Allow to define custom expectation message for type with #[serde(expecting = "...")]

Closes #1883

serde/src/private/de.rs

@@ -824,15 +824,17 @@ mod content {
     /// Not public API.
     pub struct TaggedContentVisitor<'de, T> {
         tag_name: &'static str,
+        expecting: &'static str,
         value: PhantomData<TaggedContent<'de, T>>,
     }
 
     impl<'de, T> TaggedContentVisitor<'de, T> {
         /// Visitor for the content of an internally tagged enum with the given
         /// tag name.
-        pub fn new(name: &'static str) -> Self {
+        pub fn new(name: &'static str, expecting: &'static str) -> Self {
             TaggedContentVisitor {
                 tag_name: name,
+                expecting: expecting,
                 value: PhantomData,
             }
         }
@@ -861,7 +863,7 @@ mod content {
         type Value = TaggedContent<'de, T>;
 
         fn expecting(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
-            fmt.write_str("internally tagged enum")
+            fmt.write_str(self.expecting)
         }
 
         fn visit_seq<S>(self, mut seq: S) -> Result<Self::Value, S::Error>

serde_derive/src/de.rs

@@ -399,6 +399,7 @@ fn deserialize_unit_struct(params: &Parameters, cattrs: &attr::Container) -> Fra
     let type_name = cattrs.name().deserialize_name();
 
     let expecting = format!("unit struct {}", params.type_name());
+    let expecting = cattrs.expecting().unwrap_or(&expecting);
 
     quote_block! {
         struct __Visitor;
@@ -456,6 +457,7 @@ fn deserialize_tuple(
         Some(variant_ident) => format!("tuple variant {}::{}", params.type_name(), variant_ident),
         None => format!("tuple struct {}", params.type_name()),
     };
+    let expecting = cattrs.expecting().unwrap_or(&expecting);
 
     let nfields = fields.len();
 
@@ -542,6 +544,7 @@ fn deserialize_tuple_in_place(
         Some(variant_ident) => format!("tuple variant {}::{}", params.type_name(), variant_ident),
         None => format!("tuple struct {}", params.type_name()),
     };
+    let expecting = cattrs.expecting().unwrap_or(&expecting);
 
     let nfields = fields.len();
 
@@ -630,6 +633,7 @@ fn deserialize_seq(
     } else {
         format!("{} with {} elements", expecting, deserialized_count)
     };
+    let expecting = cattrs.expecting().unwrap_or(&expecting);
 
     let mut index_in_seq = 0_usize;
     let let_values = vars.clone().zip(fields).map(|(var, field)| {
@@ -732,6 +736,7 @@ fn deserialize_seq_in_place(
     } else {
         format!("{} with {} elements", expecting, deserialized_count)
     };
+    let expecting = cattrs.expecting().unwrap_or(&expecting);
 
     let mut index_in_seq = 0usize;
     let write_values = fields.iter().map(|field| {
@@ -907,6 +912,7 @@ fn deserialize_struct(
         Some(variant_ident) => format!("struct variant {}::{}", params.type_name(), variant_ident),
         None => format!("struct {}", params.type_name()),
     };
+    let expecting = cattrs.expecting().unwrap_or(&expecting);
 
     let visit_seq = Stmts(deserialize_seq(
         &type_path, params, fields, true, cattrs, &expecting,
@@ -1048,6 +1054,7 @@ fn deserialize_struct_in_place(
         Some(variant_ident) => format!("struct variant {}::{}", params.type_name(), variant_ident),
         None => format!("struct {}", params.type_name()),
     };
+    let expecting = cattrs.expecting().unwrap_or(&expecting);
 
     let visit_seq = Stmts(deserialize_seq_in_place(params, fields, cattrs, &expecting));
 
@@ -1200,6 +1207,7 @@ fn deserialize_externally_tagged_enum(
 
     let type_name = cattrs.name().deserialize_name();
     let expecting = format!("enum {}", params.type_name());
+    let expecting = cattrs.expecting().unwrap_or(&expecting);
 
     let (variants_stmt, variant_visitor) = prepare_enum_variant_enum(variants, cattrs);
 
@@ -1309,6 +1317,9 @@ fn deserialize_internally_tagged_enum(
             }
         });
 
