Set up some trybuild tests

Cargo.toml

@@ -14,6 +14,8 @@ thiserror-impl = { version = "=1.0.0", path = "impl" }
 
 [dev-dependencies]
 anyhow = "1.0"
+rustversion = "1.0"
+trybuild = "1.0"
 
 [workspace]
 members = ["impl"]

tests/compiletest.rs

@@ -0,0 +1,6 @@
+#[rustversion::attr(not(nightly), ignore)]
+#[test]
+fn ui() {
+    let t = trybuild::TestCases::new();
+    t.compile_fail("tests/ui/*.rs");
+}

tests/ui/duplicate-fmt.rs

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

tests/ui/duplicate-fmt.stderr

@@ -0,0 +1,5 @@
+error: only one #[error(...)] attribute is allowed
+ --> $DIR/duplicate-fmt.rs:5:1
+  |
+5 | #[error("...")]
+  | ^^^^^^^^^^^^^^^

tests/ui/missing-fmt.rs

@@ -0,0 +1,10 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum Error {
+    #[error("...")]
+    A(usize),
+    B(usize),
+}
+
+fn main() {}

tests/ui/missing-fmt.stderr

@@ -0,0 +1,5 @@
+error: missing #[error("...")] display attribute
+ --> $DIR/missing-fmt.rs:7:5
+  |
+7 |     B(usize),
+  |     ^^^^^^^^

tests/ui/union.rs

@@ -0,0 +1,9 @@
+use thiserror::Error;
+
+#[derive(Error)]
+pub union U {
+    msg: &'static str,
+    num: usize,
+}
+
+fn main() {}

tests/ui/union.stderr

@@ -0,0 +1,8 @@
+error: union as errors are not supported
+ --> $DIR/union.rs:4:1
+  |
+4 | / pub union U {
+5 | |     msg: &'static str,
+6 | |     num: usize,
+7 | | }
+  | |_^