Merge pull request #2558 from Mingun/correct-span

Improve error reporting about mismatched signature in `with` and `default` attributes

serde_derive/src/de.rs

@@ -371,7 +371,11 @@ fn deserialize_transparent(cont: &Container, params: &Parameters) -> Fragment {
         } else {
             let value = match field.attrs.default() {
                 attr::Default::Default => quote!(_serde::__private::Default::default()),
-                attr::Default::Path(path) => quote!(#path()),
+                // If #path returns wrong type, error will be reported here (^^^^^).
+                // We attach span of the path to the function so it will be reported
+                // on the #[serde(default = "...")]
+                //                          ^^^^^
+                attr::Default::Path(path) => quote_spanned!(path.span()=> #path()),
                 attr::Default::None => quote!(_serde::__private::PhantomData),
             };
             quote!(#member: #value)
@@ -757,7 +761,11 @@ fn deserialize_seq(
         attr::Default::Default => Some(quote!(
             let __default: Self::Value = _serde::__private::Default::default();
         )),
-        attr::Default::Path(path) => Some(quote!(
+        // If #path returns wrong type, error will be reported here (^^^^^).
+        // We attach span of the path to the function so it will be reported
+        // on the #[serde(default = "...")]
+        //                          ^^^^^
+        attr::Default::Path(path) => Some(quote_spanned!(path.span()=>
             let __default: Self::Value = #path();
         )),
         attr::Default::None => {
@@ -839,7 +847,11 @@ fn deserialize_seq_in_place(
         attr::Default::Default => Some(quote!(
             let __default: #this_type #ty_generics = _serde::__private::Default::default();
         )),
-        attr::Default::Path(path) => Some(quote!(
+        // If #path returns wrong type, error will be reported here (^^^^^).
+        // We attach span of the path to the function so it will be reported
+        // on the #[serde(default = "...")]
+        //                          ^^^^^
+        attr::Default::Path(path) => Some(quote_spanned!(path.span()=>
             let __default: #this_type #ty_generics = #path();
         )),
         attr::Default::None => {
@@ -873,7 +885,11 @@ fn deserialize_newtype_struct(
             }
         }
         Some(path) => {
-            quote! {
+            // If #path returns wrong type, error will be reported here (^^^^^).
+            // We attach span of the path to the function so it will be reported
+            // on the #[serde(with = "...")]
+            //                       ^^^^^
+            quote_spanned! {path.span()=>
                 #path(__e)?
             }
         }
@@ -2647,7 +2663,11 @@ fn deserialize_map(
         attr::Default::Default => Some(quote!(
             let __default: Self::Value = _serde::__private::Default::default();
         )),
-        attr::Default::Path(path) => Some(quote!(
+        // If #path returns wrong type, error will be reported here (^^^^^).
+        // We attach span of the path to the function so it will be reported
+        // on the #[serde(default = "...")]
+        //                          ^^^^^
+        attr::Default::Path(path) => Some(quote_spanned!(path.span()=>
             let __default: Self::Value = #path();
         )),
         attr::Default::None => {
@@ -2817,7 +2837,11 @@ fn deserialize_map_in_place(
         attr::Default::Default => Some(quote!(
             let __default: #this_type #ty_generics = _serde::__private::Default::default();
         )),
-        attr::Default::Path(path) => Some(quote!(
+        // If #path returns wrong type, error will be reported here (^^^^^).
+        // We attach span of the path to the function so it will be reported
+        // on the #[serde(default = "...")]
+        //                          ^^^^^
+        attr::Default::Path(path) => Some(quote_spanned!(path.span()=>
             let __default: #this_type #ty_generics = #path();
         )),
         attr::Default::None => {
@@ -2856,6 +2880,13 @@ fn wrap_deserialize_with(
         split_with_de_lifetime(params);
     let delife = params.borrowed.de_lifetime();
 