+    let expecting = format!("internally tagged enum {}", params.type_name());
+    let expecting = cattrs.expecting().unwrap_or(&expecting);
+
     quote_block! {
         #variant_visitor
 
@@ -1316,7 +1327,7 @@ fn deserialize_internally_tagged_enum(
 
         let __tagged = try!(_serde::Deserializer::deserialize_any(
             __deserializer,
-            _serde::private::de::TaggedContentVisitor::<__Field>::new(#tag)));
+            _serde::private::de::TaggedContentVisitor::<__Field>::new(#tag, #expecting)));
 
         match __tagged.tag {
             #(#variant_arms)*
@@ -1359,6 +1370,7 @@ fn deserialize_adjacently_tagged_enum(
         .collect();
 
     let expecting = format!("adjacently tagged enum {}", params.type_name());
+    let expecting = cattrs.expecting().unwrap_or(&expecting);
     let type_name = cattrs.name().deserialize_name();
     let deny_unknown_fields = cattrs.deny_unknown_fields();
 
@@ -1648,6 +1660,7 @@ fn deserialize_untagged_enum(
         "data did not match any variant of untagged enum {}",
         params.type_name()
     );
+    let fallthrough_msg = cattrs.expecting().unwrap_or(&fallthrough_msg);
 
     quote_block! {
         let __content = try!(<_serde::private::de::Content as _serde::Deserialize>::deserialize(__deserializer));
@@ -1905,6 +1918,7 @@ fn deserialize_generated_identifier(
         is_variant,
         fallthrough,
         !is_variant && cattrs.has_flatten(),
+        None,
     ));
 
     let lifetime = if !is_variant && cattrs.has_flatten() {
@@ -2012,6 +2026,7 @@ fn deserialize_custom_identifier(
         is_variant,
         fallthrough,
         false,
+        cattrs.expecting(),
     ));
 
     quote_block! {
@@ -2042,6 +2057,7 @@ fn deserialize_identifier(
     is_variant: bool,
     fallthrough: Option<TokenStream>,
     collect_other_fields: bool,
+    expecting: Option<&str>,
 ) -> Fragment {
     let mut flat_fields = Vec::new();
     for (_, ident, aliases) in fields {
@@ -2066,11 +2082,11 @@ fn deserialize_identifier(
         .map(|(_, ident, _)| quote!(#this::#ident))
         .collect();
 
-    let expecting = if is_variant {
+    let expecting = expecting.unwrap_or(if is_variant {
         "variant identifier"
     } else {
         "field identifier"
-    };
+    });
 
     let index_expecting = if is_variant { "variant" } else { "field" };
 

serde_derive/src/internals/attr.rs

@@ -223,6 +223,8 @@ pub struct Container {
     has_flatten: bool,
     serde_path: Option<syn::Path>,
     is_packed: bool,
+    /// Error message generated when type can't be deserialized
+    expecting: Option<String>,
 }
 
 /// Styles of representing an enum.
@@ -305,6 +307,7 @@ impl Container {
         let mut field_identifier = BoolAttr::none(cx, FIELD_IDENTIFIER);
         let mut variant_identifier = BoolAttr::none(cx, VARIANT_IDENTIFIER);
         let mut serde_path = Attr::none(cx, CRATE);
+        let mut expecting = Attr::none(cx, EXPECTING);
 
         for meta_item in item
             .attrs
@@ -575,6 +578,13 @@ impl Container {
                     }
                 }
 
+                // Parse `#[serde(expecting = "a message")]`
+                Meta(NameValue(m)) if m.path == EXPECTING => {
+                    if let Ok(s) = get_lit_str(cx, EXPECTING, &m.lit) {
+                        expecting.set(&m.path, s.value());
+                    }
+                }
+
                 Meta(meta_item) => {
                     let path = meta_item
                         .path()
@@ -627,6 +637,7 @@ impl Container {
             has_flatten: false,
             serde_path: serde_path.get(),
             is_packed,
+            expecting: expecting.get(),
         }
     }
 
