Rollup merge of rust-lang#128666 - pitaj:intrinsic-overflow_checks, r=BoxyUwU

Add `overflow_checks` intrinsic

This adds an intrinsic which allows code in a pre-built library to inherit the overflow checks option from a crate depending on it. This enables code in the standard library to explicitly change behavior based on whether `overflow_checks` are enabled, regardless of the setting used when standard library was compiled.

This is very similar to the `ub_checks` intrinsic, and refactors the two to use a common mechanism.

The primary use case for this is to allow the new `RangeFrom` iterator to yield the maximum element before overflowing, as requested [here](rust-lang#125687 (comment)). This PR includes a working `IterRangeFrom` implementation based on this new intrinsic that exhibits the desired behavior.

[Prior discussion on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/Ability.20to.20select.20code.20based.20on.20.60overflow_checks.60.3F)

Author: Zalathar

compiler/rustc_public/src/mir/body.rs

@@ -642,8 +642,7 @@ impl Rvalue {
                     .ok_or_else(|| error!("Expected a `RigidTy` but found: {place_ty:?}"))
             }
             Rvalue::NullaryOp(NullOp::OffsetOf(..), _) => Ok(Ty::usize_ty()),
-            Rvalue::NullaryOp(NullOp::ContractChecks, _)
-            | Rvalue::NullaryOp(NullOp::UbChecks, _) => Ok(Ty::bool_ty()),
+            Rvalue::NullaryOp(NullOp::RuntimeChecks(_), _) => Ok(Ty::bool_ty()),
             Rvalue::Aggregate(ak, ops) => match *ak {
                 AggregateKind::Array(ty) => Ty::try_new_array(ty, ops.len() as u64),
                 AggregateKind::Tuple => Ok(Ty::new_tuple(
@@ -1024,10 +1023,18 @@ pub enum CastKind {
 pub enum NullOp {
     /// Returns the offset of a field.
     OffsetOf(Vec<(VariantIdx, FieldIdx)>),
+    /// Codegen conditions for runtime checks.
+    RuntimeChecks(RuntimeChecks),
+}
+
+#[derive(Clone, Debug, Eq, PartialEq, Hash, Serialize)]
+pub enum RuntimeChecks {
     /// cfg!(ub_checks), but at codegen time
     UbChecks,
     /// cfg!(contract_checks), but at codegen time
     ContractChecks,
+    /// cfg!(overflow_checks), but at codegen time
+    OverflowChecks,
 }
 
 impl Operand {

compiler/rustc_public/src/unstable/convert/stable/mir.rs

@@ -322,12 +322,16 @@ impl<'tcx> Stable<'tcx> for mir::NullOp<'tcx> {
         cx: &CompilerCtxt<'cx, BridgeTys>,
     ) -> Self::T {
         use rustc_middle::mir::NullOp::*;
+        use rustc_middle::mir::RuntimeChecks::*;
         match self {
             OffsetOf(indices) => crate::mir::NullOp::OffsetOf(
                 indices.iter().map(|idx| idx.stable(tables, cx)).collect(),
             ),
-            UbChecks => crate::mir::NullOp::UbChecks,
-            ContractChecks => crate::mir::NullOp::ContractChecks,
+            RuntimeChecks(op) => crate::mir::NullOp::RuntimeChecks(match op {
+                UbChecks => crate::mir::RuntimeChecks::UbChecks,
+                ContractChecks => crate::mir::RuntimeChecks::ContractChecks,
+                OverflowChecks => crate::mir::RuntimeChecks::OverflowChecks,
+            }),
         }
     }
 }