+    // If #deserialize_with returns wrong type, error will be reported here (^^^^^).
+    // We attach span of the path to the function so it will be reported
+    // on the #[serde(with = "...")]
+    //                       ^^^^^
+    let value = quote_spanned! {deserialize_with.span()=>
+        #deserialize_with(__deserializer)?
+    };
     let wrapper = quote! {
         #[doc(hidden)]
         struct __DeserializeWith #de_impl_generics #where_clause {
@@ -2870,7 +2901,7 @@ fn wrap_deserialize_with(
                 __D: _serde::Deserializer<#delife>,
             {
                 _serde::__private::Ok(__DeserializeWith {
-                    value: #deserialize_with(__deserializer)?,
+                    value: #value,
                     phantom: _serde::__private::PhantomData,
                     lifetime: _serde::__private::PhantomData,
                 })
@@ -2961,7 +2992,11 @@ fn expr_is_missing(field: &Field, cattrs: &attr::Container) -> Fragment {
             return quote_expr!(#func());
         }
         attr::Default::Path(path) => {
-            return quote_expr!(#path());
+            // If #path returns wrong type, error will be reported here (^^^^^).
+            // We attach span of the path to the function so it will be reported
+            // on the #[serde(default = "...")]
+            //                          ^^^^^
+            return Fragment::Expr(quote_spanned!(path.span()=> #path()));
         }
         attr::Default::None => { /* below */ }
     }
@@ -3004,6 +3039,10 @@ fn expr_is_missing_seq(
             return quote_spanned!(span=> #assign_to _serde::__private::Default::default());
         }
         attr::Default::Path(path) => {
+            // If #path returns wrong type, error will be reported here (^^^^^).
+            // We attach span of the path to the function so it will be reported
+            // on the #[serde(default = "...")]
+            //                          ^^^^^
             return quote_spanned!(path.span()=> #assign_to #path());
         }
         attr::Default::None => { /* below */ }

serde_derive/src/internals/attr.rs

@@ -8,6 +8,7 @@ use std::iter::FromIterator;
 use syn::meta::ParseNestedMeta;
 use syn::parse::ParseStream;
 use syn::punctuated::Punctuated;
+use syn::spanned::Spanned;
 use syn::{parse_quote, token, Ident, Lifetime, Token};
 
 // This module handles parsing of `#[serde(...)]` attributes. The entrypoints
@@ -888,13 +889,13 @@ impl Variant {
                         ser_path
                             .path
                             .segments
-                            .push(Ident::new("serialize", Span::call_site()).into());
+                            .push(Ident::new("serialize", ser_path.span()).into());
                         serialize_with.set(&meta.path, ser_path);
                         let mut de_path = path;
                         de_path
                             .path
                             .segments
-                            .push(Ident::new("deserialize", Span::call_site()).into());
+                            .push(Ident::new("deserialize", de_path.span()).into());
                         deserialize_with.set(&meta.path, de_path);
                     }
                 } else if meta.path == SERIALIZE_WITH {
@@ -1170,13 +1171,13 @@ impl Field {
                         ser_path
                             .path
                             .segments
-                            .push(Ident::new("serialize", Span::call_site()).into());
+                            .push(Ident::new("serialize", ser_path.span()).into());
                         serialize_with.set(&meta.path, ser_path);
                         let mut de_path = path;
                         de_path
                             .path
                             .segments
-                            .push(Ident::new("deserialize", Span::call_site()).into());
+                            .push(Ident::new("deserialize", de_path.span()).into());
                         deserialize_with.set(&meta.path, de_path);
                     }
                 } else if meta.path == BOUND {

serde_derive/src/ser.rs

@@ -1220,9 +1220,15 @@ fn wrap_serialize_with(
         })
     });
 
