Make compile error message point to field type instead of call site for "not implemented Serialize trait"

serde_derive/src/ser.rs

@@ -311,7 +311,7 @@ fn serialize_struct_tag_field(cattrs: &attr::Container, struct_trait: &StructTra
     match cattrs.tag() {
         attr::TagType::Internal { tag } => {
             let type_name = cattrs.name().serialize_name();
-            let func = struct_trait.serialize_field(Span::call_site());
+            let func = struct_trait.serialize_field(None);
             quote! {
                 #func(&mut __serde_state, #tag, #type_name)?;
             }
@@ -1081,8 +1081,7 @@ fn serialize_tuple_struct_visitor(
                 field_expr = wrap_serialize_field_with(params, field.ty, path, &field_expr);
             }
 
-            let span = field.original.span();
-            let func = tuple_trait.serialize_element(span);
+            let func = tuple_trait.serialize_element(field);
             let ser = quote! {
                 #func(&mut __serde_state, #field_expr)?;
             };
@@ -1131,7 +1130,8 @@ fn serialize_struct_visitor(
                     #func(&#field_expr, _serde::__private::ser::FlatMapSerializer(&mut __serde_state))?;
                 }
             } else {
-                let func = struct_trait.serialize_field(span);
+                let func = struct_trait.serialize_field(Some(field));
+
                 quote! {
                     #func(&mut __serde_state, #key_expr, #field_expr)?;
                 }
@@ -1300,16 +1300,28 @@ enum StructTrait {
 }
 
 impl StructTrait {
-    fn serialize_field(&self, span: Span) -> TokenStream {
+    fn serialize_field(&self, field: Option<&Field>) -> TokenStream {
+        let span = field
+            .map(|field| field.original.span())
+            .unwrap_or_else(Span::call_site);
+
+        let ty = field.and_then(ser_field_expr_type_hint).into_iter();
+
         match *self {
             StructTrait::SerializeMap => {
-                quote_spanned!(span=> _serde::ser::SerializeMap::serialize_entry)
+                quote_spanned! {span=>
+                    _serde::ser::SerializeMap::serialize_entry #( ::<_, #ty> )*
+                }
             }
             StructTrait::SerializeStruct => {
-                quote_spanned!(span=> _serde::ser::SerializeStruct::serialize_field)
+                quote_spanned! {span=>
+                    _serde::ser::SerializeStruct::serialize_field #( ::<#ty> )*
+                }
             }
             StructTrait::SerializeStructVariant => {
-                quote_spanned!(span=> _serde::ser::SerializeStructVariant::serialize_field)
+                quote_spanned! {span=>
+                    _serde::ser::SerializeStructVariant::serialize_field #( ::<#ty> )*
+                }
             }
         }
     }
@@ -1334,17 +1346,41 @@ enum TupleTrait {
 }
 
 impl TupleTrait {
-    fn serialize_element(&self, span: Span) -> TokenStream {
+    fn serialize_element(&self, field: &Field) -> TokenStream {
+        let span = field.original.span();
+        let ty = ser_field_expr_type_hint(field).into_iter();
+
         match *self {
             TupleTrait::SerializeTuple => {
-                quote_spanned!(span=> _serde::ser::SerializeTuple::serialize_element)
+                quote_spanned! {span=>
+                    _serde::ser::SerializeTuple::serialize_element #( ::<#ty> )*
+                }
             }
             TupleTrait::SerializeTupleStruct => {
-                quote_spanned!(span=> _serde::ser::SerializeTupleStruct::serialize_field)
+                quote_spanned! {span=>
+                    _serde::ser::SerializeTupleStruct::serialize_field #( ::<#ty> )*
+                }
             }
             TupleTrait::SerializeTupleVariant => {
-                quote_spanned!(span=> _serde::ser::SerializeTupleVariant::serialize_field)
+                quote_spanned! {span=>
+                    _serde::ser::SerializeTupleVariant::serialize_field #( ::<#ty> )*
+                }
             }
         }
     }
 }
+
+/// Returns the field's type unless `serialize_with` is enabled.
+/// This type hint should be used in a turbofish expression to hint the
+/// compiler where the type of the field comes from. This way an error
+/// that "field's type doesn't implement Serialize" will point to the type
+/// of the field directly.
+fn ser_field_expr_type_hint<'a>(field: &Field<'a>) -> Option<&'a syn::Type> {
+    // If `serialize_with` is enabled the type of the serialized field
+    // expression will not correlate with the type of the field.
+    if field.attrs.serialize_with().is_some() {
+        return None;
+    }
+
+    Some(field.ty)
+}

test_suite/tests/ui/on_unimplemented.stderr → test_suite/tests/ui/on-unimplemented/fn_param.stderr

