[dev.typeparams] cmd/compile/internal/importer, types2: initial check-in of types2 and importer

This is a copy of the importer and types2 (unreviewed) prototype version
excluding the testdata directory containing tests (see below). Each file
is marked with the comment

// UNREVIEWED

on the first line. The plan is to check in this code wholesale (it runs and
passes all tests) and then review the code file-by-file via subsequent CLs
and remove the "// UNREVIEWED" comments as we review the files.

Since most tests are unchanged from the original go/types, the next CL will
commit those tests as they don't need to be reviewed again. (Eventually we
may want to factor them out and share them from a single place, e.g. the
test directory.)

The existing file fmtmap_test.go was updated.

Change-Id: I9bd0ad1a7e7188b501423483a44d18e623c0fe71
Reviewed-on: https://go-review.googlesource.com/c/go/+/263624
Trust: Robert Griesemer <gri@golang.org>
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

1 files changed, 714 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/types2/resolver.go b/src/cmd/compile/internal/types2/resolver.go
new file mode 100644
index 0000000000..b116888bf2
--- /dev/null
+++ b/src/cmd/compile/internal/types2/resolver.go
@@ -0,0 +1,714 @@
+// UNREVIEWED
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types2
+
+import (
+	"cmd/compile/internal/syntax"
+	"fmt"
+	"go/constant"
+	"sort"
+	"strconv"
+	"strings"
+	"unicode"
+)
+
+// A declInfo describes a package-level const, type, var, or func declaration.
+type declInfo struct {
+	file  *Scope           // scope of file containing this declaration
+	lhs   []*Var           // lhs of n:1 variable declarations, or nil
+	vtyp  syntax.Expr      // type, or nil (for const and var declarations only)
+	init  syntax.Expr      // init/orig expression, or nil (for const and var declarations only)
+	tdecl *syntax.TypeDecl // type declaration, or nil
+	fdecl *syntax.FuncDecl // func declaration, or nil
+
+	// The deps field tracks initialization expression dependencies.
+	deps map[Object]bool // lazily initialized
+}
+
+// hasInitializer reports whether the declared object has an initialization
+// expression or function body.
+func (d *declInfo) hasInitializer() bool {
+	return d.init != nil || d.fdecl != nil && d.fdecl.Body != nil
+}
+
+// addDep adds obj to the set of objects d's init expression depends on.
+func (d *declInfo) addDep(obj Object) {
+	m := d.deps
+	if m == nil {
+		m = make(map[Object]bool)
+		d.deps = m
+	}
+	m[obj] = true
+}
+
+// arity checks that the lhs and rhs of a const or var decl
+// have a matching number of names and initialization values.
+// If inherited is set, the initialization values are from
+// another (constant) declaration.
+func (check *Checker) arity(pos syntax.Pos, names []*syntax.Name, inits []syntax.Expr, constDecl, inherited bool) {
+	l := len(names)
+	r := len(inits)
+
+	switch {
+	case l < r:
+		n := inits[l]
+		if inherited {
+			check.errorf(pos, "extra init expr at %s", n.Pos())
+		} else {
+			check.errorf(n, "extra init expr %s", n)
+		}
+	case l > r && (constDecl || r != 1): // if r == 1 it may be a multi-valued function and we can't say anything yet
+		n := names[r]
+		check.errorf(n, "missing init expr for %s", n.Value)
+	}
+}
+
+func validatedImportPath(path string) (string, error) {
+	s, err := strconv.Unquote(path)
+	if err != nil {
+		return "", err
+	}
+	if s == "" {
+		return "", fmt.Errorf("empty string")
+	}
+	const illegalChars = `!"#$%&'()*,:;<=>?[\]^{|}` + "`\uFFFD"
+	for _, r := range s {
+		if !unicode.IsGraphic(r) || unicode.IsSpace(r) || strings.ContainsRune(illegalChars, r) {
+			return s, fmt.Errorf("invalid character %#U", r)
+		}
+	}
+	return s, nil
+}
+
+// declarePkgObj declares obj in the package scope, records its ident -> obj mapping,
+// and updates check.objMap. The object must not be a function or method.