-    quote!({
+    // If #serialize_with returns wrong type, error will be reported on here.
+    // We attach span of the path to this piece so error will be reported
+    // on the #[serde(with = "...")]
+    //                       ^^^^^
+    quote_spanned!(serialize_with.span()=> {
         #[doc(hidden)]
         struct __SerializeWith #wrapper_impl_generics #where_clause {
+            // If #field_tys is empty, `values` does not used
+            #[allow(dead_code)]
             values: (#(&'__a #field_tys, )*),
             phantom: _serde::__private::PhantomData<#this_type #ty_generics>,
         }

test_suite/tests/ui/default-attribute/incorrect_type_enum_adjacently_tagged.rs

@@ -0,0 +1,22 @@
+//! Ensures that error message points to the path in attribute
+use serde_derive::Deserialize;
+
+#[derive(Deserialize)]
+#[serde(tag = "tag", content = "content")]
+enum Enum {
+    // Newtype variants does not use the provided path, so it is forbidden here
+    // Newtype(#[serde(default = "main")] u8),
+    Tuple(
+        u8,
+        #[serde(default = "main")] i8,
+    ),
+    Struct {
+        #[serde(default = "main")]
+        f1: u8,
+        f2: u8,
+        #[serde(default = "main")]
+        f3: i8,
+    },
+}
+
+fn main() {}

test_suite/tests/ui/default-attribute/incorrect_type_enum_adjacently_tagged.stderr

@@ -0,0 +1,35 @@
+error[E0308]: `match` arms have incompatible types
+  --> tests/ui/default-attribute/incorrect_type_enum_adjacently_tagged.rs:11:27
+   |
+4  | #[derive(Deserialize)]
+   |          -----------
+   |          |
+   |          this is found to be of type `i8`
+   |          `match` arms have incompatible types
+...
+11 |         #[serde(default = "main")] i8,
+   |                           ^^^^^^ expected `i8`, found `()`
+
+error[E0308]: `match` arms have incompatible types
+  --> tests/ui/default-attribute/incorrect_type_enum_adjacently_tagged.rs:14:27
+   |
+4  | #[derive(Deserialize)]
+   |          -----------
+   |          |
+   |          this is found to be of type `u8`
+   |          `match` arms have incompatible types
+...
+14 |         #[serde(default = "main")]
+   |                           ^^^^^^ expected `u8`, found `()`
+
+error[E0308]: `match` arms have incompatible types
+  --> tests/ui/default-attribute/incorrect_type_enum_adjacently_tagged.rs:17:27
+   |
+4  | #[derive(Deserialize)]
+   |          -----------
+   |          |
+   |          this is found to be of type `i8`
+   |          `match` arms have incompatible types
+...
+17 |         #[serde(default = "main")]
+   |                           ^^^^^^ expected `i8`, found `()`

test_suite/tests/ui/default-attribute/incorrect_type_enum_externally_tagged.rs

@@ -0,0 +1,21 @@
+//! Ensures that error message points to the path in attribute
+use serde_derive::Deserialize;
+
+#[derive(Deserialize)]
+enum Enum {
+    // Newtype variants does not use the provided path, so it is forbidden here
+    // Newtype(#[serde(default = "main")] u8),
+    Tuple(
+        u8,
+        #[serde(default = "main")] i8,
+    ),
+    Struct {
+        #[serde(default = "main")]
+        f1: u8,
+        f2: u8,
+        #[serde(default = "main")]
+        f3: i8,
+    },
+}
+
+fn main() {}

test_suite/tests/ui/default-attribute/incorrect_type_enum_externally_tagged.stderr

@@ -0,0 +1,35 @@
+error[E0308]: `match` arms have incompatible types
+  --> tests/ui/default-attribute/incorrect_type_enum_externally_tagged.rs:10:27
+   |
+4  | #[derive(Deserialize)]
+   |          -----------
+   |          |
+   |          this is found to be of type `i8`
+   |          `match` arms have incompatible types
+...
+10 |         #[serde(default = "main")] i8,
+   |                           ^^^^^^ expected `i8`, found `()`
+
+error[E0308]: `match` arms have incompatible types
+  --> tests/ui/default-attribute/incorrect_type_enum_externally_tagged.rs:13:27
+   |
+4  | #[derive(Deserialize)]
+   |          -----------
+   |          |
+   |          this is found to be of type `u8`
+   |          `match` arms have incompatible types
+...
+13 |         #[serde(default = "main")]
+   |                           ^^^^^^ expected `u8`, found `()`
+
+error[E0308]: `match` arms have incompatible types
+  --> tests/ui/default-attribute/incorrect_type_enum_externally_tagged.rs:16:27
+   |
+4  | #[derive(Deserialize)]
+   |          -----------
+   |          |
+   |          this is found to be of type `i8`
+   |          `match` arms have incompatible types
+...
+16 |         #[serde(default = "main")]
+   |                           ^^^^^^ expected `i8`, found `()`