@@ -1,5 +1,5 @@
 error[E0277]: the trait bound `MyStruct: Serialize` is not satisfied
-  --> tests/ui/on_unimplemented.rs:21:15
+  --> tests/ui/on-unimplemented/fn_param.rs:21:15
    |
 21 |     to_string(&MyStruct);
    |     --------- ^^^^^^^^^ the trait `Serialize` is not implemented for `MyStruct`
@@ -19,7 +19,7 @@ error[E0277]: the trait bound `MyStruct: Serialize` is not satisfied
              (T0, T1, T2, T3, T4)
            and $N others
 note: required by a bound in `to_string`
-  --> tests/ui/on_unimplemented.rs:6:8
+  --> tests/ui/on-unimplemented/fn_param.rs:6:8
    |
 4  | fn to_string<T>(_: &T) -> String
    |    --------- required by a bound in this function
@@ -28,7 +28,7 @@ note: required by a bound in `to_string`
    |        ^^^^^^^^^ required by this bound in `to_string`
 
 error[E0277]: the trait bound `MyStruct: Deserialize<'_>` is not satisfied
-  --> tests/ui/on_unimplemented.rs:22:23
+  --> tests/ui/on-unimplemented/fn_param.rs:22:23
    |
 22 |     let _: MyStruct = from_str("");
    |                       ^^^^^^^^^^^^ the trait `Deserialize<'_>` is not implemented for `MyStruct`
@@ -46,7 +46,7 @@ error[E0277]: the trait bound `MyStruct: Deserialize<'_>` is not satisfied
              (T0, T1, T2, T3)
            and $N others
 note: required by a bound in `from_str`
-  --> tests/ui/on_unimplemented.rs:13:8
+  --> tests/ui/on-unimplemented/fn_param.rs:13:8
    |
 11 | fn from_str<'de, T>(_: &'de str) -> T
    |    -------- required by a bound in this function

test_suite/tests/ui/on-unimplemented/struct_field.rs

@@ -0,0 +1,11 @@
+use serde_derive::{Deserialize, Serialize};
+
+struct NoImpls;
+
+#[derive(Serialize, Deserialize)]
+struct S {
+    x1: u32,
+    x2: NoImpls,
+}
+
+fn main() {}

