SA4000: add exceptions for functions from math/rand

Closes gh-1087

(cherry picked from commit c4bd432)

Author: dominikh

staticcheck/lint.go

@@ -1360,6 +1360,9 @@ func CheckLhsRhsIdentical(pass *analysis.Pass) (interface{}, error) {
 	// happily flags fn() == fn() – so far, we've had nobody complain
 	// about a false positive, and it's caught several bugs in real
 	// code.
+	//
+	// We special case functions from the math/rand package. Someone ran
+	// into the following false positive: "rand.Intn(2) - rand.Intn(2), which I wrote to generate values {-1, 0, 1} with {0.25, 0.5, 0.25} probability."
 	fn := func(node ast.Node) {
 		op := node.(*ast.BinaryExpr)
 		switch op.Op {
@@ -1399,6 +1402,38 @@ func CheckLhsRhsIdentical(pass *analysis.Pass) (interface{}, error) {
 			// 0 == 0 are slim.
 			return
 		}
+
+		if expr, ok := op.X.(*ast.CallExpr); ok {
+			call := code.CallName(pass, expr)
+			switch call {
+			case "math/rand.Int",
+				"math/rand.Int31",
+				"math/rand.Int31n",
+				"math/rand.Int63",
+				"math/rand.Int63n",
+				"math/rand.Intn",
+				"math/rand.Uint32",
+				"math/rand.Uint64",
+				"math/rand.ExpFloat64",
+				"math/rand.Float32",
+				"math/rand.Float64",
+				"math/rand.NormFloat64",
+				"(*math/rand.Rand).Int",
+				"(*math/rand.Rand).Int31",
+				"(*math/rand.Rand).Int31n",
+				"(*math/rand.Rand).Int63",
+				"(*math/rand.Rand).Int63n",
+				"(*math/rand.Rand).Intn",
+				"(*math/rand.Rand).Uint32",
+				"(*math/rand.Rand).Uint64",
+				"(*math/rand.Rand).ExpFloat64",
+				"(*math/rand.Rand).Float32",
+				"(*math/rand.Rand).Float64",
+				"(*math/rand.Rand).NormFloat64":
+				return
+			}
+		}
+
 		report.Report(pass, op, fmt.Sprintf("identical expressions on the left and right side of the '%s' operator", op.Op))
 	}
 	code.Preorder(pass, fn, (*ast.BinaryExpr)(nil))

staticcheck/testdata/src/CheckLhsRhsIdentical/CheckLhsRhsIdentical.go

@@ -1,5 +1,7 @@
 package pkg
 
+import "math/rand"
+
 type Float float64
 
 type Floats [5]float64
@@ -65,3 +67,34 @@ func fn(a int, s []int, f1 float64, f2 Float, fs Floats, is Ints, t1 T1, t2 T2)
 		println()
 	}
 }
+
+func fn2() {
+	_ = rand.Int() - rand.Int()
+	_ = rand.Int31() - rand.Int31()
+	_ = rand.Int31n(0) - rand.Int31n(0)
+	_ = rand.Int63() - rand.Int63()
+	_ = rand.Int63n(0) - rand.Int63n(0)
+	_ = rand.Intn(0) - rand.Intn(0)
+	_ = rand.Uint32() - rand.Uint32()
+	_ = rand.Uint64() - rand.Uint64()
+	_ = rand.ExpFloat64() - rand.ExpFloat64()
+	_ = rand.Float32() - rand.Float32()
+	_ = rand.Float64() - rand.Float64()
+	_ = rand.NormFloat64() - rand.NormFloat64()
+
+	var rng *rand.Rand
+	_ = rng.Int() - rng.Int()
+	_ = rng.Int31() - rng.Int31()
+	_ = rng.Int31n(0) - rng.Int31n(0)
+	_ = rng.Int63() - rng.Int63()
+	_ = rng.Int63n(0) - rng.Int63n(0)
+	_ = rng.Intn(0) - rng.Intn(0)
+	_ = rng.Uint32() - rng.Uint32()
+	_ = rng.Uint64() - rng.Uint64()
+	_ = rng.ExpFloat64() - rng.ExpFloat64()
+	_ = rng.Float32() - rng.Float32()
+	_ = rng.Float64() - rng.Float64()
+	_ = rng.NormFloat64() - rng.NormFloat64()
+
+	_ = rand.NewSource(0) == rand.NewSource(0) // want `identical expressions`
+}