@@ -702,6 +713,12 @@ impl Container {
         self.custom_serde_path()
             .map_or_else(|| Cow::Owned(parse_quote!(_serde)), Cow::Borrowed)
     }
+
+    /// Error message generated when type can't be deserialized.
+    /// If `None`, default message will be used
+    pub fn expecting(&self) -> Option<&str> {
+        self.expecting.as_ref().map(String::as_ref)
+    }
 }
 
 fn decide_tag(

serde_derive/src/internals/symbol.rs

@@ -35,6 +35,7 @@ pub const TRY_FROM: Symbol = Symbol("try_from");
 pub const UNTAGGED: Symbol = Symbol("untagged");
 pub const VARIANT_IDENTIFIER: Symbol = Symbol("variant_identifier");
 pub const WITH: Symbol = Symbol("with");
+pub const EXPECTING: Symbol = Symbol("expecting");
 
 impl PartialEq<Symbol> for Ident {
     fn eq(&self, word: &Symbol) -> bool {

test_suite/tests/test_annotations.rs

@@ -2633,3 +2633,200 @@ fn test_flatten_any_after_flatten_struct() {
         ],
     );
 }
+
+/// https://github.com/serde-rs/serde/issues/1883
+#[test]
+fn test_expecting_message() {
+    #[derive(Deserialize, PartialEq, Debug)]
+    #[serde(expecting = "something strange...")]
+    struct Unit;
+
+    #[derive(Deserialize)]
+    #[serde(expecting = "something strange...")]
+    struct Newtype(bool);
+
+    #[derive(Deserialize)]
+    #[serde(expecting = "something strange...")]
+    struct Tuple(u32, bool);
+
+    #[derive(Deserialize)]
+    #[serde(expecting = "something strange...")]
+    struct Struct {
+        question: String,
+        answer: u32,
+    }
+
+    assert_de_tokens_error::<Unit>(
+        &[Token::Str("Unit")],
+        r#"invalid type: string "Unit", expected something strange..."#
+    );
+
+    assert_de_tokens_error::<Newtype>(
+        &[Token::Str("Newtype")],
+        r#"invalid type: string "Newtype", expected something strange..."#
+    );
+
+    assert_de_tokens_error::<Tuple>(
+        &[Token::Str("Tuple")],
+        r#"invalid type: string "Tuple", expected something strange..."#
+    );
+
+    assert_de_tokens_error::<Struct>(
+        &[Token::Str("Struct")],
+        r#"invalid type: string "Struct", expected something strange..."#
+    );
+}
+
+#[test]
+fn test_expecting_message_externally_tagged_enum() {
+    #[derive(Deserialize)]
+    #[serde(expecting = "something strange...")]
+    enum Enum {
+        ExternallyTagged,
+    }
+
+    assert_de_tokens_error::<Enum>(
+        &[
+            Token::Str("ExternallyTagged"),
+        ],
+        r#"invalid type: string "ExternallyTagged", expected something strange..."#
+    );
+
+    // Check that #[serde(expecting = "...")] doesn't affect variant identifier error message
+    assert_de_tokens_error::<Enum>(
+        &[
+            Token::Enum { name: "Enum" },
+            Token::Unit,
+        ],
+        r#"invalid type: unit value, expected variant identifier"#
+    );
+}
+
+#[test]
+fn test_expecting_message_internally_tagged_enum() {
+    #[derive(Deserialize)]
+    #[serde(tag = "tag")]
+    #[serde(expecting = "something strange...")]
+    enum Enum {
+        InternallyTagged,
+    }
+
+    assert_de_tokens_error::<Enum>(
+        &[
+            Token::Str("InternallyTagged"),
+        ],
+        r#"invalid type: string "InternallyTagged", expected something strange..."#
+    );
+
+    // Check that #[serde(expecting = "...")] doesn't affect variant identifier error message
+    assert_de_tokens_error::<Enum>(
+        &[
+            Token::Map { len: None },
+            Token::Str("tag"),
+            Token::Unit,
+        ],
+        r#"invalid type: unit value, expected variant identifier"#
+    );
+}
+
+#[test]
+fn test_expecting_message_adjacently_tagged_enum() {
+    #[derive(Deserialize)]
+    #[serde(tag = "tag", content = "content")]
+    #[serde(expecting = "something strange...")]
+    enum Enum {
+        AdjacentlyTagged,
+    }
+
+    assert_de_tokens_error::<Enum>(
+        &[
+            Token::Str("AdjacentlyTagged"),
+        ],
+        r#"invalid type: string "AdjacentlyTagged", expected something strange..."#
+    );
+
+    assert_de_tokens_error::<Enum>(
+        &[
+            Token::Map { len: None },
+            Token::Unit,
+        ],
+        r#"invalid type: unit value, expected "tag", "content", or other ignored fields"#
+    );
+
+    // Check that #[serde(expecting = "...")] doesn't affect variant identifier error message
+    assert_de_tokens_error::<Enum>(
+        &[
+            Token::Map { len: None },
+            Token::Str("tag"),
+            Token::Unit,
+        ],
+        r#"invalid type: unit value, expected variant identifier"#
+    );
+}
+
+#[test]
+fn test_expecting_message_untagged_tagged_enum() {
+    #[derive(Deserialize)]
+    #[serde(untagged)]
+    #[serde(expecting = "something strange...")]
+    enum Enum {
+        Untagged,
+    }
+
+    assert_de_tokens_error::<Enum>(
+        &[
+            Token::Str("Untagged"),
+        ],
+        r#"something strange..."#
+    );
+}
+
+#[test]
+fn test_expecting_message_identifier_enum() {
+    #[derive(Deserialize)]
+    #[serde(field_identifier)]
+    #[serde(expecting = "something strange...")]
+    enum FieldEnum {
+        Field,
+    }
+
+    #[derive(Deserialize)]
+    #[serde(variant_identifier)]
+    #[serde(expecting = "something strange...")]
+    enum VariantEnum {
+        Variant,
+    }
+
+    assert_de_tokens_error::<FieldEnum>(
+        &[
+            Token::Unit,
+        ],
+        r#"invalid type: unit value, expected something strange..."#
+    );
+
+    assert_de_tokens_error::<FieldEnum>(
+        &[
+            Token::Enum { name: "FieldEnum" },
+            Token::Str("Unknown"),
+            Token::None,
+        ],
+        r#"invalid type: map, expected something strange..."#
+    );
+
+
+    assert_de_tokens_error::<VariantEnum>(
+        &[
+            Token::Unit,
+        ],
+        r#"invalid type: unit value, expected something strange..."#
+    );
+
+    assert_de_tokens_error::<VariantEnum>(
+        &[
+            Token::Enum { name: "VariantEnum" },
+            Token::Str("Unknown"),
+            Token::None,
+        ],
+        r#"invalid type: map, expected something strange..."#
+    );
+}

