go/cmd/vet/bool.go

// Copyright 2014 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 boolean condition tests.

package main

import (
	"go/ast"
	"go/token"
)

func init() {
	register("bool",
		"check for mistakes involving boolean operators",
		checkBool,
		binaryExpr)
}

func checkBool(f *File, n ast.Node) {
	e := n.(*ast.BinaryExpr)

	var op boolOp
	switch e.Op {
	case token.LOR:
		op = or
	case token.LAND:
		op = and
	default:
		return
	}

	comm := op.commutativeSets(e)
	for _, exprs := range comm {
		op.checkRedundant(f, exprs)
		op.checkSuspect(f, exprs)
	}
}

type boolOp struct {
	name  string
	tok   token.Token // token corresponding to this operator
	badEq token.Token // token corresponding to the equality test that should not be used with this operator
}

var (
	or  = boolOp{"or", token.LOR, token.NEQ}
	and = boolOp{"and", token.LAND, token.EQL}
)

// commutativeSets returns all side effect free sets of
// expressions in e that are connected by op.
// For example, given 'a || b || f() || c || d' with the or op,
// commutativeSets returns {{b, a}, {d, c}}.
func (op boolOp) commutativeSets(e *ast.BinaryExpr) [][]ast.Expr {
	exprs := op.split(e)

	// Partition the slice of expressions into commutative sets.
	i := 0
	var sets [][]ast.Expr
	for j := 0; j <= len(exprs); j++ {
		if j == len(exprs) || hasSideEffects(exprs[j]) {
			if i < j {
				sets = append(sets, exprs[i:j])
			}
			i = j + 1
		}
	}

	return sets
}

// checkRedundant checks for expressions of the form
//   e && e
//   e || e
// Exprs must contain only side effect free expressions.
func (op boolOp) checkRedundant(f *File, exprs []ast.Expr) {
	seen := make(map[string]bool)
	for _, e := range exprs {
		efmt := f.gofmt(e)
		if seen[efmt] {
			f.Badf(e.Pos(), "redundant %s: %s %s %s", op.name, efmt, op.tok, efmt)
		} else {
			seen[efmt] = true
		}
	}
}

// checkSuspect checks for expressions of the form
//   x != c1 || x != c2
//   x == c1 && x == c2
// where c1 and c2 are constant expressions.
// If c1 and c2 are the same then it's redundant;
// if c1 and c2 are different then it's always true or always false.
// Exprs must contain only side effect free expressions.
func (op boolOp) checkSuspect(f *File, exprs []ast.Expr) {
	// seen maps from expressions 'x' to equality expressions 'x != c'.
	seen := make(map[string]string)

	for _, e := range exprs {
		bin, ok := e.(*ast.BinaryExpr)
		if !ok || bin.Op != op.badEq {
			continue
		}

		// In order to avoid false positives, restrict to cases
		// in which one of the operands is constant. We're then
		// interested in the other operand.
		// In the rare case in which both operands are constant
		// (e.g. runtime.GOOS and "windows"), we'll only catch
		// mistakes if the LHS is repeated, which is how most
		// code is written.
		var x ast.Expr
		switch {
		case f.pkg.types[bin.Y].Value != nil:
			x = bin.X
		case f.pkg.types[bin.X].Value != nil:
			x = bin.Y
		default:
			continue
		}

		// e is of the form 'x != c' or 'x == c'.
		xfmt := f.gofmt(x)
		efmt := f.gofmt(e)
		if prev, found := seen[xfmt]; found {
			// checkRedundant handles the case in which efmt == prev.
			if efmt != prev {
				f.Badf(e.Pos(), "suspect %s: %s %s %s", op.name, efmt, op.tok, prev)
			}
		} else {
			seen[xfmt] = efmt
		}
	}
}

// hasSideEffects reports whether evaluation of e has side effects.
func hasSideEffects(e ast.Expr) bool {
	safe := true
	ast.Inspect(e, func(node ast.Node) bool {
		switch n := node.(type) {
		// Using CallExpr here will catch conversions
		// as well as function and method invocations.
		// We'll live with the false negatives for now.
		case *ast.CallExpr:
			safe = false
			return false
		case *ast.UnaryExpr:
			if n.Op == token.ARROW {
				safe = false
				return false
			}
		}
		return true
	})
	return !safe
}

// split returns a slice of all subexpressions in e that are connected by op.
// For example, given 'a || (b || c) || d' with the or op,
// split returns []{d, c, b, a}.
func (op boolOp) split(e ast.Expr) (exprs []ast.Expr) {
	for {
		e = unparen(e)
		if b, ok := e.(*ast.BinaryExpr); ok && b.Op == op.tok {
			exprs = append(exprs, op.split(b.Y)...)
			e = b.X
		} else {
			exprs = append(exprs, e)
			break
		}
	}
	return
}

// unparen returns e with any enclosing parentheses stripped.
func unparen(e ast.Expr) ast.Expr {
	for {
		p, ok := e.(*ast.ParenExpr)
		if !ok {
			return e
		}
		e = p.X
	}
}