add #[allow(deprecated)] to derive implementations

Allow deprecated in the `Serialize`/`Deserialize`
derive implementations. This allows you to
deprecate structs, enums, struct fields, or enum
variants and not get compiler warnings/errors
about use of deprecated thing. We only do this
if `#[deprecated]` or `#[allow(deprecated)]` exist
on the root object or the variants of the root
object (if it is an enum).

Resolves #2195

test_suite/tests/deprecated.rs

@@ -0,0 +1,34 @@
+#![deny(deprecated)]
+#![allow(dead_code)]
+
+use serde_derive::{Deserialize, Serialize};
+
+/// deprecated enum
+#[derive(Serialize, Deserialize)]
+#[deprecated]
+enum E1 {
+    A,
+    B,
+}
+
+/// deprecated struct
+#[derive(Serialize, Deserialize)]
+#[deprecated]
+struct S1 {
+    a: bool,
+}
+
+/// deprecated enum variant
+#[derive(Serialize, Deserialize)]
+enum E2 {
+    A,
+    #[deprecated]
+    B,
+}
+
+/// deprecated struct field
+#[derive(Serialize, Deserialize)]
+struct S2 {
+    #[deprecated]
+    a: bool,
+}

test_suite/tests/ui/deprecated/deprecated_de_with.rs

@@ -0,0 +1,20 @@
+#![deny(deprecated)]
+
+use serde::Deserializer;
+use serde_derive::Deserialize;
+
+#[derive(Deserialize)]
+pub struct Struct {
+    #[serde(deserialize_with = "deprecated_with")]
+    pub field: i32,
+}
+
+#[deprecated]
+fn deprecated_with<'de, D>(_deserializer: D) -> Result<i32, D::Error>
+where
+    D: Deserializer<'de>,
+{
+    unimplemented!()
+}
+
+fn main() {}

test_suite/tests/ui/deprecated/deprecated_de_with.stderr

@@ -0,0 +1,11 @@
+error: use of deprecated function `deprecated_with`
+ --> tests/ui/deprecated/deprecated_de_with.rs:8:32
+  |
+8 |     #[serde(deserialize_with = "deprecated_with")]
+  |                                ^^^^^^^^^^^^^^^^^
+  |
+note: the lint level is defined here
+ --> tests/ui/deprecated/deprecated_de_with.rs:1:9
+  |
+1 | #![deny(deprecated)]
+  |         ^^^^^^^^^^

test_suite/tests/ui/deprecated/deprecated_ser_with.rs

@@ -0,0 +1,20 @@
+#![deny(deprecated)]
+
+use serde::Serializer;
+use serde_derive::Serialize;
+
+#[derive(Serialize)]
+pub struct Struct {
+    #[serde(serialize_with = "deprecated_with")]
+    pub field: i32,
+}
+
+#[deprecated]
+fn deprecated_with<S>(_field: &i32, _serializer: S) -> Result<S::Ok, S::Error>
+where
+    S: Serializer,
+{
+    unimplemented!()
+}
+
+fn main() {}

test_suite/tests/ui/deprecated/deprecated_ser_with.stderr

@@ -0,0 +1,11 @@
+error: use of deprecated function `deprecated_with`
+ --> tests/ui/deprecated/deprecated_ser_with.rs:8:30
+  |
+8 |     #[serde(serialize_with = "deprecated_with")]
+  |                              ^^^^^^^^^^^^^^^^^
+  |
+note: the lint level is defined here
+ --> tests/ui/deprecated/deprecated_ser_with.rs:1:9
+  |
+1 | #![deny(deprecated)]
+  |         ^^^^^^^^^^