Assume ubchecks

compiler/rustc_mir_transform/src/simplify.rs

@@ -78,13 +78,10 @@ impl SimplifyCfg {
 
 pub(super) fn simplify_cfg<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
     if CfgSimplifier::new(tcx, body).simplify() {
-        // `simplify` returns that it changed something. We must invalidate the CFG caches as they
-        // are not consistent with the modified CFG any more.
         body.basic_blocks.invalidate_cfg_cache();
     }
     remove_dead_blocks(body);
-
-    // FIXME: Should probably be moved into some kind of pass manager
+    remove_runtime_checks_assumes(body);
     body.basic_blocks.as_mut_preserves_cfg().shrink_to_fit();
 }
 
@@ -334,6 +331,20 @@ impl<'a, 'tcx> CfgSimplifier<'a, 'tcx> {
     }
 }
 
+fn remove_runtime_checks_assumes<'tcx>(body: &mut Body<'tcx>) {
+    for block in body.basic_blocks.as_mut_preserves_cfg() {
+        for statement in &mut block.statements {
+            if let StatementKind::Intrinsic(box NonDivergingIntrinsic::Assume(operand)) =
+                &statement.kind
+            {
+                if matches!(operand, Operand::RuntimeChecks(_)) {
+                    statement.make_nop(true);
+                }
+            }
+        }
+    }
+}
+
 pub(super) fn simplify_duplicate_switch_targets(terminator: &mut Terminator<'_>) {
     if let TerminatorKind::SwitchInt { targets, .. } = &mut terminator.kind {
         let otherwise = targets.otherwise();

tests/mir-opt/ub_checks_assume_removals.rs

@@ -0,0 +1,18 @@
+//@ test-mir-pass: SimplifyCfg-final
+//@ compile-flags: -Zmir-opt-level=1
+
+#![crate_type = "lib"]
+#![feature(core_intrinsics)]
+
+// This test ensures that `assume(RuntimeChecks)`
+// is not emitted in MIR after SimplifyCfg.
+
+// EMIT_MIR_FOR_EACH ub_checks_assume_removals.test SimplifyCfg-final
+pub unsafe fn test(ptr: *const u8) -> u8 {
+    ptr.read()
+}
+
+// EMIT_MIR_FOR_EACH ub_checks_assume_removals.with_unchecked SimplifyCfg-final
+pub unsafe fn with_unchecked(x: i32, y: i32) -> i32 {
+    x.unchecked_add(y)
+}