test_suite/tests/ui/default-attribute/incorrect_type_enum_internally_tagged.rs

@@ -0,0 +1,19 @@
+//! Ensures that error message points to the path in attribute
+use serde_derive::Deserialize;
+
+#[derive(Deserialize)]
+#[serde(tag = "tag")]
+enum Enum {
+    // Newtype variants does not use the provided path, so it is forbidden here
+    // Newtype(#[serde(default = "main")] u8),
+    // Tuple variants does not supported in internally tagged enums
+    Struct {
+        #[serde(default = "main")]
+        f1: u8,
+        f2: u8,
+        #[serde(default = "main")]
+        f3: i8,
+    },
+}
+
+fn main() {}

test_suite/tests/ui/default-attribute/incorrect_type_enum_internally_tagged.stderr

@@ -0,0 +1,23 @@
+error[E0308]: `match` arms have incompatible types
+  --> tests/ui/default-attribute/incorrect_type_enum_internally_tagged.rs:11:27
+   |
+4  | #[derive(Deserialize)]
+   |          -----------
+   |          |
+   |          this is found to be of type `u8`
+   |          `match` arms have incompatible types
+...
+11 |         #[serde(default = "main")]
+   |                           ^^^^^^ expected `u8`, found `()`
+
+error[E0308]: `match` arms have incompatible types
+  --> tests/ui/default-attribute/incorrect_type_enum_internally_tagged.rs:14:27
+   |
+4  | #[derive(Deserialize)]
+   |          -----------
+   |          |
+   |          this is found to be of type `i8`
+   |          `match` arms have incompatible types
+...
+14 |         #[serde(default = "main")]
+   |                           ^^^^^^ expected `i8`, found `()`

test_suite/tests/ui/default-attribute/incorrect_type_enum_untagged.rs

@@ -0,0 +1,22 @@
+//! Ensures that error message points to the path in attribute
+use serde_derive::Deserialize;
+
+#[derive(Deserialize)]
+#[serde(untagged)]
+enum Enum {
+    // Newtype variants does not use the provided path, so it is forbidden here
+    // Newtype(#[serde(default = "main")] u8),
+    Tuple(
+        u8,
+        #[serde(default = "main")] i8,
+    ),
+    Struct {
+        #[serde(default = "main")]
+        f1: u8,
+        f2: u8,
+        #[serde(default = "main")]
+        f3: i8,
+    },
+}
+
+fn main() {}

test_suite/tests/ui/default-attribute/incorrect_type_enum_untagged.stderr

@@ -0,0 +1,35 @@
+error[E0308]: `match` arms have incompatible types
+  --> tests/ui/default-attribute/incorrect_type_enum_untagged.rs:11:27
+   |
+4  | #[derive(Deserialize)]
+   |          -----------
+   |          |
+   |          this is found to be of type `i8`
+   |          `match` arms have incompatible types
+...
+11 |         #[serde(default = "main")] i8,
+   |                           ^^^^^^ expected `i8`, found `()`
+
+error[E0308]: `match` arms have incompatible types
+  --> tests/ui/default-attribute/incorrect_type_enum_untagged.rs:14:27
+   |
+4  | #[derive(Deserialize)]
+   |          -----------
+   |          |
+   |          this is found to be of type `u8`
+   |          `match` arms have incompatible types
+...
+14 |         #[serde(default = "main")]
+   |                           ^^^^^^ expected `u8`, found `()`
+
+error[E0308]: `match` arms have incompatible types
+  --> tests/ui/default-attribute/incorrect_type_enum_untagged.rs:17:27
+   |
+4  | #[derive(Deserialize)]
+   |          -----------
+   |          |
+   |          this is found to be of type `i8`
+   |          `match` arms have incompatible types
+...
+17 |         #[serde(default = "main")]
+   |                           ^^^^^^ expected `i8`, found `()`

