diff --git a/src/eval.rs b/src/eval.rs
index 95c24f0171..511163a2e7 100644
--- a/src/eval.rs
+++ b/src/eval.rs
@@ -130,7 +130,7 @@ pub struct MiriConfig {
     /// Whether Stacked Borrows retagging should recurse into fields of datatypes.
     pub retag_fields: bool,
     /// The location of a shared object file to load when calling external functions
-    /// TODO! consider allowing users to specify paths to multiple SO files, or to a directory
+    /// FIXME! consider allowing users to specify paths to multiple SO files, or to a directory
     pub external_so_file: Option<PathBuf>,
 }
 
diff --git a/src/shims/ffi_support.rs b/src/shims/ffi_support.rs
index 3bc4940451..f1ae1e7d3f 100644
--- a/src/shims/ffi_support.rs
+++ b/src/shims/ffi_support.rs
@@ -149,7 +149,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                     },
                 _ => {}
             }
-            // TODO ellen! deal with all the other return types
+            // FIXME ellen! deal with all the other return types
             throw_unsup_format!("unsupported return type to external C function: {:?}", link_name);
         }
     }
@@ -245,6 +245,10 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
 
 #[derive(Debug, Clone)]
 /// Enum of supported arguments to external C functions.
+// We introduce this enum instead of just calling `ffi::arg` and storing a list
+// of `libffi::high::Arg` directly, because the `libffi::high::Arg` just wraps a reference
+// to the value it represents: https://docs.rs/libffi/latest/libffi/high/call/struct.Arg.html
+// and we need to store a copy of the value, and pass a reference to this copy to C instead.
 pub enum CArg {
     /// 8-bit signed integer.
     Int8(i8),