add comments

compiler/rustc_const_eval/src/const_eval/valtrees.rs

@@ -272,7 +272,8 @@ pub(crate) fn eval_to_valtree<'tcx>(
 
 /// Converts a `ValTree` to a `ConstValue`, which is needed after mir
 /// construction has finished.
-// FIXME Merge `valtree_to_const_value` and `valtree_into_mplace` into one function
+// FIXME(valtrees): Merge `valtree_to_const_value` and `valtree_into_mplace` into one function
+// FIXME(valtrees): Accept `ty::Value` instead of `Ty` and `ty::ValTree` separately.
 #[instrument(skip(tcx), level = "debug", ret)]
 pub fn valtree_to_const_value<'tcx>(
     tcx: TyCtxt<'tcx>,

compiler/rustc_middle/src/ty/consts.rs

@@ -222,6 +222,8 @@ impl<'tcx> Const<'tcx> {
     }
 
     /// Attempts to convert to a value.
+    ///
+    /// Note that this does not evaluate the constant.
     pub fn try_to_value(self) -> Option<ty::Value<'tcx>> {
         match self.kind() {
             ty::ConstKind::Value(cv) => Some(cv),
@@ -229,6 +231,10 @@ impl<'tcx> Const<'tcx> {
         }
     }
 
+    /// Convenience method to extract the value of a usize constant,
+    /// useful to get the length of an array type.
+    ///
+    /// Note that this does not evaluate the constant.
     #[inline]
     pub fn try_to_target_usize(self, tcx: TyCtxt<'tcx>) -> Option<u64> {
         self.try_to_value()?.try_to_target_usize(tcx)

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -1785,6 +1785,7 @@ pub trait PrettyPrinter<'tcx>: Printer<'tcx> + fmt::Write {
         Ok(())
     }
 
+    // FIXME(valtrees): Accept `ty::Value` instead of `Ty` and `ty::ValTree` separately.
     fn pretty_print_const_valtree(
         &mut self,
         valtree: ty::ValTree<'tcx>,

compiler/rustc_mir_build/src/thir/pattern/const_to_pat.rs

@@ -214,6 +214,7 @@ impl<'tcx> ConstToPat<'tcx> {
     }
 
     // Recursive helper for `to_pat`; invoke that (instead of calling this directly).
+    // FIXME(valtrees): Accept `ty::Value` instead of `Ty` and `ty::ValTree` separately.
     #[instrument(skip(self), level = "debug")]
     fn valtree_to_pat(&self, cv: ValTree<'tcx>, ty: Ty<'tcx>) -> Box<Pat<'tcx>> {
         let span = self.span;