Add more documentation

Author: oli-obk

src/librustc_mir/const_eval.rs

@@ -16,7 +16,7 @@ use rustc::ty::layout::{self, LayoutOf, VariantIdx};
 use rustc::ty::subst::Subst;
 use rustc::traits::Reveal;
 use rustc_data_structures::fx::FxHashMap;
-use crate::interpret::eval_nulary_intrinsic;
+use crate::interpret::eval_nullary_intrinsic;
 
 use syntax::source_map::{Span, DUMMY_SP};
 
@@ -607,13 +607,15 @@ pub fn const_eval_provider<'tcx>(
         }
     }
 
+    // We call `const_eval` for zero arg intrinsics, too, in order to cache their value.
+    // Catch such calls and evaluate them instead of trying to load a constant's MIR.
     if let ty::InstanceDef::Intrinsic(def_id) = key.value.instance.def {
         let ty = key.value.instance.ty(tcx);
         let substs = match ty.sty {
             ty::FnDef(_, substs) => substs,
             _ => bug!("intrinsic with type {:?}", ty),
         };
-        return Ok(eval_nulary_intrinsic(tcx, key.param_env, def_id, substs));
+        return Ok(eval_nullary_intrinsic(tcx, key.param_env, def_id, substs));
     }
 
     tcx.const_eval_raw(key).and_then(|val| {

src/librustc_mir/interpret/intrinsics.rs

@@ -38,8 +38,9 @@ fn numeric_intrinsic<'tcx, Tag>(
     Ok(Scalar::from_uint(bits_out, size))
 }
 
-
-crate fn eval_nulary_intrinsic<'tcx>(
+/// The logic for all nullary intrinsics is implemented here. These intrinsics don't get evaluated
+/// inside an `InterpCx` and instead have their value computed directly from rustc internal info.
+crate fn eval_nullary_intrinsic<'tcx>(
     tcx: TyCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     def_id: DefId,

src/librustc_mir/interpret/intrinsics/type_name.rs

@@ -214,7 +214,7 @@ impl Write for AbsolutePathPrinter<'_> {
 }
 
 /// Directly returns an `Allocation` containing an absolute path representation of the given type.
-pub(crate) fn alloc_type_name<'tcx>(
+crate fn alloc_type_name<'tcx>(
     tcx: TyCtxt<'tcx>,
     ty: Ty<'tcx>
 ) -> &'tcx Allocation {

src/librustc_mir/interpret/mod.rs

@@ -36,4 +36,4 @@ pub use self::validity::RefTracking;
 
 pub use self::intern::intern_const_alloc_recursive;
 
-pub(crate) use self::intrinsics::eval_nulary_intrinsic;
+crate use self::intrinsics::eval_nullary_intrinsic;