Auto merge of #148766 - cjgillot:mir-const-runtime-checks, r=RalfJung,saethlin

Replace Rvalue::NullaryOp by a variant in mir::Operand.

Based on rust-lang/rust#148151

This PR fully removes the MIR `Rvalue::NullaryOp`. After rust-lang/rust#148151, it was only useful for runtime checks like `ub_checks`, `contract_checks` and `overflow_checks`.

These are "runtime" checks, boolean constants that may only be `true` in codegen. It depends on a rustc flag passed to codegen, so we need to represent those flags cross-crate.

This PR replaces those runtime checks by special variants in MIR `ConstValue`. This allows code that expects constants to manipulate those as such, even if we may not always be able to evaluate them to actual scalars.

Author: bors

clippy_utils/src/mir/possible_borrower.rs

@@ -110,7 +110,7 @@ impl<'tcx> mir::visit::Visitor<'tcx> for PossibleBorrowerVisitor<'_, '_, 'tcx> {
                             immutable_borrowers.push(p.local);
                         }
                     },
-                    mir::Operand::Constant(..) => (),
+                    mir::Operand::Constant(..) | mir::Operand::RuntimeChecks(..) => (),
                 }
             }
 
@@ -151,7 +151,7 @@ fn rvalue_locals(rvalue: &mir::Rvalue<'_>, mut visit: impl FnMut(mir::Local)) {
 
     let mut visit_op = |op: &mir::Operand<'_>| match op {
         mir::Operand::Copy(p) | mir::Operand::Move(p) => visit(p.local),
-        mir::Operand::Constant(..) => (),
+        mir::Operand::Constant(..) | mir::Operand::RuntimeChecks(..) => (),
     };
 
     match rvalue {

clippy_utils/src/qualify_min_const_fn.rs

@@ -13,7 +13,7 @@ use rustc_infer::infer::TyCtxtInferExt;
 use rustc_infer::traits::Obligation;
 use rustc_lint::LateContext;
 use rustc_middle::mir::{
-    Body, CastKind, NonDivergingIntrinsic, NullOp, Operand, Place, ProjectionElem, Rvalue, Statement, StatementKind,
+    Body, CastKind, NonDivergingIntrinsic, Operand, Place, ProjectionElem, Rvalue, Statement, StatementKind,
     Terminator, TerminatorKind,
 };
 use rustc_middle::traits::{BuiltinImplSource, ImplSource, ObligationCause};
@@ -194,7 +194,7 @@ fn check_rvalue<'tcx>(
                 ))
             }
         },
-        Rvalue::NullaryOp(NullOp::RuntimeChecks(_)) | Rvalue::ShallowInitBox(_, _) => Ok(()),
+        Rvalue::ShallowInitBox(_, _) => Ok(()),
         Rvalue::UnaryOp(_, operand) => {
             let ty = operand.ty(body, cx.tcx);
             if ty.is_integral() || ty.is_bool() {
@@ -277,6 +277,7 @@ fn check_operand<'tcx>(
             Some(_) => Err((span, "cannot access `static` items in const fn".into())),
             None => Ok(()),
         },
+        Operand::RuntimeChecks(..) => Ok(()),
     }
 }