+func (check *Checker) declarePkgObj(ident *syntax.Name, obj Object, d *declInfo) {
+	assert(ident.Value == obj.Name())
+
+	// spec: "A package-scope or file-scope identifier with name init
+	// may only be declared to be a function with this (func()) signature."
+	if ident.Value == "init" {
+		check.errorf(ident, "cannot declare init - must be func")
+		return
+	}
+
+	// spec: "The main package must have package name main and declare
+	// a function main that takes no arguments and returns no value."
+	if ident.Value == "main" && check.pkg.name == "main" {
+		check.errorf(ident, "cannot declare main - must be func")
+		return
+	}
+
+	check.declare(check.pkg.scope, ident, obj, nopos)
+	check.objMap[obj] = d
+	obj.setOrder(uint32(len(check.objMap)))
+}
+
+// filename returns a filename suitable for debugging output.
+func (check *Checker) filename(fileNo int) string {
+	file := check.files[fileNo]
+	if pos := file.Pos(); pos.IsKnown() {
+		// return check.fset.File(pos).Name()
+		// TODO(gri) do we need the actual file name here?
+		return pos.RelFilename()
+	}
+	return fmt.Sprintf("file[%d]", fileNo)
+}
+
+func (check *Checker) importPackage(pos syntax.Pos, path, dir string) *Package {
+	// If we already have a package for the given (path, dir)
+	// pair, use it instead of doing a full import.
+	// Checker.impMap only caches packages that are marked Complete
+	// or fake (dummy packages for failed imports). Incomplete but
+	// non-fake packages do require an import to complete them.
+	key := importKey{path, dir}
+	imp := check.impMap[key]
+	if imp != nil {
+		return imp
+	}
+
+	// no package yet => import it
+	if path == "C" && (check.conf.FakeImportC || check.conf.go115UsesCgo) {
+		imp = NewPackage("C", "C")
+		imp.fake = true // package scope is not populated
+		imp.cgo = check.conf.go115UsesCgo
+	} else {
+		// ordinary import
+		var err error
+		if importer := check.conf.Importer; importer == nil {
+			err = fmt.Errorf("Config.Importer not installed")
+		} else if importerFrom, ok := importer.(ImporterFrom); ok {
+			imp, err = importerFrom.ImportFrom(path, dir, 0)
+			if imp == nil && err == nil {
+				err = fmt.Errorf("Config.Importer.ImportFrom(%s, %s, 0) returned nil but no error", path, dir)
+			}
+		} else {
+			imp, err = importer.Import(path)
+			if imp == nil && err == nil {
+				err = fmt.Errorf("Config.Importer.Import(%s) returned nil but no error", path)
+			}
+		}
+		// make sure we have a valid package name
+		// (errors here can only happen through manipulation of packages after creation)
+		if err == nil && imp != nil && (imp.name == "_" || imp.name == "") {
+			err = fmt.Errorf("invalid package name: %q", imp.name)
+			imp = nil // create fake package below
+		}
+		if err != nil {
+			check.errorf(pos, "could not import %s (%s)", path, err)
+			if imp == nil {
+				// create a new fake package
+				// come up with a sensible package name (heuristic)
+				name := path
+				if i := len(name); i > 0 && name[i-1] == '/' {
+					name = name[:i-1]
+				}
+				if i := strings.LastIndex(name, "/"); i >= 0 {
+					name = name[i+1:]
+				}
+				imp = NewPackage(path, name)
+			}
+			// continue to use the package as best as we can
+			imp.fake = true // avoid follow-up lookup failures
+		}
+	}
+
+	// package should be complete or marked fake, but be cautious
+	if imp.complete || imp.fake {
+		check.impMap[key] = imp
+		check.pkgCnt[imp.name]++
+		return imp
+	}
+
+	// something went wrong (importer may have returned incomplete package without error)
+	return nil
+}
+
+// collectObjects collects all file and package objects and inserts them
+// into their respective scopes. It also performs imports and associates
+// methods with receiver base type names.
