Improve UI test coverage

With this commit I believe I've covered all `compile_error!`-based
errors.

test_suite/tests/ui/transparent/enum.rs

@@ -0,0 +1,8 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)]
+#[serde(transparent)]
+enum E {}
+
+fn main() {}

test_suite/tests/ui/transparent/enum.stderr

@@ -0,0 +1,9 @@
+error: #[serde(transparent)] is not allowed on an enum
+ --> $DIR/enum.rs:5:1
+  |
+5 | / #[serde(transparent)]
+6 | | enum E {}
+  | |_________^
+
+error: aborting due to previous error
+

test_suite/tests/ui/transparent/unit_struct.rs

@@ -0,0 +1,8 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)]
+#[serde(transparent)]
+struct S;
+
+fn main() {}

test_suite/tests/ui/transparent/unit_struct.stderr

@@ -0,0 +1,9 @@
+error: #[serde(transparent)] is not allowed on a unit struct
+ --> $DIR/unit_struct.rs:5:1
+  |
+5 | / #[serde(transparent)]
+6 | | struct S;
+  | |_________^
+
+error: aborting due to previous error
+

test_suite/tests/ui/transparent/with_from.rs

@@ -0,0 +1,10 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)]
+#[serde(transparent, from = "u64")]
+struct S {
+    a: u8,
+}
+
+fn main() {}

test_suite/tests/ui/transparent/with_from.stderr

@@ -0,0 +1,11 @@
+error: #[serde(transparent)] is not allowed with #[serde(from = "...")]
+ --> $DIR/with_from.rs:5:1
+  |
+5 | / #[serde(transparent, from = "u64")]
+6 | | struct S {
+7 | |     a: u8,
+8 | | }
+  | |_^
+
+error: aborting due to previous error
+

test_suite/tests/ui/transparent/with_into.rs

@@ -0,0 +1,10 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)]
+#[serde(transparent, into = "u64")]
+struct S {
+    a: u8,
+}
+
+fn main() {}

test_suite/tests/ui/transparent/with_into.stderr

@@ -0,0 +1,11 @@
+error: #[serde(transparent)] is not allowed with #[serde(into = "...")]
+ --> $DIR/with_into.rs:5:1
+  |
+5 | / #[serde(transparent, into = "u64")]
+6 | | struct S {
+7 | |     a: u8,
+8 | | }
+  | |_^
+
+error: aborting due to previous error
+