Add tests of #[serde(default)] attribute for units and unions

Author: Mingun

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")]
+  |         ^^^^^^^^^^^^^^^^^^^^^