+func (check *Checker) collectObjects() {
+	pkg := check.pkg
+
+	// pkgImports is the set of packages already imported by any package file seen
+	// so far. Used to avoid duplicate entries in pkg.imports. Allocate and populate
+	// it (pkg.imports may not be empty if we are checking test files incrementally).
+	// Note that pkgImports is keyed by package (and thus package path), not by an
+	// importKey value. Two different importKey values may map to the same package
+	// which is why we cannot use the check.impMap here.
+	var pkgImports = make(map[*Package]bool)
+	for _, imp := range pkg.imports {
+		pkgImports[imp] = true
+	}
+
+	type methodInfo struct {
+		obj  *Func        // method
+		ptr  bool         // true if pointer receiver
+		recv *syntax.Name // receiver type name
+	}
+	var methods []methodInfo // collected methods with valid receivers and non-blank _ names
+	var fileScopes []*Scope
+	for fileNo, file := range check.files {
+		// The package identifier denotes the current package,
+		// but there is no corresponding package object.
+		check.recordDef(file.PkgName, nil)
+
+		fileScope := NewScope(check.pkg.scope, startPos(file), endPos(file), check.filename(fileNo))
+		fileScopes = append(fileScopes, fileScope)
+		check.recordScope(file, fileScope)
+
+		// determine file directory, necessary to resolve imports
+		// FileName may be "" (typically for tests) in which case
+		// we get "." as the directory which is what we would want.
+		fileDir := dir(file.PkgName.Pos().RelFilename()) // TODO(gri) should this be filename?
+
+		first := -1                // index of first ConstDecl in the current group, or -1
+		var last *syntax.ConstDecl // last ConstDecl with init expressions, or nil
+		for index, decl := range file.DeclList {
+			if _, ok := decl.(*syntax.ConstDecl); !ok {
+				first = -1 // we're not in a constant declaration
+			}
+
+			switch s := decl.(type) {
+			case *syntax.ImportDecl:
+				// import package
+				path, err := validatedImportPath(s.Path.Value)
+				if err != nil {
+					check.errorf(s.Path, "invalid import path (%s)", err)
+					continue
+				}
+
+				imp := check.importPackage(s.Path.Pos(), path, fileDir)
+				if imp == nil {
+					continue
+				}
+
+				// add package to list of explicit imports
+				// (this functionality is provided as a convenience
+				// for clients; it is not needed for type-checking)
+				if !pkgImports[imp] {
+					pkgImports[imp] = true
+					pkg.imports = append(pkg.imports, imp)
+				}
+
+				// local name overrides imported package name
+				name := imp.name
+				if s.LocalPkgName != nil {
+					name = s.LocalPkgName.Value
+					if path == "C" {
+						// match cmd/compile (not prescribed by spec)
+						check.errorf(s.LocalPkgName, `cannot rename import "C"`)
+						continue
+					}
+					if name == "init" {
+						check.errorf(s.LocalPkgName, "cannot declare init - must be func")
+						continue
+					}
+				}
+
+				obj := NewPkgName(s.Pos(), pkg, name, imp)
+				if s.LocalPkgName != nil {
+					// in a dot-import, the dot represents the package
+					check.recordDef(s.LocalPkgName, obj)
+				} else {
+					check.recordImplicit(s, obj)
+				}
+
+				if path == "C" {
+					// match cmd/compile (not prescribed by spec)
+					obj.used = true
+				}
+
+				// add import to file scope
+				if name == "." {
+					// merge imported scope with file scope
+					for _, obj := range imp.scope.elems {
+						// A package scope may contain non-exported objects,
+						// do not import them!
