4..85846a1348 100644
--- a/src/go/internal/gcimporter/gcimporter_test.go
+++ b/src/go/internal/gcimporter/gcimporter_test.go
@@ -165,7 +165,7 @@ func TestImportedTypes(t *testing.T) {
 			continue
 		}
 
-		got := types.ObjectString(pkg, obj)
+		got := types.ObjectString(obj, types.RelativeTo(pkg))
 		if got != test.want {
 			t.Errorf("%s: got %q; want %q", test.name, got, test.want)
 		}
diff --git a/src/go/types/errors.go b/src/go/types/errors.go
index 0a9dd0e19b..0c0049b1f3 100644
--- a/src/go/types/errors.go
+++ b/src/go/types/errors.go
@@ -23,6 +23,13 @@ func unreachable() {
 	panic("unreachable")
 }
 
+func (check *Checker) qualifier(pkg *Package) string {
+	if pkg != check.pkg {
+		return pkg.path
+	}
+	return ""
+}
+
 func (check *Checker) sprintf(format string, args ...interface{}) string {
 	for i, arg := range args {
 		switch a := arg.(type) {
@@ -31,15 +38,15 @@ func (check *Checker) sprintf(format string, args ...interface{}) string {
 		case operand:
 			panic("internal error: should always pass *operand")
 		case *operand:
-			arg = operandString(check.pkg, a)
+			arg = operandString(a, check.qualifier)
 		case token.Pos:
 			arg = check.fset.Position(a).String()
 		case ast.Expr:
 			arg = ExprString(a)
 		case Object:
-			arg = ObjectString(check.pkg, a)
+			arg = ObjectString(a, check.qualifier)
 		case Type:
-			arg = TypeString(check.pkg, a)
+			arg = TypeString(a, check.qualifier)
 		}
 		args[i] = arg
 	}
diff --git a/src/go/types/example_test.go b/src/go/types/example_test.go
index 997aed33d1..8882e5063a 100644
--- a/src/go/types/example_test.go
+++ b/src/go/types/example_test.go
@@ -199,7 +199,7 @@ func fib(x int) int {
 	for obj, uses := range usesByObj {
 		sort.Strings(uses)
 		item := fmt.Sprintf("%s:\n  defined at %s\n  used at %s",
-			types.ObjectString(pkg, obj),
+			types.ObjectString(obj, types.RelativeTo(pkg)),
 			fset.Position(obj.Pos()),
 			strings.Join(uses, ", "))
 		items = append(items, item)
diff --git a/src/go/types/object.go b/src/go/types/object.go
index ebbd760df4..62b39c6b81 100644
--- a/src/go/types/object.go
+++ b/src/go/types/object.go
@@ -207,7 +207,7 @@ func NewFunc(pos token.Pos, pkg *Package, name string, sig *Signature) *Func {
 // function or method obj.
 func (obj *Func) FullName() string {
 	var buf bytes.Buffer
-	writeFuncName(&buf, nil, obj)
+	writeFuncName(&buf, obj, nil)
 	return buf.String()
 }
 
@@ -241,7 +241,7 @@ type Nil struct {
 	object
 }
 
-func writeObject(buf *bytes.Buffer, this *Package, obj Object) {
+func writeObject(buf *bytes.Buffer, obj Object, qf Qualifier) {
 	typ := obj.Type()
 	switch obj := obj.(type) {
 	case *PkgName:
@@ -267,9 +267,9 @@ func writeObject(buf *bytes.Buffer, this *Package, obj Object) {
 
 	case *Func:
 		buf.WriteString("func ")
-		writeFuncName(buf, this, obj)
+		writeFuncName(buf, obj, qf)
 		if typ != nil {
-			WriteSignature(buf, this, typ.(*Signature))
+			WriteSignature(buf, typ.(*Signature), qf)
 		}
 		return
 
