lsp/source: create zero value ast.Exprs

In support of https://golang.org/cl/224960, add utility functions that
create zero value expressions for a given types.Type. There are probably
some bugs here but they pass the basic tests in that CL.

Known problems: it skips through type aliases, and can't handle
anonymous structs/interfaces. Hopefully it's pretty unusual to return
those.

Change-Id: I69a388d9ce9bd60bc230e4a3d2b30f581ec25698
Reviewed-on: https://go-review.googlesource.com/c/tools/+/225177
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rohan Challa <rohan@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

internal/lsp/source/util.go

@@ -16,6 +16,7 @@ import (
 	"sort"
 	"strings"
 
+	"golang.org/x/tools/go/ast/astutil"
 	"golang.org/x/tools/internal/lsp/protocol"
 	"golang.org/x/tools/internal/span"
 	errors "golang.org/x/xerrors"
@@ -774,3 +775,81 @@ func formatZeroValue(T types.Type, qf types.Qualifier) string {
 		return types.TypeString(T, qf) + "{}"
 	}
 }
+
+func zeroValue(fset *token.FileSet, f *ast.File, pkg *types.Package, typ types.Type) ast.Expr {
+	under := typ
+	if n, ok := typ.(*types.Named); ok {
+		under = n.Underlying()
+	}
+	switch u := under.(type) {
+	case *types.Basic:
+		switch {
+		case u.Info()&types.IsNumeric != 0:
+			return &ast.BasicLit{Kind: token.INT, Value: "0"}
+		case u.Info()&types.IsBoolean != 0:
+			return &ast.Ident{Name: "false"}
+		case u.Info()&types.IsString != 0:
+			return &ast.BasicLit{Kind: token.STRING, Value: `""`}
+		default:
+			panic("unknown basic type")
+		}
+	case *types.Chan, *types.Interface, *types.Map, *types.Pointer, *types.Signature, *types.Slice:
+		return ast.NewIdent("nil")
+	case *types.Struct:
+		texpr := typeExpr(fset, f, pkg, typ) // typ because we want the name here.
+		if texpr == nil {
+			return nil
+		}
+		return &ast.CompositeLit{
+			Type: texpr,
+		}
+	case *types.Array:
+		texpr := typeExpr(fset, f, pkg, u.Elem())
+		if texpr == nil {
+			return nil
+		}
+		return &ast.CompositeLit{
+			Type: &ast.ArrayType{
+				Elt: texpr,
+				Len: &ast.BasicLit{Kind: token.INT, Value: fmt.Sprintf("%v", u.Len())},
+			},
+		}
+	}
+	return nil
+}
+
+func typeExpr(fset *token.FileSet, f *ast.File, pkg *types.Package, typ types.Type) ast.Expr {
+	switch t := typ.(type) {
+	case *types.Basic:
+		switch t.Kind() {
+		case types.UnsafePointer:
+			return &ast.SelectorExpr{X: ast.NewIdent("unsafe"), Sel: ast.NewIdent("Pointer")}
+		default:
+			return ast.NewIdent(t.Name())
+		}
+	case *types.Named:
+		if t.Obj().Pkg() == pkg {
+			return ast.NewIdent(t.Obj().Name())
+		}
+		pkgName := t.Obj().Pkg().Name()
+		// If the file already imports the package under another name, use that.
+		for _, group := range astutil.Imports(fset, f) {
+			for _, cand := range group {
+				if strings.Trim(cand.Path.Value, `"`) == t.Obj().Pkg().Path() {
+					if cand.Name != nil && cand.Name.Name != "" {
+						pkgName = cand.Name.Name
+					}
+				}
+			}
+		}
+		if pkgName == "." {
+			return ast.NewIdent(t.Obj().Name())
+		}
+		return &ast.SelectorExpr{
+			X:   ast.NewIdent(pkgName),
+			Sel: ast.NewIdent(t.Obj().Name()),
+		}
+	default:
+		return nil // TODO: anonymous structs, but who does that
+	}
+}