+						if obj.Exported() {
+							// declare dot-imported object
+							// (Do not use check.declare because it modifies the object
+							// via Object.setScopePos, which leads to a race condition;
+							// the object may be imported into more than one file scope
+							// concurrently. See issue #32154.)
+							if alt := fileScope.Insert(obj); alt != nil {
+								check.errorf(s.LocalPkgName, "%s redeclared in this block", obj.Name())
+								check.reportAltDecl(alt)
+							}
+						}
+					}
+					// add position to set of dot-import positions for this file
+					// (this is only needed for "imported but not used" errors)
+					check.addUnusedDotImport(fileScope, imp, s.Pos())
+				} else {
+					// declare imported package object in file scope
+					// (no need to provide s.LocalPkgName since we called check.recordDef earlier)
+					check.declare(fileScope, nil, obj, nopos)
+				}
+
+			case *syntax.ConstDecl:
+				// iota is the index of the current constDecl within the group
+				if first < 0 || file.DeclList[index-1].(*syntax.ConstDecl).Group != s.Group {
+					first = index
+					last = nil
+				}
+				iota := constant.MakeInt64(int64(index - first))
+
+				// determine which initialization expressions to use
+				inherited := true
+				switch {
+				case s.Type != nil || s.Values != nil:
+					last = s
+					inherited = false
+				case last == nil:
+					last = new(syntax.ConstDecl) // make sure last exists
+					inherited = false
+				}
+
+				// declare all constants
+				values := unpackExpr(last.Values)
+				for i, name := range s.NameList {
+					obj := NewConst(name.Pos(), pkg, name.Value, nil, iota)
+
+					var init syntax.Expr
+					if i < len(values) {
+						init = values[i]
+					}
+
+					d := &declInfo{file: fileScope, vtyp: last.Type, init: init}
+					check.declarePkgObj(name, obj, d)
+				}
+
+				// Constants must always have init values.
+				check.arity(s.Pos(), s.NameList, values, true, inherited)
+
+			case *syntax.VarDecl:
+				lhs := make([]*Var, len(s.NameList))
+				// If there's exactly one rhs initializer, use
+				// the same declInfo d1 for all lhs variables
+				// so that each lhs variable depends on the same
+				// rhs initializer (n:1 var declaration).
+				var d1 *declInfo
+				if _, ok := s.Values.(*syntax.ListExpr); !ok {
+					// The lhs elements are only set up after the for loop below,
+					// but that's ok because declarePkgObj only collects the declInfo
+					// for a later phase.
+					d1 = &declInfo{file: fileScope, lhs: lhs, vtyp: s.Type, init: s.Values}
+				}
+
+				// declare all variables
+				values := unpackExpr(s.Values)
+				for i, name := range s.NameList {
+					obj := NewVar(name.Pos(), pkg, name.Value, nil)
+					lhs[i] = obj
+
+					d := d1
+					if d == nil {
+						// individual assignments
+						var init syntax.Expr
+						if i < len(values) {
+							init = values[i]
+						}
+						d = &declInfo{file: fileScope, vtyp: s.Type, init: init}
+					}
+
+					check.declarePkgObj(name, obj, d)
+				}
+
+				// If we have no type, we must have values.
