`slice::from_raw_parts` returns a different address in const context

LegionMammal978 · December 10, 2022, 4:29pm

Hmm, it looks like this is a particular optimization that the compiler uses to treat &str and &[u8] as immediate values.

github.com

rust-lang/rust/blob/1.65.0/compiler/rustc_const_eval/src/const_eval/eval_queries.rs#L118-L136


      
          // We do not have value optimizations for everything.
          // Only scalars and slices, since they are very common.
          // Note that further down we turn scalars of uninitialized bits back to `ByRef`. These can result
          // from scalar unions that are initialized with one of their zero sized variants. We could
          // instead allow `ConstValue::Scalar` to store `ScalarMaybeUninit`, but that would affect all
          // the usual cases of extracting e.g. a `usize`, without there being a real use case for the
          // `Undef` situation.
          let try_as_immediate = match op.layout.abi {
              Abi::Scalar(abi::Scalar::Initialized { .. }) => true,
              Abi::ScalarPair(..) => match op.layout.ty.kind() {
                  ty::Ref(_, inner, _) => match *inner.kind() {
                      ty::Slice(elem) => elem == ecx.tcx.types.u8,
                      ty::Str => true,
                      _ => false,
                  },
                  _ => false,
              },
              _ => false,
          };

So the &[u8] is treated as a by-value empty slice, which gets converted into a pointer into (presumably) the area where all the strings end up.

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`slice::from_raw_parts` returns a different address in const context

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