@@ -291,39 +291,52 @@ func writeObject(buf *bytes.Buffer, this *Package, obj Object) {
 
 	buf.WriteByte(' ')
 
-	// For package-level objects, package-qualify the name,
-	// except for intra-package references (this != nil).
-	if pkg := obj.Pkg(); pkg != nil && this != pkg && pkg.scope.Lookup(obj.Name()) == obj {
-		buf.WriteString(pkg.path)
-		buf.WriteByte('.')
+	// For package-level objects, qualify the name.
+	if obj.Pkg() != nil && obj.Pkg().scope.Lookup(obj.Name()) == obj {
+		writePackage(buf, obj.Pkg(), qf)
 	}
 	buf.WriteString(obj.Name())
 	if typ != nil {
 		buf.WriteByte(' ')
-		WriteType(buf, this, typ)
+		WriteType(buf, typ, qf)
+	}
+}
+
+func writePackage(buf *bytes.Buffer, pkg *Package, qf Qualifier) {
+	if pkg == nil {
+		return
+	}
+	var s string
+	if qf != nil {
+		s = qf(pkg)
+	} else {
+		s = pkg.Path()
+	}
+	if s != "" {
+		buf.WriteString(s)
+		buf.WriteByte('.')
 	}
 }
 
 // ObjectString returns the string form of obj.
-// Object and type names are printed package-qualified
-// only if they do not belong to this package.
-//
-func ObjectString(this *Package, obj Object) string {
+// The Qualifier controls the printing of
+// package-level objects, and may be nil.
+func ObjectString(obj Object, qf Qualifier) string {
 	var buf bytes.Buffer
-	writeObject(&buf, this, obj)
+	writeObject(&buf, obj, qf)
 	return buf.String()
 }
 
-func (obj *PkgName) String() string  { return ObjectString(nil, obj) }
-func (obj *Const) String() string    { return ObjectString(nil, obj) }
-func (obj *TypeName) String() string { return ObjectString(nil, obj) }
-func (obj *Var) String() string      { return ObjectString(nil, obj) }
-func (obj *Func) String() string     { return ObjectString(nil, obj) }
-func (obj *Label) String() string    { return ObjectString(nil, obj) }
-func (obj *Builtin) String() string  { return ObjectString(nil, obj) }
-func (obj *Nil) String() string      { return ObjectString(nil, obj) }
+func (obj *PkgName) String() string  { return ObjectString(obj, nil) }
+func (obj *Const) String() string    { return ObjectString(obj, nil) }
+func (obj *TypeName) String() string { return ObjectString(obj, nil) }
+func (obj *Var) String() string      { return ObjectString(obj, nil) }
+func (obj *Func) String() string     { return ObjectString(obj, nil) }
+func (obj *Label) String() string    { return ObjectString(obj, nil) }
+func (obj *Builtin) String() string  { return ObjectString(obj, nil) }
+func (obj *Nil) String() string      { return ObjectString(obj, nil) }
 
-func writeFuncName(buf *bytes.Buffer, this *Package, f *Func) {
+func writeFuncName(buf *bytes.Buffer, f *Func, qf Qualifier) {
 	if f.typ != nil {
 		sig := f.typ.(*Signature)
 		if recv := sig.Recv(); recv != nil {
@@ -335,13 +348,12 @@ func writeFuncName(buf *bytes.Buffer, this *Package, f *Func) {
 				// Don't print it in full.
 				buf.WriteString("interface")
 			} else {
-				WriteType(buf, this, recv.Type())
+				WriteType(buf, recv.Type(), qf)
 			}
 			buf.WriteByte(')')
 			buf.WriteByte('.')
-		} else if f.pkg != nil && f.pkg != this {
-			buf.WriteString(f.pkg.path)
-			buf.WriteByte('.')
+		} else if f.pkg != nil {
+			writePackage(buf, f.pkg, qf)
 		}
 	}
 	buf.WriteString(f.name)
diff --git a/src/go/types/operand.go b/src/go/types/operand.go
index 8d167067d5..a7d3b0aaee 100644
--- a/src/go/types/operand.go
+++ b/src/go/types/operand.go
@@ -93,7 +93,7 @@ func (x *operand) pos() token.Pos {
 // commaok    <expr> (<untyped kind> <mode>                    )
 // commaok    <expr> (               <mode>       of type <typ>)
 //
-func operandString(this *Package, x *operand) string {
+func operandString(x *operand, qf Qualifier) string {
 	var buf bytes.Buffer
 