test_suite/tests/ui/default-attribute/incorrect_type_newtype.rs

@@ -0,0 +1,8 @@
+//! Ensures that error message points to the path in attribute
+use serde_derive::Deserialize;
+
+#[derive(Deserialize)]
+#[serde(default = "main")]
+struct Newtype(#[serde(default = "main")] u8);
+
+fn main() {}

test_suite/tests/ui/default-attribute/incorrect_type_newtype.stderr

@@ -0,0 +1,37 @@
+error[E0308]: mismatched types
+ --> tests/ui/default-attribute/incorrect_type_newtype.rs:5:19
+  |
+5 | #[serde(default = "main")]
+  |                   ^^^^^^
+  |                   |
+  |                   expected `Newtype`, found `()`
+  |                   expected due to this
+
+error[E0308]: `match` arms have incompatible types
+ --> tests/ui/default-attribute/incorrect_type_newtype.rs:6:34
+  |
+4 | #[derive(Deserialize)]
+  |          -----------
+  |          |
+  |          this is found to be of type `u8`
+  |          `match` arms have incompatible types
+5 | #[serde(default = "main")]
+6 | struct Newtype(#[serde(default = "main")] u8);
+  |                                  ^^^^^^ expected `u8`, found `()`
+
+error[E0308]: mismatched types
+ --> tests/ui/default-attribute/incorrect_type_newtype.rs:5:19
+  |
+5 | #[serde(default = "main")]
+  |                   ^^^^^^ expected `Newtype`, found `()`
+6 | struct Newtype(#[serde(default = "main")] u8);
+  |        ------- expected due to this
+
+error[E0308]: mismatched types
+ --> tests/ui/default-attribute/incorrect_type_newtype.rs:6:34
+  |
+4 | #[derive(Deserialize)]
+  |          ----------- expected due to the type of this binding
+5 | #[serde(default = "main")]
+6 | struct Newtype(#[serde(default = "main")] u8);
+  |                                  ^^^^^^ expected `u8`, found `()`

test_suite/tests/ui/default-attribute/incorrect_type_struct.rs

@@ -0,0 +1,14 @@
+//! Ensures that error message points to the path in attribute
+use serde_derive::Deserialize;
+
+#[derive(Deserialize)]
+#[serde(default = "main")]
+struct Struct {
+    #[serde(default = "main")]
+    f1: u8,
+    f2: u8,
+    #[serde(default = "main")]
+    f3: i8,
+}
+
+fn main() {}

