error on failed assumptions

src/error.rs

@@ -43,6 +43,7 @@ pub enum EvalError<'tcx> {
     CalledClosureAsFunction,
     VtableForArgumentlessMethod,
     ModifiedConstantMemory,
+    AssumptionNotHeld,
 }
 
 pub type EvalResult<'tcx, T> = Result<T, EvalError<'tcx>>;
@@ -97,6 +98,8 @@ impl<'tcx> Error for EvalError<'tcx> {
                 "tried to call a vtable function without arguments",
             EvalError::ModifiedConstantMemory =>
                 "tried to modify constant memory",
+            EvalError::AssumptionNotHeld =>
+                "`assume` argument was false"
         }
     }
 

src/interpreter/terminator.rs

@@ -284,8 +284,11 @@ impl<'a, 'tcx> EvalContext<'a, 'tcx> {
             "sub_with_overflow" => self.intrinsic_with_overflow(mir::BinOp::Sub, &args[0], &args[1], dest, dest_layout)?,
             "mul_with_overflow" => self.intrinsic_with_overflow(mir::BinOp::Mul, &args[0], &args[1], dest, dest_layout)?,
 
-            // FIXME: turn into an assertion to catch wrong `assume` that would cause UB in llvm
-            "assume" => {}
+            "assume" => {
+                if !self.memory.read_bool(args_ptrs[0])? {
+                    return Err(EvalError::AssumptionNotHeld);
+                }
+            }
 
             "copy_nonoverlapping" => {
                 let elem_ty = substs.type_at(0);

tests/compile-fail/assume.rs

@@ -0,0 +1,10 @@
+#![feature(core_intrinsics)]
+
+fn main() {
+    let x = 5;
+    unsafe {
+        std::intrinsics::assume(x < 10);
+        std::intrinsics::assume(x > 1);
+        std::intrinsics::assume(x > 42); //~ ERROR: `assume` argument was false
+    }
+}