+				if s.Type == nil || values != nil {
+					check.arity(s.Pos(), s.NameList, values, false, false)
+				}
+
+			case *syntax.TypeDecl:
+				obj := NewTypeName(s.Name.Pos(), pkg, s.Name.Value, nil)
+				check.declarePkgObj(s.Name, obj, &declInfo{file: fileScope, tdecl: s})
+
+			case *syntax.FuncDecl:
+				d := s // TODO(gri) get rid of this
+				name := d.Name.Value
+				obj := NewFunc(d.Name.Pos(), pkg, name, nil)
+				if d.Recv == nil {
+					// regular function
+					if name == "init" {
+						if d.TParamList != nil {
+							//check.softErrorf(d.TParamList.Pos(), "func init must have no type parameters")
+							check.softErrorf(d.Name, "func init must have no type parameters")
+						}
+						if t := d.Type; len(t.ParamList) != 0 || len(t.ResultList) != 0 {
+							check.softErrorf(d, "func init must have no arguments and no return values")
+						}
+						// don't declare init functions in the package scope - they are invisible
+						obj.parent = pkg.scope
+						check.recordDef(d.Name, obj)
+						// init functions must have a body
+						if d.Body == nil {
+							// TODO(gri) make this error message consistent with the others above
+							check.softErrorf(obj.pos, "missing function body")
+						}
+					} else {
+						check.declare(pkg.scope, d.Name, obj, nopos)
+					}
+				} else {
+					// method
+					// d.Recv != nil
+					if !methodTypeParamsOk && len(d.TParamList) != 0 {
+						//check.invalidASTf(d.TParamList.Pos(), "method must have no type parameters")
+						check.invalidASTf(d, "method must have no type parameters")
+					}
+					ptr, recv, _ := check.unpackRecv(d.Recv.Type, false)
+					// (Methods with invalid receiver cannot be associated to a type, and
+					// methods with blank _ names are never found; no need to collect any
+					// of them. They will still be type-checked with all the other functions.)
+					if recv != nil && name != "_" {
+						methods = append(methods, methodInfo{obj, ptr, recv})
+					}
+					check.recordDef(d.Name, obj)
+				}
+				info := &declInfo{file: fileScope, fdecl: d}
+				// Methods are not package-level objects but we still track them in the
+				// object map so that we can handle them like regular functions (if the
+				// receiver is invalid); also we need their fdecl info when associating
+				// them with their receiver base type, below.
+				check.objMap[obj] = info
+				obj.setOrder(uint32(len(check.objMap)))
+
+			default:
+				check.invalidASTf(s, "unknown syntax.Decl node %T", s)
+			}
+		}
+	}
+
+	// verify that objects in package and file scopes have different names
+	for _, scope := range fileScopes {
+		for _, obj := range scope.elems {
+			if alt := pkg.scope.Lookup(obj.Name()); alt != nil {
+				if pkg, ok := obj.(*PkgName); ok {
+					check.errorf(alt, "%s already declared through import of %s", alt.Name(), pkg.Imported())
+					check.reportAltDecl(pkg)
+				} else {
+					check.errorf(alt, "%s already declared through dot-import of %s", alt.Name(), obj.Pkg())
+					// TODO(gri) dot-imported objects don't have a position; reportAltDecl won't print anything
+					check.reportAltDecl(obj)
+				}
+			}
+		}
+	}
+
+	// Now that we have all package scope objects and all methods,
+	// associate methods with receiver base type name where possible.
+	// Ignore methods that have an invalid receiver. They will be
+	// type-checked later, with regular functions.
+	if methods != nil {
+		check.methods = make(map[*TypeName][]*Func)
+		for i := range methods {
+			m := &methods[i]
+			// Determine the receiver base type and associate m with it.
+			ptr, base := check.resolveBaseTypeName(m.ptr, m.recv)
+			if base != nil {
+				m.obj.hasPtrRecv = ptr
+				check.methods[base] = append(check.methods[base], m.obj)
+			}
+		}
+	}
+}
+
+// unpackRecv unpacks a receiver type and returns its components: ptr indicates whether
+// rtyp is a pointer receiver, rname is the receiver type name, and tparams are its
+// type parameters, if any. The type parameters are only unpacked if unpackParams is
+// set. If rname is nil, the receiver is unusable (i.e., the source has a bug which we
+// cannot easily work around).
