simple: reverse "len(x) > 0" as "len(x) == 0"

Rather than "len(x) <= 0", which is technically correct,
but not actually what most Go programmers would write,
since funcs like len or cap can never return negative integers.

Do the same for cap and copy, which never return negative ints either.

Fixes #1422.

simple/lint.go

@@ -546,7 +546,7 @@ func CheckIfReturn(pass *analysis.Pass) (interface{}, error) {
 		cond := m1.State["cond"].(ast.Expr)
 		origCond := cond
 		if ret1.Name == "false" {
-			cond = negate(cond)
+			cond = negate(pass, cond)
 		}
 		report.Report(pass, n1,
 			fmt.Sprintf("should use 'return %s' instead of 'if %s { return %s }; return %s'",
@@ -558,7 +558,7 @@ func CheckIfReturn(pass *analysis.Pass) (interface{}, error) {
 	return nil, nil
 }
 
-func negate(expr ast.Expr) ast.Expr {
+func negate(pass *analysis.Pass, expr ast.Expr) ast.Expr {
 	switch expr := expr.(type) {
 	case *ast.BinaryExpr:
 		out := *expr
@@ -568,7 +568,14 @@ func negate(expr ast.Expr) ast.Expr {
 		case token.LSS:
 			out.Op = token.GEQ
 		case token.GTR:
-			out.Op = token.LEQ
+			// Some builtins never return negative ints; "len(x) <= 0" should be "len(x) == 0".
+			if call, ok := expr.X.(*ast.CallExpr); ok &&
+				code.IsCallToAny(pass, call, "len", "cap", "copy") &&
+				code.IsIntegerLiteral(pass, expr.Y, constant.MakeInt64(0)) {
+				out.Op = token.EQL
+			} else {
+				out.Op = token.LEQ
+			}
 		case token.NEQ:
 			out.Op = token.EQL
 		case token.LEQ:

simple/testdata/src/example.com/CheckIfReturn/if-return.go

@@ -153,3 +153,10 @@ func fn21(x bool) bool {
 	}
 	return false
 }
+
+func fn22(x string) bool {
+	if len(x) > 0 { //@ diag(`should use 'return len(x) == 0'`)
+		return false
+	}
+	return true
+}