go/analysis/passes/stdmethods: rewrite check to use go/types

Now that vet can rely on go/types, there's no reason to do extra work to
avoid using it. The rewrite lets us get rid of the field list flattening
code, as well as the slight verbosity that comes with go/printer.

While at it, make the testdata/method.go expected errors be more
specific, to make sure that we're not breaking the warnings that are
printed.

This change was originally made to cmd/vet in
https://go-review.googlesource.com/c/148919

Change-Id: I123e64d369e521199712c9807583c53d428534ac
Reviewed-on: https://go-review.googlesource.com/c/149418
Reviewed-by: Michael Matloob <matloob@golang.org>

go/analysis/passes/stdmethods/stdmethods.go

@@ -7,10 +7,7 @@
 package stdmethods
 
 import (
-	"bytes"
-	"fmt"
 	"go/ast"
-	"go/printer"
 	"go/token"
 	"go/types"
 	"strings"
@@ -95,12 +92,12 @@ func run(pass *analysis.Pass) (interface{}, error) {
 		switch n := n.(type) {
 		case *ast.FuncDecl:
 			if n.Recv != nil {
-				canonicalMethod(pass, n.Name, n.Type)
+				canonicalMethod(pass, n.Name)
 			}
 		case *ast.InterfaceType:
 			for _, field := range n.Methods.List {
 				for _, id := range field.Names {
-					canonicalMethod(pass, id, field.Type.(*ast.FuncType))
+					canonicalMethod(pass, id)
 				}
 			}
 		}
@@ -108,7 +105,7 @@ func run(pass *analysis.Pass) (interface{}, error) {
 	return nil, nil
 }
 
-func canonicalMethod(pass *analysis.Pass, id *ast.Ident, t *ast.FuncType) {
+func canonicalMethod(pass *analysis.Pass, id *ast.Ident) {
 	// Expected input/output.
 	expect, ok := canonicalMethods[id.Name]
 	if !ok {
@@ -116,11 +113,9 @@ func canonicalMethod(pass *analysis.Pass, id *ast.Ident, t *ast.FuncType) {
 	}
 
 	// Actual input/output
-	args := typeFlatten(t.Params.List)
-	var results []ast.Expr
-	if t.Results != nil {
-		results = typeFlatten(t.Results.List)
-	}
+	sign := pass.TypesInfo.Defs[id].Type().(*types.Signature)
+	args := sign.Params()
+	results := sign.Results()
 
 	// Do the =s (if any) all match?
 	if !matchParams(pass, expect.args, args, "=") || !matchParams(pass, expect.results, results, "=") {
@@ -136,11 +131,7 @@ func canonicalMethod(pass *analysis.Pass, id *ast.Ident, t *ast.FuncType) {
 			expectFmt += " (" + argjoin(expect.results) + ")"
 		}
 
-		var buf bytes.Buffer
-		if err := printer.Fprint(&buf, pass.Fset, t); err != nil {
-			fmt.Fprintf(&buf, "<%s>", err)
-		}
-		actual := buf.String()
+		actual := sign.String()
 		actual = strings.TrimPrefix(actual, "func")
 		actual = id.Name + actual
 
@@ -159,45 +150,27 @@ func argjoin(x []string) string {
 	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 matchParams(pass *analysis.Pass, expect []string, actual []ast.Expr, prefix string) bool {
+func matchParams(pass *analysis.Pass, expect []string, actual *types.Tuple, prefix string) bool {
 	for i, x := range expect {
 		if !strings.HasPrefix(x, prefix) {
 			continue
 		}
-		if i >= len(actual) {
+		if i >= actual.Len() {
 			return false
 		}
-		if !matchParamType(pass.Fset, pass.Pkg, x, actual[i]) {
+		if !matchParamType(pass.Fset, pass.Pkg, x, actual.At(i).Type()) {
 			return false
 		}
 	}
-	if prefix == "" && len(actual) > len(expect) {
+	if prefix == "" && actual.Len() > len(expect) {
 		return false
 	}
 	return true
 }
 
 // Does this one type match?
-func matchParamType(fset *token.FileSet, pkg *types.Package, expect string, actual ast.Expr) bool {
+func matchParamType(fset *token.FileSet, pkg *types.Package, expect string, actual types.Type) bool {
 	expect = strings.TrimPrefix(expect, "=")
 	// Strip package name if we're in that package.
 	if n := len(pkg.Name()); len(expect) > n && expect[:n] == pkg.Name() && expect[n] == '.' {
@@ -205,7 +178,5 @@ func matchParamType(fset *token.FileSet, pkg *types.Package, expect string, actu
 	}
 
 	// Overkill but easy.
-	var buf bytes.Buffer
-	printer.Fprint(&buf, fset, actual)
-	return buf.String() == expect
+	return actual.String() == expect
 }

go/analysis/passes/stdmethods/testdata/src/a/a.go

@@ -8,7 +8,7 @@ import "fmt"
 
 type T int
 
-func (T) Scan(x fmt.ScanState, c byte) {} // want "should have signature Scan"
+func (T) Scan(x fmt.ScanState, c byte) {} // want `should have signature Scan\(fmt\.ScanState, rune\) error`
 
 func (T) Format(fmt.State, byte) {} // want `should have signature Format\(fmt.State, rune\)`
 
@@ -19,5 +19,5 @@ func (U) Format(byte) {} // no error: first parameter must be fmt.State to trigg
 func (U) GobDecode() {} // want `should have signature GobDecode\(\[\]byte\) error`
 
 type I interface {
-	ReadByte() byte // want "should have signature ReadByte"
+	ReadByte() byte // want `should have signature ReadByte\(\) \(byte, error\)`
 }