+func (check *Checker) unpackRecv(rtyp syntax.Expr, unpackParams bool) (ptr bool, rname *syntax.Name, tparams []*syntax.Name) {
+L: // unpack receiver type
+	// This accepts invalid receivers such as ***T and does not
+	// work for other invalid receivers, but we don't care. The
+	// validity of receiver expressions is checked elsewhere.
+	for {
+		switch t := rtyp.(type) {
+		case *syntax.ParenExpr:
+			rtyp = t.X
+		// case *ast.StarExpr:
+		// 	rtyp = t.X
+		case *syntax.Operation:
+			if t.Op != syntax.Mul || t.Y != nil {
+				break
+			}
+			rtyp = t.X
+		default:
+			break L
+		}
+	}
+
+	// unpack type parameters, if any
+	if ptyp, _ := rtyp.(*syntax.IndexExpr); ptyp != nil {
+		rtyp = ptyp.X
+		if unpackParams {
+			for _, arg := range unpackExpr(ptyp.Index) {
+				var par *syntax.Name
+				switch arg := arg.(type) {
+				case *syntax.Name:
+					par = arg
+				case *syntax.BadExpr:
+					// ignore - error already reported by parser
+				case nil:
+					check.invalidASTf(ptyp, "parameterized receiver contains nil parameters")
+				default:
+					check.errorf(arg, "receiver type parameter %s must be an identifier", arg)
+				}
+				if par == nil {
+					par = newName(arg.Pos(), "_")
+				}
+				tparams = append(tparams, par)
+			}
+
+		}
+	}
+
+	// unpack receiver name
+	if name, _ := rtyp.(*syntax.Name); name != nil {
+		rname = name
+	}
+
+	return
+}
+
+// resolveBaseTypeName returns the non-alias base type name for typ, and whether
+// there was a pointer indirection to get to it. The base type name must be declared
+// in package scope, and there can be at most one pointer indirection. If no such type
+// name exists, the returned base is nil.
+func (check *Checker) resolveBaseTypeName(seenPtr bool, typ syntax.Expr) (ptr bool, base *TypeName) {
+	// Algorithm: Starting from a type expression, which may be a name,
+	// we follow that type through alias declarations until we reach a
+	// non-alias type name. If we encounter anything but pointer types or
+	// parentheses we're done. If we encounter more than one pointer type
+	// we're done.
+	ptr = seenPtr
+	var seen map[*TypeName]bool
+	for {
+		typ = unparen(typ)
+
+		// check if we have a pointer type
+		// if pexpr, _ := typ.(*ast.StarExpr); pexpr != nil {
+		if pexpr, _ := typ.(*syntax.Operation); pexpr != nil && pexpr.Op == syntax.Mul && pexpr.Y == nil {
+			// if we've already seen a pointer, we're done
+			if ptr {
+				return false, nil
+			}
+			ptr = true
+			typ = unparen(pexpr.X) // continue with pointer base type
+		}
+
+		// typ must be a name
+		name, _ := typ.(*syntax.Name)
+		if name == nil {
+			return false, nil
+		}
+
+		// name must denote an object found in the current package scope
+		// (note that dot-imported objects are not in the package scope!)
+		obj := check.pkg.scope.Lookup(name.Value)
+		if obj == nil {
+			return false, nil
+		}
+
+		// the object must be a type name...
+		tname, _ := obj.(*TypeName)
+		if tname == nil {
+			return false, nil
+		}
+
+		// ... which we have not seen before
+		if seen[tname] {
+			return false, nil
+		}
+
+		// we're done if tdecl defined tname as a new type
+		// (rather than an alias)
+		tdecl := check.objMap[tname].tdecl // must exist for objects in package scope
+		if !tdecl.Alias {
+			return ptr, tname
+		}
+
+		// otherwise, continue resolving
+		typ = tdecl.Type
+		if seen == nil {
+			seen = make(map[*TypeName]bool)
+		}
+		seen[tname] = true
+	}
+}
+
+// packageObjects typechecks all package objects, but not function bodies.