test_suite/tests/ui/on-unimplemented/struct_field.stderr

@@ -0,0 +1,166 @@
+error[E0277]: the trait bound `NoImpls: Serialize` is not satisfied
+ --> tests/ui/on-unimplemented/struct_field.rs:8:9
+  |
+8 |     x2: NoImpls,
+  |         ^^^^^^^ the trait `Serialize` is not implemented for `NoImpls`
+  |
+  = note: for local types consider adding `#[derive(serde::Serialize)]` to your `NoImpls` type
+  = note: for types from other crates check whether the crate offers a `serde` feature flag
+  = help: the following other types implement trait `Serialize`:
+            &'a T
+            &'a mut T
+            ()
+            (T,)
+            (T0, T1)
+            (T0, T1, T2)
+            (T0, T1, T2, T3)
+            (T0, T1, T2, T3, T4)
+          and $N others
+note: required by a bound in `_::_serde::ser::SerializeStruct::serialize_field`
+ --> $WORKSPACE/serde/src/ser/mod.rs
+  |
+  |     fn serialize_field<T>(&mut self, key: &'static str, value: &T) -> Result<(), Self::Error>
+  |        --------------- required by a bound in this associated function
+  |     where
+  |         T: ?Sized + Serialize;
+  |                     ^^^^^^^^^ required by this bound in `SerializeStruct::serialize_field`
+
+error[E0277]: the trait bound `NoImpls: Deserialize<'_>` is not satisfied
+ --> tests/ui/on-unimplemented/struct_field.rs:8:9
+  |
+8 |     x2: NoImpls,
+  |         ^^^^^^^ the trait `Deserialize<'_>` is not implemented for `NoImpls`
+  |
+  = note: for local types consider adding `#[derive(serde::Deserialize)]` to your `NoImpls` type
+  = note: for types from other crates check whether the crate offers a `serde` feature flag
+  = help: the following other types implement trait `Deserialize<'de>`:
+            &'a Path
+            &'a [u8]
+            &'a str
+            ()
+            (T,)
+            (T0, T1)
+            (T0, T1, T2)
+            (T0, T1, T2, T3)
+          and $N others
+note: required by a bound in `next_element`
+ --> $WORKSPACE/serde/src/de/mod.rs
+  |
+  |     fn next_element<T>(&mut self) -> Result<Option<T>, Self::Error>
+  |        ------------ required by a bound in this associated function
+  |     where
+  |         T: Deserialize<'de>,
+  |            ^^^^^^^^^^^^^^^^ required by this bound in `SeqAccess::next_element`
+
+error[E0277]: the trait bound `NoImpls: Deserialize<'_>` is not satisfied
+ --> tests/ui/on-unimplemented/struct_field.rs:8:9
+  |
+8 |     x2: NoImpls,
+  |         ^^^^^^^ the trait `Deserialize<'_>` is not implemented for `NoImpls`
+  |
+  = note: for local types consider adding `#[derive(serde::Deserialize)]` to your `NoImpls` type
+  = note: for types from other crates check whether the crate offers a `serde` feature flag
+  = help: the following other types implement trait `Deserialize<'de>`:
+            &'a Path
+            &'a [u8]
+            &'a str
+            ()
+            (T,)
+            (T0, T1)
+            (T0, T1, T2)
+            (T0, T1, T2, T3)
+          and $N others
+note: required by a bound in `next_value`
+ --> $WORKSPACE/serde/src/de/mod.rs
+  |
+  |     fn next_value<V>(&mut self) -> Result<V, Self::Error>
+  |        ---------- required by a bound in this associated function
+  |     where
+  |         V: Deserialize<'de>,
+  |            ^^^^^^^^^^^^^^^^ required by this bound in `MapAccess::next_value`
+
+error[E0277]: the trait bound `NoImpls: Deserialize<'_>` is not satisfied
+ --> tests/ui/on-unimplemented/struct_field.rs:5:21
+  |
+5 | #[derive(Serialize, Deserialize)]
+  |                     ^^^^^^^^^^^ the trait `Deserialize<'_>` is not implemented for `NoImpls`
+  |
+  = note: for local types consider adding `#[derive(serde::Deserialize)]` to your `NoImpls` type
+  = note: for types from other crates check whether the crate offers a `serde` feature flag
+  = help: the following other types implement trait `Deserialize<'de>`:
+            &'a Path
+            &'a [u8]
+            &'a str
+            ()
+            (T,)
+            (T0, T1)
+            (T0, T1, T2)
+            (T0, T1, T2, T3)
+          and $N others
+note: required by a bound in `_::_serde::__private::de::missing_field`
+ --> $WORKSPACE/serde/src/private/de.rs
+  |
+  | pub fn missing_field<'de, V, E>(field: &'static str) -> Result<V, E>
+  |        ------------- required by a bound in this function
+  | where
+  |     V: Deserialize<'de>,
+  |        ^^^^^^^^^^^^^^^^ required by this bound in `missing_field`
+  = note: this error originates in the derive macro `Deserialize` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: the trait bound `NoImpls: Deserialize<'_>` is not satisfied
+ --> tests/ui/on-unimplemented/struct_field.rs:5:21
+  |
+5 | #[derive(Serialize, Deserialize)]
+  |                     ^^^^^^^^^^^ the trait `Deserialize<'_>` is not implemented for `NoImpls`, which is required by `InPlaceSeed<'_, NoImpls>: DeserializeSeed<'_>`
+  |
+  = note: for local types consider adding `#[derive(serde::Deserialize)]` to your `NoImpls` type
+  = note: for types from other crates check whether the crate offers a `serde` feature flag
+  = help: the following other types implement trait `Deserialize<'de>`:
+            &'a Path
+            &'a [u8]
+            &'a str
+            ()
+            (T,)
+            (T0, T1)
+            (T0, T1, T2)
+            (T0, T1, T2, T3)
+          and $N others
+  = note: required for `InPlaceSeed<'_, NoImpls>` to implement `DeserializeSeed<'_>`
+note: required by a bound in `next_element_seed`
+ --> $WORKSPACE/serde/src/de/mod.rs
+  |
+  |     fn next_element_seed<T>(&mut self, seed: T) -> Result<Option<T::Value>, Self::Error>
+  |        ----------------- required by a bound in this associated function
+  |     where
+  |         T: DeserializeSeed<'de>;
+  |            ^^^^^^^^^^^^^^^^^^^^ required by this bound in `SeqAccess::next_element_seed`
+  = note: this error originates in the derive macro `Deserialize` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: the trait bound `NoImpls: Deserialize<'_>` is not satisfied
+ --> tests/ui/on-unimplemented/struct_field.rs:5:21
+  |
+5 | #[derive(Serialize, Deserialize)]
+  |                     ^^^^^^^^^^^ the trait `Deserialize<'_>` is not implemented for `NoImpls`, which is required by `InPlaceSeed<'_, NoImpls>: DeserializeSeed<'_>`
+  |
+  = note: for local types consider adding `#[derive(serde::Deserialize)]` to your `NoImpls` type
+  = note: for types from other crates check whether the crate offers a `serde` feature flag
+  = help: the following other types implement trait `Deserialize<'de>`:
+            &'a Path
+            &'a [u8]
+            &'a str
+            ()
+            (T,)
+            (T0, T1)
+            (T0, T1, T2)
+            (T0, T1, T2, T3)
+          and $N others
+  = note: required for `InPlaceSeed<'_, NoImpls>` to implement `DeserializeSeed<'_>`
+note: required by a bound in `next_value_seed`
+ --> $WORKSPACE/serde/src/de/mod.rs
+  |
+  |     fn next_value_seed<V>(&mut self, seed: V) -> Result<V::Value, Self::Error>
+  |        --------------- required by a bound in this associated function
+  |     where
+  |         V: DeserializeSeed<'de>;
+  |            ^^^^^^^^^^^^^^^^^^^^ required by this bound in `MapAccess::next_value_seed`
+  = note: this error originates in the derive macro `Deserialize` (in Nightly builds, run with -Z macro-backtrace for more info)

test_suite/tests/ui/on-unimplemented/struct_variant.rs

@@ -0,0 +1,10 @@
+use serde_derive::{Deserialize, Serialize};
+
+struct NoImpls;
+
+#[derive(Serialize, Deserialize)]
+enum E {
+    S { x1: u32, x2: NoImpls },
+}
+
+fn main() {}