 	var expr string
@@ -104,7 +104,7 @@ func operandString(this *Package, x *operand) string {
 		case builtin:
 			expr = predeclaredFuncs[x.id].name
 		case typexpr:
-			expr = TypeString(this, x.typ)
+			expr = TypeString(x.typ, qf)
 		case constant:
 			expr = x.val.String()
 		}
@@ -146,7 +146,7 @@ func operandString(this *Package, x *operand) string {
 	if hasType {
 		if x.typ != Typ[Invalid] {
 			buf.WriteString(" of type ")
-			WriteType(&buf, this, x.typ)
+			WriteType(&buf, x.typ, qf)
 		} else {
 			buf.WriteString(" with invalid type")
 		}
@@ -161,7 +161,7 @@ func operandString(this *Package, x *operand) string {
 }
 
 func (x *operand) String() string {
-	return operandString(nil, x)
+	return operandString(x, nil)
 }
 
 // setConst sets x to the untyped constant for literal lit.
diff --git a/src/go/types/selection.go b/src/go/types/selection.go
index 1c7016550a..124e0d39f0 100644
--- a/src/go/types/selection.go
+++ b/src/go/types/selection.go
@@ -105,18 +105,18 @@ func (s *Selection) Index() []int { return s.index }
 // x to f in x.f.
 func (s *Selection) Indirect() bool { return s.indirect }
 
-func (s *Selection) String() string { return SelectionString(nil, s) }
+func (s *Selection) String() string { return SelectionString(s, nil) }
 
 // SelectionString returns the string form of s.
-// Type names are printed package-qualified
-// only if they do not belong to this package.
+// The Qualifier controls the printing of
+// package-level objects, and may be nil.
 //
 // Examples:
 //	"field (T) f int"
 //	"method (T) f(X) Y"
 //	"method expr (T) f(X) Y"
 //
-func SelectionString(this *Package, s *Selection) string {
+func SelectionString(s *Selection, qf Qualifier) string {
 	var k string
 	switch s.kind {
 	case FieldVal:
@@ -131,13 +131,13 @@ func SelectionString(this *Package, s *Selection) string {
 	var buf bytes.Buffer
 	buf.WriteString(k)
 	buf.WriteByte('(')
-	WriteType(&buf, this, s.Recv())
+	WriteType(&buf, s.Recv(), qf)
 	fmt.Fprintf(&buf, ") %s", s.obj.Name())
 	if T := s.Type(); s.kind == FieldVal {
 		buf.WriteByte(' ')
-		WriteType(&buf, this, T)
+		WriteType(&buf, T, qf)
 	} else {
-		WriteSignature(&buf, this, T.(*Signature))
+		WriteSignature(&buf, T.(*Signature), qf)
 	}
 	return buf.String()
 }
diff --git a/src/go/types/type.go b/src/go/types/type.go
index 3d1af20a10..1df8b45b28 100644
--- a/src/go/types/type.go
+++ b/src/go/types/type.go
@@ -441,14 +441,14 @@ func (t *Map) Underlying() Type       { return t }
 func (t *Chan) Underlying() Type      { return t }
 func (t *Named) Underlying() Type     { return t.underlying }
 