+func (check *Checker) packageObjects() {
+	// process package objects in source order for reproducible results
+	objList := make([]Object, len(check.objMap))
+	i := 0
+	for obj := range check.objMap {
+		objList[i] = obj
+		i++
+	}
+	sort.Sort(inSourceOrder(objList))
+
+	// add new methods to already type-checked types (from a prior Checker.Files call)
+	for _, obj := range objList {
+		if obj, _ := obj.(*TypeName); obj != nil && obj.typ != nil {
+			check.collectMethods(obj)
+		}
+	}
+
+	// We process non-alias declarations first, in order to avoid situations where
+	// the type of an alias declaration is needed before it is available. In general
+	// this is still not enough, as it is possible to create sufficiently convoluted
+	// recursive type definitions that will cause a type alias to be needed before it
+	// is available (see issue #25838 for examples).
+	// As an aside, the cmd/compiler suffers from the same problem (#25838).
+	var aliasList []*TypeName
+	// phase 1
+	for _, obj := range objList {
+		// If we have a type alias, collect it for the 2nd phase.
+		if tname, _ := obj.(*TypeName); tname != nil && check.objMap[tname].tdecl.Alias {
+			aliasList = append(aliasList, tname)
+			continue
+		}
+
+		check.objDecl(obj, nil)
+	}
+	// phase 2
+	for _, obj := range aliasList {
+		check.objDecl(obj, nil)
+	}
+
+	// At this point we may have a non-empty check.methods map; this means that not all
+	// entries were deleted at the end of typeDecl because the respective receiver base
+	// types were not found. In that case, an error was reported when declaring those
+	// methods. We can now safely discard this map.
+	check.methods = nil
+}
+
+// inSourceOrder implements the sort.Sort interface.
+type inSourceOrder []Object
+
+func (a inSourceOrder) Len() int           { return len(a) }
+func (a inSourceOrder) Less(i, j int) bool { return a[i].order() < a[j].order() }
+func (a inSourceOrder) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
+
+// unusedImports checks for unused imports.
+func (check *Checker) unusedImports() {
+	// if function bodies are not checked, packages' uses are likely missing - don't check
+	if check.conf.IgnoreFuncBodies {
+		return
+	}
+
+	// spec: "It is illegal (...) to directly import a package without referring to
+	// any of its exported identifiers. To import a package solely for its side-effects
+	// (initialization), use the blank identifier as explicit package name."
+
+	// check use of regular imported packages
+	for _, scope := range check.pkg.scope.children /* file scopes */ {
+		for _, obj := range scope.elems {
+			if obj, ok := obj.(*PkgName); ok {
+				// Unused "blank imports" are automatically ignored
+				// since _ identifiers are not entered into scopes.
+				if !obj.used {
+					path := obj.imported.path
+					base := pkgName(path)
+					if obj.name == base {
+						check.softErrorf(obj.pos, "%q imported but not used", path)
+					} else {
+						check.softErrorf(obj.pos, "%q imported but not used as %s", path, obj.name)
+					}
+				}
+			}
+		}
+	}
+
+	// check use of dot-imported packages
+	for _, unusedDotImports := range check.unusedDotImports {
+		for pkg, pos := range unusedDotImports {
+			check.softErrorf(pos, "%q imported but not used", pkg.path)
+		}
+	}
+}
+
+// pkgName returns the package name (last element) of an import path.
+func pkgName(path string) string {
+	if i := strings.LastIndex(path, "/"); i >= 0 {
+		path = path[i+1:]
+	}
+	return path
+}
+
+// dir makes a good-faith attempt to return the directory
+// portion of path. If path is empty, the result is ".".
+// (Per the go/build package dependency tests, we cannot import
+// path/filepath and simply use filepath.Dir.)
+func dir(path string) string {
+	if i := strings.LastIndexAny(path, `/\`); i > 0 {
+		return path[:i]
+	}
+	// i <= 0
+	return "."
+}