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// Copyright 2010 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.
// This file contains the code to check canonical methods.
package main
import (
"fmt"
"go/ast"
"go/printer"
"strings"
)
func init() {
register("methods",
"check that canonically named methods are canonically defined",
checkCanonicalMethod,
funcDecl, interfaceType)
}
type MethodSig struct {
args []string
results []string
}
// canonicalMethods lists the input and output types for Go methods
// that are checked using dynamic interface checks. Because the
// checks are dynamic, such methods would not cause a compile error
// if they have the wrong signature: instead the dynamic check would
// fail, sometimes mysteriously. If a method is found with a name listed
// here but not the input/output types listed here, vet complains.
//
// A few of the canonical methods have very common names.
// For example, a type might implement a Scan method that
// has nothing to do with fmt.Scanner, but we still want to check
// the methods that are intended to implement fmt.Scanner.
// To do that, the arguments that have a = prefix are treated as
// signals that the canonical meaning is intended: if a Scan
// method doesn't have a fmt.ScanState as its first argument,
// we let it go. But if it does have a fmt.ScanState, then the
// rest has to match.
var canonicalMethods = map[string]MethodSig{
// "Flush": {{}, {"error"}}, // http.Flusher and jpeg.writer conflict
"Format": {[]string{"=fmt.State", "rune"}, []string{}}, // fmt.Formatter
"GobDecode": {[]string{"[]byte"}, []string{"error"}}, // gob.GobDecoder
"GobEncode": {[]string{}, []string{"[]byte", "error"}}, // gob.GobEncoder
"MarshalJSON": {[]string{}, []string{"[]byte", "error"}}, // json.Marshaler
"MarshalXML": {[]string{"*xml.Encoder", "xml.StartElement"}, []string{"error"}}, // xml.Marshaler
"ReadByte": {[]string{}, []string{"byte", "error"}}, // io.ByteReader
"ReadFrom": {[]string{"=io.Reader"}, []string{"int64", "error"}}, // io.ReaderFrom
"ReadRune": {[]string{}, []string{"rune", "int", "error"}}, // io.RuneReader
"Scan": {[]string{"=fmt.ScanState", "rune"}, []string{"error"}}, // fmt.Scanner
"Seek": {[]string{"=int64", "int"}, []string{"int64", "error"}}, // io.Seeker
"UnmarshalJSON": {[]string{"[]byte"}, []string{"error"}}, // json.Unmarshaler
"UnmarshalXML": {[]string{"*xml.Decoder", "xml.StartElement"}, []string{"error"}}, // xml.Unmarshaler
"UnreadByte": {[]string{}, []string{"error"}},
"UnreadRune": {[]string{}, []string{"error"}},
"WriteByte": {[]string{"byte"}, []string{"error"}}, // jpeg.writer (matching bufio.Writer)
"WriteTo": {[]string{"=io.Writer"}, []string{"int64", "error"}}, // io.WriterTo
}
func checkCanonicalMethod(f *File, node ast.Node) {
switch n := node.(type) {
case *ast.FuncDecl:
if n.Recv != nil {
canonicalMethod(f, n.Name, n.Type)
}
case *ast.InterfaceType:
for _, field := range n.Methods.List {
for _, id := range field.Names {
canonicalMethod(f, id, field.Type.(*ast.FuncType))
}
}
}
}
func canonicalMethod(f *File, id *ast.Ident, t *ast.FuncType) {
// Expected input/output.
expect, ok := canonicalMethods[id.Name]
if !ok {
return
}
// Actual input/output
args := typeFlatten(t.Params.List)
var results []ast.Expr
if t.Results != nil {
results = typeFlatten(t.Results.List)
}
// Do the =s (if any) all match?
if !f.matchParams(expect.args, args, "=") || !f.matchParams(expect.results, results, "=") {
return
}
// Everything must match.
if !f.matchParams(expect.args, args, "") || !f.matchParams(expect.results, results, "") {
expectFmt := id.Name + "(" + argjoin(expect.args) + ")"
if len(expect.results) == 1 {
expectFmt += " " + argjoin(expect.results)
} else if len(expect.results) > 1 {
expectFmt += " (" + argjoin(expect.results) + ")"
}
f.b.Reset()
if err := printer.Fprint(&f.b, f.fset, t); err != nil {
fmt.Fprintf(&f.b, "<%s>", err)
}
actual := f.b.String()
actual = strings.TrimPrefix(actual, "func")
actual = id.Name + actual
f.Badf(id.Pos(), "method %s should have signature %s", actual, expectFmt)
}
}
func argjoin(x []string) string {
y := make([]string, len(x))
for i, s := range x {
if s[0] == '=' {
s = s[1:]
}
y[i] = s
}
return strings.Join(y, ", ")
}
// Turn parameter list into slice of types
// (in the ast, types are Exprs).
// Have to handle f(int, bool) and f(x, y, z int)
// so not a simple 1-to-1 conversion.
func typeFlatten(l []*ast.Field) []ast.Expr {
var t []ast.Expr
for _, f := range l {
if len(f.Names) == 0 {
t = append(t, f.Type)
continue
}
for range f.Names {
t = append(t, f.Type)
}
}
return t
}
// Does each type in expect with the given prefix match the corresponding type in actual?
func (f *File) matchParams(expect []string, actual []ast.Expr, prefix string) bool {
for i, x := range expect {
if !strings.HasPrefix(x, prefix) {
continue
}
if i >= len(actual) {
return false
}
if !f.matchParamType(x, actual[i]) {
return false
}
}
if prefix == "" && len(actual) > len(expect) {
return false
}
return true
}
// Does this one type match?
func (f *File) matchParamType(expect string, actual ast.Expr) bool {
expect = strings.TrimPrefix(expect, "=")
// Strip package name if we're in that package.
if n := len(f.file.Name.Name); len(expect) > n && expect[:n] == f.file.Name.Name && expect[n] == '.' {
expect = expect[n+1:]
}
// Overkill but easy.
f.b.Reset()
printer.Fprint(&f.b, f.fset, actual)
return f.b.String() == expect
}
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