test_suite/tests/test_gen.rs

@@ -723,6 +723,55 @@ fn test_gen() {
     }
 
     deriving!(&'a str);
+
+    // https://github.com/serde-rs/serde/issues/1883
+    #[derive(Deserialize)]
+    #[serde(expecting = "a message")]
+    struct CustomMessageUnit;
+
+    #[derive(Deserialize)]
+    #[serde(expecting = "a message")]
+    struct CustomMessageNewtype(bool);
+
+    #[derive(Deserialize)]
+    #[serde(expecting = "a message")]
+    struct CustomMessageTuple(u32, bool);
+
+    #[derive(Deserialize)]
+    #[serde(expecting = "a message")]
+    struct CustomMessageStruct {
+        question: String,
+        answer: u32,
+    }
+
+    #[derive(Deserialize)]
+    #[serde(expecting = "a message")]
+    enum CustomMessageExternallyTaggedEnum {}
+
+    #[derive(Deserialize)]
+    #[serde(tag = "tag")]
+    #[serde(expecting = "a message")]
+    enum CustomMessageInternallyTaggedEnum {}
+
+    #[derive(Deserialize)]
+    #[serde(tag = "tag", content = "content")]
+    #[serde(expecting = "a message")]
+    enum CustomMessageAdjacentlyTaggedEnum {}
+
+    #[derive(Deserialize)]
+    #[serde(untagged)]
+    #[serde(expecting = "a message")]
+    enum CustomMessageUntaggedEnum {}
+
+    #[derive(Deserialize)]
+    #[serde(field_identifier)]
+    #[serde(expecting = "a message")]
+    enum CustomMessageFieldIdentifierEnum {}
+
+    #[derive(Deserialize)]
+    #[serde(variant_identifier)]
+    #[serde(expecting = "a message")]
+    enum CustomMessageVariantIdentifierEnum {}
 }
 
 //////////////////////////////////////////////////////////////////////////