Fix borrow error on pre-NLL compilers

    error[E0506]: cannot assign to `missing_content` because it is borrowed
        --> serde_derive/src/de.rs:1414:9
         |
    1388 |           .filter_map(|(i, variant)| {
         |                       -------------- borrow of `missing_content` occurs here
    ...
    1414 | /         missing_content = quote! {
    1415 | |             match __field {
    1416 | |                 #(#missing_content_arms)*
    1417 | |                 #missing_content_fallthrough
    1418 | |             }
    1419 | |         };
         | |_________^ assignment to borrowed `missing_content` occurs here

    error[E0502]: cannot borrow `missing_content_fallthrough` as immutable because it is also borrowed as mutable
        --> serde_derive/src/de.rs:1414:27
         |
    1388 |           .filter_map(|(i, variant)| {
         |                       -------------- mutable borrow occurs here
    ...
    1404 |                       missing_content_fallthrough = quote!(_ => #missing_content);
         |                       --------------------------- previous borrow occurs due to use of `missing_content_fallthrough` in closure
    ...
    1414 |           missing_content = quote! {
         |  ___________________________^
    1415 | |             match __field {
    1416 | |                 #(#missing_content_arms)*
    1417 | |                 #missing_content_fallthrough
    1418 | |             }
    1419 | |         };
         | |_________^ immutable borrow occurs here
    ...
    1622 |   }
         |   - mutable borrow ends here
         |
         = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

serde_derive/src/de.rs

@@ -1381,7 +1381,7 @@ fn deserialize_adjacently_tagged_enum(
         _serde::export::Err(<__A::Error as _serde::de::Error>::missing_field(#content))
     };
     let mut missing_content_fallthrough = quote!();
-    let mut missing_content_arms = variants
+    let missing_content_arms = variants
         .iter()
         .enumerate()
         .filter(|&(_, variant)| !variant.attrs.skip_deserializing())
@@ -1409,8 +1409,8 @@ fn deserialize_adjacently_tagged_enum(
                 __Field::#variant_index => #arm,
             })
         })
-        .peekable();
-    if missing_content_arms.peek().is_some() {
+        .collect::<Vec<_>>();
+    if !missing_content_arms.is_empty() {
         missing_content = quote! {
             match __field {
                 #(#missing_content_arms)*