Add trybuild tests for transparent feature

tests/ui/duplicate-transparent.rs

@@ -0,0 +1,8 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error(transparent)]
+#[error(transparent)]
+pub struct Error(anyhow::Error);
+
+fn main() {}

tests/ui/duplicate-transparent.stderr

@@ -0,0 +1,5 @@
+error: duplicate #[error(transparent)] attribute
+ --> $DIR/duplicate-transparent.rs:5:1
+  |
+5 | #[error(transparent)]
+  | ^^^^^^^^^^^^^^^^^^^^^

tests/ui/transparent-display.rs

@@ -0,0 +1,8 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error(transparent)]
+#[error("...")]
+pub struct Error(anyhow::Error);
+
+fn main() {}

tests/ui/transparent-display.stderr

@@ -0,0 +1,5 @@
+error: cannot have both #[error(transparent)] and a display attribute
+ --> $DIR/transparent-display.rs:5:1
+  |
+5 | #[error("...")]
+  | ^^^^^^^^^^^^^^^

tests/ui/transparent-enum-many.rs

@@ -0,0 +1,9 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum Error {
+    #[error(transparent)]
+    Other(anyhow::Error, String),
+}
+
+fn main() {}

tests/ui/transparent-enum-many.stderr

@@ -0,0 +1,6 @@
+error: #[error(transparent)] requires exactly one field
+ --> $DIR/transparent-enum-many.rs:5:5
+  |
+5 | /     #[error(transparent)]
+6 | |     Other(anyhow::Error, String),
+  | |________________________________^

tests/ui/transparent-enum-source.rs

@@ -0,0 +1,9 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum Error {
+    #[error(transparent)]
+    Other(#[source] anyhow::Error),
+}
+
+fn main() {}

tests/ui/transparent-enum-source.stderr

@@ -0,0 +1,5 @@
+error: transparent variant can't contain #[source]
+ --> $DIR/transparent-enum-source.rs:6:11
+  |
+6 |     Other(#[source] anyhow::Error),
+  |           ^^^^^^^^^

tests/ui/transparent-struct-many.rs

@@ -0,0 +1,10 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error(transparent)]
+pub struct Error {
+    inner: anyhow::Error,
+    what: String,
+}
+
+fn main() {}

tests/ui/transparent-struct-many.stderr

@@ -0,0 +1,5 @@
+error: #[error(transparent)] requires exactly one field
+ --> $DIR/transparent-struct-many.rs:4:1
+  |
+4 | #[error(transparent)]
+  | ^^^^^^^^^^^^^^^^^^^^^

tests/ui/transparent-struct-source.rs

@@ -0,0 +1,7 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error(transparent)]
+pub struct Error(#[source] anyhow::Error);
+
+fn main() {}

tests/ui/transparent-struct-source.stderr

@@ -0,0 +1,5 @@
+error: transparent error struct can't contain #[source]
+ --> $DIR/transparent-struct-source.rs:5:18
+  |
+5 | pub struct Error(#[source] anyhow::Error);
+  |                  ^^^^^^^^^