Add `tag_for_variant` query

This query allows for sharing code between `rustc_const_eval` and
`rustc_transmutability`.

Also moves `DummyMachine` to `rustc_const_eval`.

3 files changed, 214 insertions, 1 deletions

diff --git a/compiler/rustc_const_eval/src/const_eval/dummy_machine.rs b/compiler/rustc_const_eval/src/const_eval/dummy_machine.rs
new file mode 100644
index 00000000000..ba2e2a1e353
--- /dev/null
+++ b/compiler/rustc_const_eval/src/const_eval/dummy_machine.rs
@@ -0,0 +1,193 @@
+use crate::interpret::{self, HasStaticRootDefId, ImmTy, Immediate, InterpCx, PointerArithmetic};
+use rustc_middle::mir::interpret::{AllocId, ConstAllocation, InterpResult};
+use rustc_middle::mir::*;
+use rustc_middle::query::TyCtxtAt;
+use rustc_middle::ty;
+use rustc_middle::ty::layout::TyAndLayout;
+use rustc_span::def_id::DefId;
+
+/// Macro for machine-specific `InterpError` without allocation.
+/// (These will never be shown to the user, but they help diagnose ICEs.)
+pub macro throw_machine_stop_str($($tt:tt)*) {{
+    // We make a new local type for it. The type itself does not carry any information,
+    // but its vtable (for the `MachineStopType` trait) does.
+    #[derive(Debug)]
+    struct Zst;
+    // Printing this type shows the desired string.
+    impl std::fmt::Display for Zst {
+        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+            write!(f, $($tt)*)
+        }
+    }
+
+    impl rustc_middle::mir::interpret::MachineStopType for Zst {
+        fn diagnostic_message(&self) -> rustc_errors::DiagMessage {
+            self.to_string().into()
+        }
+
+        fn add_args(
+            self: Box<Self>,
+            _: &mut dyn FnMut(rustc_errors::DiagArgName, rustc_errors::DiagArgValue),
+        ) {}
+    }
+    throw_machine_stop!(Zst)
+}}
+
+pub struct DummyMachine;
+
+impl HasStaticRootDefId for DummyMachine {
+    fn static_def_id(&self) -> Option<rustc_hir::def_id::LocalDefId> {
+        None
+    }
+}
+
+impl<'mir, 'tcx: 'mir> interpret::Machine<'mir, 'tcx> for DummyMachine {
+    interpret::compile_time_machine!(<'mir, 'tcx>);
+    type MemoryKind = !;
+    const PANIC_ON_ALLOC_FAIL: bool = true;
+
+    #[inline(always)]
+    fn enforce_alignment(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
+        false // no reason to enforce alignment
+    }
+
+    fn enforce_validity(_ecx: &InterpCx<'mir, 'tcx, Self>, _layout: TyAndLayout<'tcx>) -> bool {
+        false
+    }
+
+    fn before_access_global(
+        _tcx: TyCtxtAt<'tcx>,
+        _machine: &Self,
+        _alloc_id: AllocId,
+        alloc: ConstAllocation<'tcx>,
+        _static_def_id: Option<DefId>,
+        is_write: bool,
+    ) -> InterpResult<'tcx> {
+        if is_write {
+            throw_machine_stop_str!("can't write to global");
+        }
+
+        // If the static allocation is mutable, then we can't const prop it as its content
+        // might be different at runtime.
+        if alloc.inner().mutability.is_mut() {
+            throw_machine_stop_str!("can't access mutable globals in ConstProp");
+        }
+
+        Ok(())
+    }
+
+    fn find_mir_or_eval_fn(
+        _ecx: &mut InterpCx<'mir, 'tcx, Self>,
+        _instance: ty::Instance<'tcx>,
+        _abi: rustc_target::spec::abi::Abi,
+        _args: &[interpret::FnArg<'tcx, Self::Provenance>],
+        _destination: &interpret::MPlaceTy<'tcx, Self::Provenance>,
+        _target: Option<BasicBlock>,
+        _unwind: UnwindAction,
+    ) -> interpret::InterpResult<'tcx, Option<(&'mir Body<'tcx>, ty::Instance<'tcx>)>> {
+        unimplemented!()
+    }
+
+    fn panic_nounwind(
+        _ecx: &mut InterpCx<'mir, 'tcx, Self>,
+        _msg: &str,
+    ) -> interpret::InterpResult<'tcx> {
+        unimplemented!()
+    }
+
+    fn call_intrinsic(
+        _ecx: &mut InterpCx<'mir, 'tcx, Self>,
+        _instance: ty::Instance<'tcx>,
+        _args: &[interpret::OpTy<'tcx, Self::Provenance>],
+        _destination: &interpret::MPlaceTy<'tcx, Self::Provenance>,
+        _target: Option<BasicBlock>,
+        _unwind: UnwindAction,
+    ) -> interpret::InterpResult<'tcx> {
+        unimplemented!()
+    }
+
+    fn assert_panic(
+        _ecx: &mut InterpCx<'mir, 'tcx, Self>,
+        _msg: &rustc_middle::mir::AssertMessage<'tcx>,
+        _unwind: UnwindAction,
+    ) -> interpret::InterpResult<'tcx> {
+        unimplemented!()
+    }
+
+    fn binary_ptr_op(
+        ecx: &InterpCx<'mir, 'tcx, Self>,
+        bin_op: BinOp,
+        left: &interpret::ImmTy<'tcx, Self::Provenance>,
+        right: &interpret::ImmTy<'tcx, Self::Provenance>,
+    ) -> interpret::InterpResult<'tcx, (ImmTy<'tcx, Self::Provenance>, bool)> {
+        use rustc_middle::mir::BinOp::*;
+        Ok(match bin_op {
+            Eq | Ne | Lt | Le | Gt | Ge => {
+                // Types can differ, e.g. fn ptrs with different `for`.
+                assert_eq!(left.layout.abi, right.layout.abi);
+                let size = ecx.pointer_size();
+                // Just compare the bits. ScalarPairs are compared lexicographically.
+                // We thus always compare pairs and simply fill scalars up with 0.
+                // If the pointer has provenance, `to_bits` will return `Err` and we bail out.
+                let left = match **left {
+                    Immediate::Scalar(l) => (l.to_bits(size)?, 0),
+                    Immediate::ScalarPair(l1, l2) => (l1.to_bits(size)?, l2.to_bits(size)?),
+                    Immediate::Uninit => panic!("we should never see uninit data here"),
+                };
+                let right = match **right {
+                    Immediate::Scalar(r) => (r.to_bits(size)?, 0),
+                    Immediate::ScalarPair(r1, r2) => (r1.to_bits(size)?, r2.to_bits(size)?),
+                    Immediate::Uninit => panic!("we should never see uninit data here"),
+                };
+                let res = match bin_op {
+                    Eq => left == right,
+                    Ne => left != right,
+                    Lt => left < right,
+                    Le => left <= right,
+                    Gt => left > right,
+                    Ge => left >= right,
+                    _ => bug!(),
+                };
+                (ImmTy::from_bool(res, *ecx.tcx), false)
+            }
+
+            // Some more operations are possible with atomics.
+            // The return value always has the provenance of the *left* operand.
+            Add | Sub | BitOr | BitAnd | BitXor => {
+                throw_machine_stop_str!("pointer arithmetic is not handled")
+            }
+
+            _ => span_bug!(ecx.cur_span(), "Invalid operator on pointers: {:?}", bin_op),
+        })
+    }
+
+    fn expose_ptr(
+        _ecx: &mut InterpCx<'mir, 'tcx, Self>,
+        _ptr: interpret::Pointer<Self::Provenance>,
+    ) -> interpret::InterpResult<'tcx> {
+        unimplemented!()
+    }
+
+    fn init_frame_extra(
+        _ecx: &mut InterpCx<'mir, 'tcx, Self>,
+        _frame: interpret::Frame<'mir, 'tcx, Self::Provenance>,
+    ) -> interpret::InterpResult<
+        'tcx,
+        interpret::Frame<'mir, 'tcx, Self::Provenance, Self::FrameExtra>,
+    > {
+        unimplemented!()
+    }
+
+    fn stack<'a>(
+        _ecx: &'a InterpCx<'mir, 'tcx, Self>,
+    ) -> &'a [interpret::Frame<'mir, 'tcx, Self::Provenance, Self::FrameExtra>] {
+        // Return an empty stack instead of panicking, as `cur_span` uses it to evaluate constants.
+        &[]
+    }
+
+    fn stack_mut<'a>(
+        _ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
+    ) -> &'a mut Vec<interpret::Frame<'mir, 'tcx, Self::Provenance, Self::FrameExtra>> {
+        unimplemented!()
+    }
+}
diff --git a/compiler/rustc_const_eval/src/const_eval/eval_queries.rs b/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
index 5a1c7cc4209..16bd0296247 100644
--- a/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
+++ b/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
@@ -3,7 +3,7 @@ use either::{Left, Right};
 use rustc_hir::def::DefKind;
 use rustc_middle::mir::interpret::{AllocId, ErrorHandled, InterpErrorInfo};
 use rustc_middle::mir::{self, ConstAlloc, ConstValue};
-use rustc_middle::query::TyCtxtAt;
+use rustc_middle::query::{Key, TyCtxtAt};
 use rustc_middle::traits::Reveal;
 use rustc_middle::ty::layout::LayoutOf;
 use rustc_middle::ty::print::with_no_trimmed_paths;
@@ -243,6 +243,24 @@ pub(crate) fn turn_into_const_value<'tcx>(
     op_to_const(&ecx, &mplace.into(), /* for diagnostics */ false)
 }
 