-func (t *Basic) String() string     { return TypeString(nil, t) }
-func (t *Array) String() string     { return TypeString(nil, t) }
-func (t *Slice) String() string     { return TypeString(nil, t) }
-func (t *Struct) String() string    { return TypeString(nil, t) }
-func (t *Pointer) String() string   { return TypeString(nil, t) }
-func (t *Tuple) String() string     { return TypeString(nil, t) }
-func (t *Signature) String() string { return TypeString(nil, t) }
-func (t *Interface) String() string { return TypeString(nil, t) }
-func (t *Map) String() string       { return TypeString(nil, t) }
-func (t *Chan) String() string      { return TypeString(nil, t) }
-func (t *Named) String() string     { return TypeString(nil, t) }
+func (t *Basic) String() string     { return TypeString(t, nil) }
+func (t *Array) String() string     { return TypeString(t, nil) }
+func (t *Slice) String() string     { return TypeString(t, nil) }
+func (t *Struct) String() string    { return TypeString(t, nil) }
+func (t *Pointer) String() string   { return TypeString(t, nil) }
+func (t *Tuple) String() string     { return TypeString(t, nil) }
+func (t *Signature) String() string { return TypeString(t, nil) }
+func (t *Interface) String() string { return TypeString(t, nil) }
+func (t *Map) String() string       { return TypeString(t, nil) }
+func (t *Chan) String() string      { return TypeString(t, nil) }
+func (t *Named) String() string     { return TypeString(t, nil) }
diff --git a/src/go/types/typestring.go b/src/go/types/typestring.go
index 6f527bbed0..bd62f4dc22 100644
--- a/src/go/types/typestring.go
+++ b/src/go/types/typestring.go
@@ -11,6 +11,33 @@ import (
 	"fmt"
 )
 
+// A Qualifier controls how named package-level objects are printed in
+// calls to TypeString, ObjectString, and SelectionString.
+//
+// These three formatting routines call the Qualifier for each
+// package-level object O, and if the Qualifier returns a non-empty
+// string p, the object is printed in the form p.O.
+// If it returns an empty string, only the object name O is printed.
+//
+// Using a nil Qualifier is equivalent to using (*Package).Path: the
+// object is qualified by the import path, e.g., "encoding/json.Marshal".
+//
+type Qualifier func(*Package) string
+
+// RelativeTo(pkg) returns a Qualifier that fully qualifies members of
+// all packages other than pkg.
+func RelativeTo(pkg *Package) Qualifier {
+	if pkg == nil {
+		return nil
+	}
+	return func(other *Package) string {
+		if pkg == other {
+			return "" // same package; unqualified
+		}
+		return other.Path()
+	}
+}
+
 // If gcCompatibilityMode is set, printing of types is modified
 // to match the representation of some types in the gc compiler:
 //
@@ -32,22 +59,22 @@ import (
 var gcCompatibilityMode bool
 
 // TypeString returns the string representation of typ.
-// Named types are printed package-qualified if they
-// do not belong to this package.
-func TypeString(this *Package, typ Type) string {
+// The Qualifier controls the printing of
+// package-level objects, and may be nil.
+func TypeString(typ Type, qf Qualifier) string {
 	var buf bytes.Buffer
-	WriteType(&buf, this, typ)
+	WriteType(&buf, typ, qf)
 	return buf.String()
 }
 
 // WriteType writes the string representation of typ to buf.
-// Named types are printed package-qualified if they
-// do not belong to this package.
-func WriteType(buf *bytes.Buffer, this *Package, typ Type) {
-	writeType(buf, this, typ, make([]Type, 8))
+// The Qualifier controls the printing of
+// package-level objects, and may be nil.
+func WriteType(buf *bytes.Buffer, typ Type, qf Qualifier) {
+	writeType(buf, typ, qf, make([]Type, 8))
 }
 