test_suite/tests/ui/default-attribute/incorrect_type_struct.stderr

@@ -0,0 +1,58 @@
+error[E0308]: mismatched types
+ --> tests/ui/default-attribute/incorrect_type_struct.rs:5:19
+  |
+5 | #[serde(default = "main")]
+  |                   ^^^^^^
+  |                   |
+  |                   expected `Struct`, found `()`
+  |                   expected due to this
+
+error[E0308]: `match` arms have incompatible types
+ --> tests/ui/default-attribute/incorrect_type_struct.rs:7:23
+  |
+4 | #[derive(Deserialize)]
+  |          -----------
+  |          |
+  |          this is found to be of type `u8`
+  |          `match` arms have incompatible types
+...
+7 |     #[serde(default = "main")]
+  |                       ^^^^^^ expected `u8`, found `()`
+
+error[E0308]: `match` arms have incompatible types
+  --> tests/ui/default-attribute/incorrect_type_struct.rs:10:23
+   |
+4  | #[derive(Deserialize)]
+   |          -----------
+   |          |
+   |          this is found to be of type `i8`
+   |          `match` arms have incompatible types
+...
+10 |     #[serde(default = "main")]
+   |                       ^^^^^^ expected `i8`, found `()`
+
+error[E0308]: mismatched types
+ --> tests/ui/default-attribute/incorrect_type_struct.rs:5:19
+  |
+5 | #[serde(default = "main")]
+  |                   ^^^^^^ expected `Struct`, found `()`
+6 | struct Struct {
+  |        ------ expected due to this
+
+error[E0308]: mismatched types
+ --> tests/ui/default-attribute/incorrect_type_struct.rs:7:23
+  |
+4 | #[derive(Deserialize)]
+  |          ----------- expected due to the type of this binding
+...
+7 |     #[serde(default = "main")]
+  |                       ^^^^^^ expected `u8`, found `()`
+
+error[E0308]: mismatched types
+  --> tests/ui/default-attribute/incorrect_type_struct.rs:10:23
+   |
+4  | #[derive(Deserialize)]
+   |          ----------- expected due to the type of this binding
+...
+10 |     #[serde(default = "main")]
+   |                       ^^^^^^ expected `i8`, found `()`

test_suite/tests/ui/default-attribute/incorrect_type_tuple.rs

@@ -0,0 +1,11 @@
+//! Ensures that error message points to the path in attribute
+use serde_derive::Deserialize;
+
+#[derive(Deserialize)]
+#[serde(default = "main")]
+struct Tuple(
+    u8,
+    #[serde(default = "main")] i8,
+);
+
+fn main() {}

test_suite/tests/ui/default-attribute/incorrect_type_tuple.stderr

@@ -0,0 +1,37 @@
+error[E0308]: mismatched types
+ --> tests/ui/default-attribute/incorrect_type_tuple.rs:5:19
+  |
+5 | #[serde(default = "main")]
+  |                   ^^^^^^
+  |                   |
+  |                   expected `Tuple`, found `()`
+  |                   expected due to this
+
+error[E0308]: `match` arms have incompatible types
+ --> tests/ui/default-attribute/incorrect_type_tuple.rs:8:23
+  |
+4 | #[derive(Deserialize)]
+  |          -----------
+  |          |
+  |          this is found to be of type `i8`
+  |          `match` arms have incompatible types
+...
+8 |     #[serde(default = "main")] i8,
+  |                       ^^^^^^ expected `i8`, found `()`
+
+error[E0308]: mismatched types
+ --> tests/ui/default-attribute/incorrect_type_tuple.rs:5:19
+  |
+5 | #[serde(default = "main")]
+  |                   ^^^^^^ expected `Tuple`, found `()`
+6 | struct Tuple(
+  |        ----- expected due to this
+
+error[E0308]: mismatched types
+ --> tests/ui/default-attribute/incorrect_type_tuple.rs:8:23
+  |
+4 | #[derive(Deserialize)]
+  |          ----------- expected due to the type of this binding
+...
+8 |     #[serde(default = "main")] i8,
+  |                       ^^^^^^ expected `i8`, found `()`

test_suite/tests/ui/default-attribute/union.rs

@@ -0,0 +1,9 @@
+use serde_derive::Deserialize;
+
+#[derive(Deserialize)]
+#[serde(default)]
+union Union {
+    f: u8,
+}
+
+fn main() {}

test_suite/tests/ui/default-attribute/union.stderr

@@ -0,0 +1,14 @@
+error: #[serde(default)] can only be used on structs
+ --> tests/ui/default-attribute/union.rs:4:9
+  |
+4 | #[serde(default)]
+  |         ^^^^^^^
+
+error: Serde does not support derive for unions
+ --> tests/ui/default-attribute/union.rs:4:1
+  |
+4 | / #[serde(default)]
+5 | | union Union {
+6 | |     f: u8,
+7 | | }
+  | |_^

