From b638e7e5c004c4484354c2d45048f9a0410f0f54 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Cl=C3=A9ment=20DOUIN?= <clement.douin@posteo.net>
Date: Wed, 30 Sep 2026 16:05:45 +0200
Subject: [PATCH] feat: add quirk_body_fld_param_value_nil

Refs: pimalaya/himalaya#649
---
 imap-codec/Cargo.toml  |  3 ++
 imap-codec/src/body.rs | 64 ++++++++++++++++++++++++++++++++++++++++--
 justfile               |  3 +-
 3 files changed, 67 insertions(+), 3 deletions(-)

diff --git a/imap-codec/Cargo.toml b/imap-codec/Cargo.toml
index c887551e..188302b4 100644
--- a/imap-codec/Cargo.toml
+++ b/imap-codec/Cargo.toml
@@ -35,6 +35,7 @@ quirk = [
     "quirk_spaces_between_addresses",
     "quirk_empty_continue_req",
     "quirk_body_fld_enc_nil_to_empty",
+    "quirk_body_fld_param_value_nil",
     "quirk_always_normalize_sequence_sets",
 ]
 # Make `\r` in `\r\n` optional.
@@ -61,6 +62,8 @@ quirk_trailing_space_search = []
 quirk_empty_continue_req = []
 # Encode NIL `body-fld-enc` as empty string.
 quirk_body_fld_enc_nil_to_empty = []
+# Encode NIL `body-fld-param` value as empty string.
+quirk_body_fld_param_value_nil = []
 # Always normalize sequence sets during encoding
 quirk_always_normalize_sequence_sets = []
 
diff --git a/imap-codec/src/body.rs b/imap-codec/src/body.rs
index b78f51cc..7a217a05 100644
--- a/imap-codec/src/body.rs
+++ b/imap-codec/src/body.rs
@@ -217,13 +217,25 @@ pub(crate) fn body_fields(input: &[u8]) -> IMAPResult<&[u8], BasicFields> {
 ///                  ")" / nil
 /// ```
 pub(crate) fn body_fld_param(input: &[u8]) -> IMAPResult<&[u8], Vec<(IString, IString)>> {
+    #[cfg(not(feature = "quirk_body_fld_param_value_nil"))]
+    let value = string;
+
+    #[cfg(feature = "quirk_body_fld_param_value_nil")]
+    let value = alt((
+        string,
+        map(nil, |_| {
+            log::warn!("Rectified NIL body-fld-param value to empty string");
+            IString::try_from("").unwrap()
+        }),
+    ));
+
     let mut parser = alt((
         delimited(
             tag(b"("),
             // Quirk: See https://github.com/emersion/go-imap/issues/557
             separated_list0(
                 sp,
-                map(tuple((string, sp, string)), |(key, _, value)| (key, value)),
+                map(tuple((string, sp, value)), |(key, _, value)| (key, value)),
             ),
             tag(b")"),
         ),
@@ -594,7 +606,11 @@ mod tests {
     };
 
     use super::*;
-    use crate::testing::{kat_inverse_response, known_answer_test_encode};
+    use crate::{
+        ResponseCodec,
+        decode::Decoder,
+        testing::{kat_inverse_response, known_answer_test_encode},
+    };
 
     #[test]
     fn test_parse_media_basic() {
@@ -790,4 +806,48 @@ mod tests {
             assert_eq!(body_fld_octets(b"-999999)").unwrap().1, 0);
         }
     }
+
+    #[test]
+    fn test_body_fld_param_nil_quirk() {
+        let input = b"(\"boundary\" NIL \"charset\" \"utf-8\"))";
+
+        #[cfg(not(feature = "quirk_body_fld_param_value_nil"))]
+        assert!(body_fld_param(input).is_err());
+
+        #[cfg(feature = "quirk_body_fld_param_value_nil")]
+        assert_eq!(
+            body_fld_param(input).unwrap(),
+            (
+                &b")"[..],
+                vec![
+                    (
+                        IString::try_from("boundary").unwrap(),
+                        IString::try_from("").unwrap(),
+                    ),
+                    (
+                        IString::try_from("charset").unwrap(),
+                        IString::try_from("utf-8").unwrap(),
+                    ),
+                ]
+            )
+        );
+    }
+
+    /// Reproducer from a Mail.ru `FETCH` response.
+    #[test]
+    fn test_body_fld_param_nil_quirk_mailru_fetch() {
+        let input = b"* 2010 FETCH (UID 154395 FLAGS (\\Seen) ENVELOPE (\"Fri, 17 Apr 2026 14:15:28 +0000\" \"subject\" ((\"name\" NIL \"local\" \"example.org\")) NIL NIL ((\"\" NIL \"local\" \"example.org\")) NIL NIL NIL \"<id@example.org>\") BODYSTRUCTURE ((\"text\" \"plain\" (\"charset\" \"US-ASCII\") NIL NIL \"8bit\" 7078 0 NIL NIL NIL NIL)(\"text\" \"html\" (\"charset\" \"utf-8\") NIL NIL \"quoted-printable\" 37865 0 NIL NIL NIL NIL) \"alternative\" (\"boundary\" NIL)))\r\n";
+
+        let got = ResponseCodec::default().decode(input);
+
+        #[cfg(not(feature = "quirk_body_fld_param_value_nil"))]
+        assert!(got.is_err());
+
+        #[cfg(feature = "quirk_body_fld_param_value_nil")]
+        {
+            let (remaining, response) = got.unwrap();
+            assert!(remaining.is_empty());
+            assert!(matches!(response, Response::Data(Data::Fetch { .. })));
+        }
+    }
 }
diff --git a/justfile b/justfile
index 9e7ae902..6d2a5620 100644
--- a/justfile
+++ b/justfile
@@ -80,7 +80,8 @@ cargo_hack mode: install_cargo_hack
         quirk_trailing_space_status,\
         quirk_spaces_between_addresses,\
         quirk_empty_continue_req,\
-        quirk_body_fld_enc_nil_to_empty\
+        quirk_body_fld_enc_nil_to_empty,\
+        quirk_body_fld_param_value_nil\
         {{ mode }}
     cargo hack check -p imap-types \
         --no-dev-deps \