-func writeType(buf *bytes.Buffer, this *Package, typ Type, visited []Type) {
+func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 	// Theoretically, this is a quadratic lookup algorithm, but in
 	// practice deeply nested composite types with unnamed component
 	// types are uncommon. This code is likely more efficient than
@@ -81,11 +108,11 @@ func writeType(buf *bytes.Buffer, this *Package, typ Type, visited []Type) {
 
 	case *Array:
 		fmt.Fprintf(buf, "[%d]", t.len)
-		writeType(buf, this, t.elem, visited)
+		writeType(buf, t.elem, qf, visited)
 
 	case *Slice:
 		buf.WriteString("[]")
-		writeType(buf, this, t.elem, visited)
+		writeType(buf, t.elem, qf, visited)
 
 	case *Struct:
 		buf.WriteString("struct{")
@@ -97,7 +124,7 @@ func writeType(buf *bytes.Buffer, this *Package, typ Type, visited []Type) {
 				buf.WriteString(f.name)
 				buf.WriteByte(' ')
 			}
-			writeType(buf, this, f.typ, visited)
+			writeType(buf, f.typ, qf, visited)
 			if tag := t.Tag(i); tag != "" {
 				fmt.Fprintf(buf, " %q", tag)
 			}
@@ -106,14 +133,14 @@ func writeType(buf *bytes.Buffer, this *Package, typ Type, visited []Type) {
 
 	case *Pointer:
 		buf.WriteByte('*')
-		writeType(buf, this, t.base, visited)
+		writeType(buf, t.base, qf, visited)
 
 	case *Tuple:
-		writeTuple(buf, this, t, false, visited)
+		writeTuple(buf, t, false, qf, visited)
 
 	case *Signature:
 		buf.WriteString("func")
-		writeSignature(buf, this, t, visited)
+		writeSignature(buf, t, qf, visited)
 
 	case *Interface:
 		// We write the source-level methods and embedded types rather
@@ -136,7 +163,7 @@ func writeType(buf *bytes.Buffer, this *Package, typ Type, visited []Type) {
 					buf.WriteString("; ")
 				}
 				buf.WriteString(m.name)
-				writeSignature(buf, this, m.typ.(*Signature), visited)
+				writeSignature(buf, m.typ.(*Signature), qf, visited)
 			}
 		} else {
 			// print explicit interface methods and embedded types
@@ -145,22 +172,22 @@ func writeType(buf *bytes.Buffer, this *Package, typ Type, visited []Type) {
 					buf.WriteString("; ")
 				}
 				buf.WriteString(m.name)
-				writeSignature(buf, this, m.typ.(*Signature), visited)
+				writeSignature(buf, m.typ.(*Signature), qf, visited)
 			}
 			for i, typ := range t.embeddeds {
 				if i > 0 || len(t.methods) > 0 {
 					buf.WriteString("; ")
 				}
-				writeType(buf, this, typ, visited)
+				writeType(buf, typ, qf, visited)
 			}
 		}
 		buf.WriteByte('}')
 
 	case *Map:
 		buf.WriteString("map[")
-		writeType(buf, this, t.key, visited)
+		writeType(buf, t.key, qf, visited)
 		buf.WriteByte(']')
-		writeType(buf, this, t.elem, visited)
+		writeType(buf, t.elem, qf, visited)
 
 	case *Chan:
 		var s string
@@ -183,7 +210,7 @@ func writeType(buf *bytes.Buffer, this *Package, typ Type, visited []Type) {
 		if parens {
 			buf.WriteByte('(')
 		}
-		writeType(buf, this, t.elem, visited)
+		writeType(buf, t.elem, qf, visited)
 		if parens {
 			buf.WriteByte(')')
 		}
@@ -191,9 +218,8 @@ func writeType(buf *bytes.Buffer, this *Package, typ Type, visited []Type) {
 	case *Named:
 		s := "<Named w/o object>"
 		if obj := t.obj; obj != nil {
-			if pkg := obj.pkg; pkg != nil && pkg != this {
-				buf.WriteString(pkg.path)
-				buf.WriteByte('.')
+			if obj.pkg != nil {
+				writePackage(buf, obj.pkg, qf)
 			}
 			// TODO(gri): function-local named types should be displayed
 			// differently from named types at package level to avoid
@@ -208,7 +234,7 @@ func writeType(buf *bytes.Buffer, this *Package, typ Type, visited []Type) {
 	}
 }
 