test_suite/tests/ui/default-attribute/union_path.rs

@@ -0,0 +1,9 @@
+use serde_derive::Deserialize;
+
+#[derive(Deserialize)]
+#[serde(default = "default_u")]
+union Union {
+    f: u8,
+}
+
+fn main() {}

test_suite/tests/ui/default-attribute/union_path.stderr

@@ -0,0 +1,14 @@
+error: #[serde(default = "...")] can only be used on structs
+ --> tests/ui/default-attribute/union_path.rs:4:9
+  |
+4 | #[serde(default = "default_u")]
+  |         ^^^^^^^^^^^^^^^^^^^^^
+
+error: Serde does not support derive for unions
+ --> tests/ui/default-attribute/union_path.rs:4:1
+  |
+4 | / #[serde(default = "default_u")]
+5 | | union Union {
+6 | |     f: u8,
+7 | | }
+  | |_^

test_suite/tests/ui/default-attribute/unit.rs

@@ -0,0 +1,7 @@
+use serde_derive::Deserialize;
+
+#[derive(Deserialize)]
+#[serde(default)]
+struct Unit;
+
+fn main() {}

test_suite/tests/ui/default-attribute/unit.stderr

@@ -0,0 +1,7 @@
+error: #[serde(default)] can only be used on structs that have fields
+ --> tests/ui/default-attribute/unit.rs:3:10
+  |
+3 | #[derive(Deserialize)]
+  |          ^^^^^^^^^^^
+  |
+  = note: this error originates in the derive macro `Deserialize` (in Nightly builds, run with -Z macro-backtrace for more info)

test_suite/tests/ui/default-attribute/unit_path.rs

@@ -0,0 +1,7 @@
+use serde_derive::Deserialize;
+
+#[derive(Deserialize)]
+#[serde(default = "default_u")]
+struct Unit;
+
+fn main() {}

test_suite/tests/ui/default-attribute/unit_path.stderr

@@ -0,0 +1,5 @@
+error: #[serde(default = "...")] can only be used on structs that have fields
+ --> tests/ui/default-attribute/unit_path.rs:4:9
+  |
+4 | #[serde(default = "default_u")]
+  |         ^^^^^^^^^^^^^^^^^^^^^

test_suite/tests/ui/with/incorrect_type.rs

@@ -0,0 +1,23 @@
+use serde_derive::{Deserialize, Serialize};
+
+mod w {
+    use serde::{Deserializer, Serializer};
+
+    pub fn deserialize<'de, D: Deserializer<'de>>(_: D) -> Result<(), D::Error> {
+        unimplemented!()
+    }
+    pub fn serialize<T, S: Serializer>(_: S) -> Result<S::Ok, S::Error> {
+        unimplemented!()
+    }
+}
+
+#[derive(Serialize, Deserialize)]
+struct W(#[serde(with = "w")] u8, u8);
+
+#[derive(Serialize, Deserialize)]
+struct S(#[serde(serialize_with = "w::serialize")] u8, u8);
+
+#[derive(Serialize, Deserialize)]
+struct D(#[serde(deserialize_with = "w::deserialize")] u8, u8);
+
+fn main() {}

