Improve error message on remote derive duplicate generics

serde_derive/Cargo.toml

@@ -24,7 +24,7 @@ proc-macro = true
 [dependencies]
 proc-macro2 = "1.0"
 quote = "1.0"
-syn = "1.0.90"
+syn = "1.0.104"
 
 [dev-dependencies]
 serde = { version = "1.0", path = "../serde" }

serde_derive/src/internals/check.rs

@@ -6,6 +6,7 @@ use syn::{Member, Type};
 /// Cross-cutting checks that require looking at more than a single attrs
 /// object. Simpler checks should happen when parsing and building the attrs.
 pub fn check(cx: &Ctxt, cont: &mut Container, derive: Derive) {
+    check_remote_generic(cx, cont);
     check_getter(cx, cont);
     check_flatten(cx, cont);
     check_identifier(cx, cont);
@@ -16,6 +17,28 @@ pub fn check(cx: &Ctxt, cont: &mut Container, derive: Derive) {
     check_from_and_try_from(cx, cont);
 }
 
+/// Remote derive definition type must have either all of the generics of the
+/// remote type:
+///
+///     #[serde(remote = "Generic")]
+///     struct Generic<T> {…}
+///
+/// or none of them, i.e. defining impls for one concrete instantiation of the
+/// remote type only:
+///
+///     #[serde(remote = "Generic<T>")]
+///     struct ConcreteDef {…}
+///
+fn check_remote_generic(cx: &Ctxt, cont: &Container) {
+    if let Some(remote) = cont.attrs.remote() {
+        let local_has_generic = !cont.generics.params.is_empty();
+        let remote_has_generic = !remote.segments.last().unwrap().arguments.is_none();
+        if local_has_generic && remote_has_generic {
+            cx.error_spanned_by(remote, "remove generic parameters from this path");
+        }
+    }
+}
+
 /// Getters are only allowed inside structs (not enums) with the `remote`
 /// attribute.
 fn check_getter(cx: &Ctxt, cont: &Container) {

serde_derive_internals/Cargo.toml

@@ -17,7 +17,7 @@ path = "lib.rs"
 [dependencies]
 proc-macro2 = "1.0"
 quote = "1.0"
-syn = { version = "1.0.90", default-features = false, features = ["derive", "parsing", "printing", "clone-impls"] }
+syn = { version = "1.0.104", default-features = false, features = ["derive", "parsing", "printing", "clone-impls"] }
 
 [package.metadata.docs.rs]
 targets = ["x86_64-unknown-linux-gnu"]

test_suite/tests/ui/remote/double_generic.stderr

@@ -1,52 +1,5 @@
-error: expected one of `:`, `@`, or `|`, found `>`
-  --> tests/ui/remote/double_generic.rs:12:19
-   |
-12 | struct StructDef<U> {
-   |                   ^ expected one of `:`, `@`, or `|`
-
-error: expected one of `!`, `)`, `,`, or `::`, found `<`
-  --> tests/ui/remote/double_generic.rs:12:17
-   |
-11 | #[serde(remote = "remote::StructGeneric<u8>")]
-   |                                             -
-   |                                             |
-   |                                             expected one of `!`, `)`, `,`, or `::`
-   |                                             help: missing `,`
-12 | struct StructDef<U> {
-   |                 ^ unexpected token
-
-error: proc-macro derive produced unparseable tokens
-  --> tests/ui/remote/double_generic.rs:10:10
-   |
-10 | #[derive(Serialize, Deserialize)]
-   |          ^^^^^^^^^
-
-error: expected one of `!`, `+`, `,`, `::`, or `>`, found `<`
-  --> tests/ui/remote/double_generic.rs:12:17
-   |
-10 | #[derive(Serialize, Deserialize)]
-   |                     -----------
-   |                     |
-   |                     while parsing this item list starting here
-   |                     the item list ends here
-11 | #[serde(remote = "remote::StructGeneric<u8>")]
-   |                                             - expected one of `!`, `+`, `,`, `::`, or `>`
-12 | struct StructDef<U> {
-   |                 ^ unexpected token
-   |
-help: you might have meant to end the type parameters here
-   |
-11 | #[serde(remote = "remote::StructGeneric<u8>">)]
-   |                                             +
-
-error: proc-macro derive produced unparseable tokens
-  --> tests/ui/remote/double_generic.rs:10:21
-   |
-10 | #[derive(Serialize, Deserialize)]
-   |                     ^^^^^^^^^^^
-
-error[E0412]: cannot find type `StructGeneric` in module `remote`
+error: remove generic parameters from this path
   --> tests/ui/remote/double_generic.rs:11:18
    |
 11 | #[serde(remote = "remote::StructGeneric<u8>")]
-   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^ not found in `remote`
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^