Added some missing conditionals for feature compilation

serde/src/private/de.rs

@@ -2158,6 +2158,7 @@ pub struct FlatMapAccess<'a, 'de: 'a, E> {
     _marker: PhantomData<E>,
 }
 
+#[cfg(any(feature = "std", feature = "alloc"))]
 impl<'a, 'de, E> FlatMapAccess<'a, 'de, E> {
     fn new(
         iter: slice::IterMut<'a, Option<(String, Content<'de>)>>,

serde/src/private/ser.rs

@@ -1283,6 +1283,7 @@ pub struct FlatMapSerializeStructVariantAsMapValue<'a, M: 'a> {
     fields: Vec<(&'static str, Content)>,
 }
 
+#[cfg(any(feature = "std", feature = "alloc"))]
 impl<'a, M> FlatMapSerializeStructVariantAsMapValue<'a, M>
     where M: SerializeMap + 'a
 {