-func writeTuple(buf *bytes.Buffer, this *Package, tup *Tuple, variadic bool, visited []Type) {
+func writeTuple(buf *bytes.Buffer, tup *Tuple, variadic bool, qf Qualifier, visited []Type) {
 	buf.WriteByte('(')
 	if tup != nil {
 		for i, v := range tup.vars {
@@ -230,12 +256,12 @@ func writeTuple(buf *bytes.Buffer, this *Package, tup *Tuple, variadic bool, vis
 					if t, ok := typ.Underlying().(*Basic); !ok || t.kind != String {
 						panic("internal error: string type expected")
 					}
-					writeType(buf, this, typ, visited)
+					writeType(buf, typ, qf, visited)
 					buf.WriteString("...")
 					continue
 				}
 			}
-			writeType(buf, this, typ, visited)
+			writeType(buf, typ, qf, visited)
 		}
 	}
 	buf.WriteByte(')')
@@ -243,14 +269,14 @@ func writeTuple(buf *bytes.Buffer, this *Package, tup *Tuple, variadic bool, vis
 
 // WriteSignature writes the representation of the signature sig to buf,
 // without a leading "func" keyword.
-// Named types are printed package-qualified if they
-// do not belong to this package.
-func WriteSignature(buf *bytes.Buffer, this *Package, sig *Signature) {
-	writeSignature(buf, this, sig, make([]Type, 8))
+// The Qualifier controls the printing of
+// package-level objects, and may be nil.
+func WriteSignature(buf *bytes.Buffer, sig *Signature, qf Qualifier) {
+	writeSignature(buf, sig, qf, make([]Type, 8))
 }
 
-func writeSignature(buf *bytes.Buffer, this *Package, sig *Signature, visited []Type) {
-	writeTuple(buf, this, sig.params, sig.variadic, visited)
+func writeSignature(buf *bytes.Buffer, sig *Signature, qf Qualifier, visited []Type) {
+	writeTuple(buf, sig.params, sig.variadic, qf, visited)
 
 	n := sig.results.Len()
 	if n == 0 {
@@ -261,10 +287,10 @@ func writeSignature(buf *bytes.Buffer, this *Package, sig *Signature, visited []
 	buf.WriteByte(' ')
 	if n == 1 && sig.results.vars[0].name == "" {
 		// single unnamed result
-		writeType(buf, this, sig.results.vars[0].typ, visited)
+		writeType(buf, sig.results.vars[0].typ, qf, visited)
 		return
 	}
 
 	// multiple or named result(s)
-	writeTuple(buf, this, sig.results, false, visited)
+	writeTuple(buf, sig.results, false, qf, visited)
 }
diff --git a/src/go/types/typestring_test.go b/src/go/types/typestring_test.go
index 9945ed8c12..913e6c735c 100644
--- a/src/go/types/typestring_test.go
+++ b/src/go/types/typestring_test.go
@@ -154,7 +154,13 @@ func TestQualifiedTypeString(t *testing.T) {
 		{NewPointer(pT), p, "*T"},
 		{NewPointer(pT), q, "*p.T"},
 	} {
-		if got := TypeString(test.this, test.typ); got != test.want {
+		qualifier := func(pkg *Package) string {
+			if pkg != test.this {
+				return pkg.Name()
+			}
+			return ""
+		}
+		if got := TypeString(test.typ, qualifier); got != test.want {
 			t.Errorf("TypeString(%s, %s) = %s, want %s",
 				test.this, test.typ, got, test.want)
 		}