test_suite/tests/ui/with/incorrect_type.stderr

@@ -0,0 +1,97 @@
+error[E0277]: the trait bound `&u8: Serializer` is not satisfied
+  --> tests/ui/with/incorrect_type.rs:14:10
+   |
+14 | #[derive(Serialize, Deserialize)]
+   |          ^^^^^^^^^ the trait `Serializer` is not implemented for `&u8`
+15 | struct W(#[serde(with = "w")] u8, u8);
+   |                         --- required by a bound introduced by this call
+   |
+   = help: the following other types implement trait `Serializer`:
+             &'a mut Formatter<'b>
+             FlatMapSerializer<'a, M>
+             _::_serde::__private::ser::content::ContentSerializer<E>
+note: required by a bound in `w::serialize`
+  --> tests/ui/with/incorrect_type.rs:9:28
+   |
+9  |     pub fn serialize<T, S: Serializer>(_: S) -> Result<S::Ok, S::Error> {
+   |                            ^^^^^^^^^^ required by this bound in `serialize`
+
+error[E0061]: this function takes 1 argument but 2 arguments were supplied
+  --> tests/ui/with/incorrect_type.rs:15:25
+   |
+15 | struct W(#[serde(with = "w")] u8, u8);
+   |                         ^^^ unexpected argument #2 of type `__S`
+   |
+note: function defined here
+  --> tests/ui/with/incorrect_type.rs:9:12
+   |
+9  |     pub fn serialize<T, S: Serializer>(_: S) -> Result<S::Ok, S::Error> {
+   |            ^^^^^^^^^                   ----
+
+error[E0277]: the trait bound `&u8: Serializer` is not satisfied
+  --> tests/ui/with/incorrect_type.rs:15:25
+   |
+15 | struct W(#[serde(with = "w")] u8, u8);
+   |                         ^^^ the trait `Serializer` is not implemented for `&u8`
+   |
+   = help: the following other types implement trait `Serializer`:
+             &'a mut Formatter<'b>
+             FlatMapSerializer<'a, M>
+             _::_serde::__private::ser::content::ContentSerializer<E>
+
+error[E0308]: `?` operator has incompatible types
+  --> tests/ui/with/incorrect_type.rs:15:25
+   |
+15 | struct W(#[serde(with = "w")] u8, u8);
+   |                         ^^^ expected `u8`, found `()`
+   |
+   = note: `?` operator cannot convert from `()` to `u8`
+
+error[E0277]: the trait bound `&u8: Serializer` is not satisfied
+  --> tests/ui/with/incorrect_type.rs:17:10
+   |
+17 | #[derive(Serialize, Deserialize)]
+   |          ^^^^^^^^^ the trait `Serializer` is not implemented for `&u8`
+18 | struct S(#[serde(serialize_with = "w::serialize")] u8, u8);
+   |                                   -------------- required by a bound introduced by this call
+   |
+   = help: the following other types implement trait `Serializer`:
+             &'a mut Formatter<'b>
+             FlatMapSerializer<'a, M>
+             _::_serde::__private::ser::content::ContentSerializer<E>
+note: required by a bound in `w::serialize`
+  --> tests/ui/with/incorrect_type.rs:9:28
+   |
+9  |     pub fn serialize<T, S: Serializer>(_: S) -> Result<S::Ok, S::Error> {
+   |                            ^^^^^^^^^^ required by this bound in `serialize`
+
+error[E0061]: this function takes 1 argument but 2 arguments were supplied
+  --> tests/ui/with/incorrect_type.rs:18:35
+   |
+18 | struct S(#[serde(serialize_with = "w::serialize")] u8, u8);
+   |                                   ^^^^^^^^^^^^^^ unexpected argument #2 of type `__S`
+   |
+note: function defined here
+  --> tests/ui/with/incorrect_type.rs:9:12
+   |
+9  |     pub fn serialize<T, S: Serializer>(_: S) -> Result<S::Ok, S::Error> {
+   |            ^^^^^^^^^                   ----
+
+error[E0277]: the trait bound `&u8: Serializer` is not satisfied
+  --> tests/ui/with/incorrect_type.rs:18:35
+   |
+18 | struct S(#[serde(serialize_with = "w::serialize")] u8, u8);
+   |                                   ^^^^^^^^^^^^^^ the trait `Serializer` is not implemented for `&u8`
+   |
+   = help: the following other types implement trait `Serializer`:
+             &'a mut Formatter<'b>
+             FlatMapSerializer<'a, M>
+             _::_serde::__private::ser::content::ContentSerializer<E>
+
+error[E0308]: `?` operator has incompatible types
+  --> tests/ui/with/incorrect_type.rs:21:37
+   |
+21 | struct D(#[serde(deserialize_with = "w::deserialize")] u8, u8);
+   |                                     ^^^^^^^^^^^^^^^^ expected `u8`, found `()`
+   |
+   = note: `?` operator cannot convert from `()` to `u8`