Fix/add doc comments and change FIXME

Author: skinnyBat

src/librustc/mir/interpret/mod.rs

@@ -149,7 +149,7 @@ pub struct GlobalId<'tcx> {
     pub promoted: Option<mir::Promoted>,
 }
 
-/// The input type for const eval queries. Const eval queries a given both the `ParamEnv` in which
+/// The input type for const eval queries. Const eval queries are given both the `ParamEnv` in which
 /// the constant is evaluated in and the identifier of the constant.
 pub type ConstEvalInput<'tcx> = Canonical<'tcx, ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>>;
 

src/librustc/ty/sty.rs

@@ -2396,8 +2396,9 @@ impl<'tcx> Const<'tcx> {
                 // HACK(eddyb) when substs contain e.g. inference variables,
                 // attempt using identity substs instead, that will succeed
                 // when the expression doesn't depend on any parameters.
-                // FIXME(eddyb) make `const_eval` a canonical query instead,
-                // that would properly handle inference variables in `substs`.
+                // FIXME(skinny121) to handle inference variables correctly, the inference context
+                // needs to be passed into here so that the inference variables can be used within
+                // the `const_eval` canonical query.
                 if substs.has_local_value() {
                     let identity_substs = InternalSubsts::identity_for_item(tcx, did);
                     // The `ParamEnv` needs to match the `identity_substs`.

src/librustc_mir/interpret/eval_context.rs

@@ -34,6 +34,9 @@ pub struct InterpCx<'infcx, 'mir, 'tcx, M: Machine<'mir, 'tcx>> {
     /// The results of the type checker, from rustc.
     pub tcx: TyCtxtAt<'tcx>,
 
+    /// The inference context for inference variables that are within this `InterpCx`. Inference
+    /// variables may be within either the `param_env` or within `substs` of frames. Many operations
+    /// outside of mir which takes values that may contain variable require this context.
     pub(super) infcx: &'infcx InferCtxt<'infcx, 'tcx>,
 
     /// Bounds in scope for polymorphic evaluations.