Smol improvements to new trybuild tests

.cargo/config.toml

@@ -0,0 +1,4 @@
+[term]
+# Set `color` to `auto` in case the user has set it to `always`, which would
+# cause divergence in compiler output and break the trybuild tests.
+color = "auto"

tree_hash/tests/compile_fail/duplicate_variant_selector.rs

@@ -0,0 +1,11 @@
+//! More than one selector in a variant-level `tree_hash` attribute must be rejected.
+use tree_hash_derive::TreeHash;
+
+#[derive(TreeHash)]
+#[tree_hash(enum_behaviour = "compatible_union")]
+enum Foo {
+    #[tree_hash(selector = "1", selector = "2")]
+    A(u8),
+}
+
+fn main() {}

tree_hash/tests/compile_fail/duplicate_variant_selector.stderr

@@ -0,0 +1,7 @@
+error: proc-macro derive panicked
+ --> tests/compile_fail/duplicate_variant_selector.rs:4:10
+  |
+4 | #[derive(TreeHash)]
+  |          ^^^^^^^^
+  |
+  = help: message: failed to parse variant attribute for "A": Duplicate field `selector`; note that `selector` must be an integer in 1..=127