test exact_div UB detection

Author: RalfJung

src/intrinsic.rs

@@ -272,7 +272,13 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                 let b = this.read_immediate(args[1])?;
                 // check x % y != 0
                 if this.binary_op(mir::BinOp::Rem, a, b)?.0.to_bits(dest.layout.size)? != 0 {
-                    return err!(ValidationFailure(format!("exact_div: {:?} cannot be divided by {:?}", a, b)));
+                    // Check if `b` is -1, which is the "min_value / -1" case.
+                    let minus1 = Scalar::from_int(-1, dest.layout.size);
+                    return if b.to_scalar().unwrap() == minus1 {
+                        err!(Intrinsic(format!("exact_div: result of dividing MIN by -1 cannot be represented")))
+                    } else {
+                        err!(Intrinsic(format!("exact_div: {:?} cannot be divided by {:?} without remainder", *a, *b)))
+                    };
                 }
                 this.binop_ignore_overflow(mir::BinOp::Div, a, b, dest)?;
             },

tests/compile-fail/exact_div1.rs

@@ -0,0 +1,5 @@
+#![feature(core_intrinsics)]
+fn main() {
+    // divison by 0
+    unsafe { std::intrinsics::exact_div(2, 0); } //~ ERROR divisor of zero
+}

tests/compile-fail/exact_div2.rs

@@ -0,0 +1,5 @@
+#![feature(core_intrinsics)]
+fn main() {
+    // divison with a remainder
+    unsafe { std::intrinsics::exact_div(2u16, 3); } //~ ERROR Scalar(0x0002) cannot be divided by Scalar(0x0003) without remainder
+}

tests/compile-fail/exact_div3.rs

@@ -0,0 +1,5 @@
+#![feature(core_intrinsics)]
+fn main() {
+    // signed divison with a remainder
+    unsafe { std::intrinsics::exact_div(-19i8, 2); } //~ ERROR Scalar(0xed) cannot be divided by Scalar(0x02) without remainder
+}

tests/compile-fail/exact_div4.rs

@@ -0,0 +1,5 @@
+#![feature(core_intrinsics)]
+fn main() {
+    // divison of min_value by -1
+    unsafe { std::intrinsics::exact_div(i64::min_value(), -1); } //~ ERROR result of dividing MIN by -1 cannot be represented
+}