+/// Computes the tag (if any) for a given type and variant.
+#[instrument(skip(tcx), level = "debug")]
+pub fn tag_for_variant_provider<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    (ty, variant_index): (Ty<'tcx>, abi::VariantIdx),
+) -> Option<ty::ScalarInt> {
+    assert!(ty.is_enum());
+
+    let ecx = InterpCx::new(
+        tcx,
+        ty.default_span(tcx),
+        ty::ParamEnv::reveal_all(),
+        crate::const_eval::DummyMachine,
+    );
+
+    ecx.tag_for_variant(ty, variant_index).unwrap().map(|(tag, _tag_field)| tag)
+}
+
 #[instrument(skip(tcx), level = "debug")]
 pub fn eval_to_const_value_raw_provider<'tcx>(
     tcx: TyCtxt<'tcx>,
diff --git a/compiler/rustc_const_eval/src/const_eval/mod.rs b/compiler/rustc_const_eval/src/const_eval/mod.rs
index d0d6adbfad0..b768c429070 100644
--- a/compiler/rustc_const_eval/src/const_eval/mod.rs
+++ b/compiler/rustc_const_eval/src/const_eval/mod.rs
@@ -7,12 +7,14 @@ use rustc_middle::ty::{self, Ty};
 
 use crate::interpret::format_interp_error;
 
+mod dummy_machine;
 mod error;
 mod eval_queries;
 mod fn_queries;
 mod machine;
 mod valtrees;
 
+pub use dummy_machine::*;
 pub use error::*;
 pub use eval_queries::*;
 pub use fn_queries::*;