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.rs

@@ -0,0 +1,17 @@
+use serde_derive::{Deserialize, Serialize};
+
+mod remote {
+    pub struct Struct<T, U> {
+        pub t: T,
+        pub u: U,
+    }
+}
+
+#[derive(Serialize, Deserialize)]
+#[serde(remote = "remote::StructGeneric<u8>")]
+struct StructDef<U> {
+    t: u8,
+    u: U,
+}
+
+fn main() {}

test_suite/tests/ui/remote/double_generic.stderr

@@ -0,0 +1,5 @@
+error: remove generic parameters from this path
+  --> tests/ui/remote/double_generic.rs:11:18
+   |
+11 | #[serde(remote = "remote::StructGeneric<u8>")]
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^