Rollup merge of rust-lang#129309 - RalfJung:CompileTimeInterpCx, r=compiler-errors

ctfe: make CompileTimeInterpCx type alias public

`CompileTimeMachine` is already public so there is no good reason to not also make this public.

Also add comment explaining why `CompileTimeMachine` is public.

Author: matthiaskrgr

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -40,7 +40,10 @@ const TINY_LINT_TERMINATOR_LIMIT: usize = 20;
 /// power of two of interpreted terminators.
 const PROGRESS_INDICATOR_START: usize = 4_000_000;
 
-/// Extra machine state for CTFE, and the Machine instance
+/// Extra machine state for CTFE, and the Machine instance.
+//
+// Should be public because out-of-tree rustc consumers need this
+// if they want to interact with constant values.
 pub struct CompileTimeMachine<'tcx> {
     /// The number of terminators that have been evaluated.
     ///
@@ -160,7 +163,7 @@ impl<K: Hash + Eq, V> interpret::AllocMap<K, V> for FxIndexMap<K, V> {
     }
 }
 
-pub(crate) type CompileTimeInterpCx<'tcx> = InterpCx<'tcx, CompileTimeMachine<'tcx>>;
+pub type CompileTimeInterpCx<'tcx> = InterpCx<'tcx, CompileTimeMachine<'tcx>>;
 
 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 pub enum MemoryKind {