Rollup merge of #100239 - RalfJung:const-prop-uninit, r=oli-obk

remove an ineffective check in const_prop

Based on https://github.com/rust-lang/rust/pull/100043, only the last two commits are new.

ConstProp has a special check when reading from a local that prevents reading uninit locals. However, if that local flows into `force_allocation`, then no check fires and evaluation proceeds. So this check is not really effective at preventing accesses to uninit locals.

With https://github.com/rust-lang/rust/pull/100043, `read_immediate` and friends always fail when reading uninit locals, so I don't see why ConstProp would need a separate check. Thus I propose we remove it. This is needed to be able to do https://github.com/rust-lang/rust/pull/100085.

compiler/rustc_const_eval/src/interpret/eval_context.rs

@@ -187,9 +187,6 @@ pub enum LocalValue<Prov: Provenance = AllocId> {
 
 impl<'tcx, Prov: Provenance + 'static> LocalState<'tcx, Prov> {
     /// Read the local's value or error if the local is not yet live or not live anymore.
-    ///
-    /// Note: This may only be invoked from the `Machine::access_local` hook and not from
-    /// anywhere else. You may be invalidating machine invariants if you do!
     #[inline]
     pub fn access(&self) -> InterpResult<'tcx, &Operand<Prov>> {
         match &self.value {

compiler/rustc_const_eval/src/interpret/machine.rs

@@ -215,23 +215,12 @@ pub trait Machine<'mir, 'tcx>: Sized {
         right: &ImmTy<'tcx, Self::Provenance>,
     ) -> InterpResult<'tcx, (Scalar<Self::Provenance>, bool, Ty<'tcx>)>;
 
-    /// Called to read the specified `local` from the `frame`.
-    /// Since reading a ZST is not actually accessing memory or locals, this is never invoked
-    /// for ZST reads.
-    #[inline]
-    fn access_local<'a>(
-        frame: &'a Frame<'mir, 'tcx, Self::Provenance, Self::FrameExtra>,
-        local: mir::Local,
-    ) -> InterpResult<'tcx, &'a Operand<Self::Provenance>>
-    where
-        'tcx: 'mir,
-    {
-        frame.locals[local].access()
-    }
-
     /// Called to write the specified `local` from the `frame`.
     /// Since writing a ZST is not actually accessing memory or locals, this is never invoked
     /// for ZST reads.
+    ///
+    /// Due to borrow checker trouble, we indicate the `frame` as an index rather than an `&mut
+    /// Frame`.
     #[inline]
     fn access_local_mut<'a>(
         ecx: &'a mut InterpCx<'mir, 'tcx, Self>,

compiler/rustc_const_eval/src/interpret/operand.rs

@@ -444,7 +444,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         }
     }
 
-    /// Read from a local. Will not actually access the local if reading from a ZST.
+    /// Read from a local.
     /// Will not access memory, instead an indirect `Operand` is returned.
     ///
     /// This is public because it is used by [priroda](https://github.com/oli-obk/priroda) to get an
@@ -456,12 +456,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         layout: Option<TyAndLayout<'tcx>>,
     ) -> InterpResult<'tcx, OpTy<'tcx, M::Provenance>> {
         let layout = self.layout_of_local(frame, local, layout)?;
-        let op = if layout.is_zst() {
-            // Bypass `access_local` (helps in ConstProp)
-            Operand::Immediate(Immediate::Uninit)
-        } else {
-            *M::access_local(frame, local)?
-        };
+        let op = *frame.locals[local].access()?;
         Ok(OpTy { op, layout, align: Some(layout.align.abi) })
     }
 

compiler/rustc_const_eval/src/interpret/place.rs

@@ -642,7 +642,7 @@ where
         // avoid force_allocation.
         let src = match self.read_immediate_raw(src)? {
             Ok(src_val) => {
-                assert!(!src.layout.is_unsized(), "cannot have unsized immediates");
+                assert!(!src.layout.is_unsized(), "cannot copy unsized immediates");
                 assert!(
                     !dest.layout.is_unsized(),
                     "the src is sized, so the dest must also be sized"

compiler/rustc_const_eval/src/interpret/projection.rs

@@ -100,6 +100,8 @@ where
         // This makes several assumptions about what layouts we will encounter; we match what
         // codegen does as good as we can (see `extract_field` in `rustc_codegen_ssa/src/mir/operand.rs`).
         let field_val: Immediate<_> = match (*base, base.layout.abi) {
+            // if the entire value is uninit, then so is the field (can happen in ConstProp)
+            (Immediate::Uninit, _) => Immediate::Uninit,
             // the field contains no information, can be left uninit
             _ if field_layout.is_zst() => Immediate::Uninit,
             // the field covers the entire type
@@ -124,6 +126,7 @@ where
                     b_val
                 })
             }
+            // everything else is a bug
             _ => span_bug!(
                 self.cur_span(),
                 "invalid field access on immediate {}, layout {:#?}",