chore: Improve no instantiation error message

sylvia-derive/src/message.rs

@@ -94,7 +94,10 @@ impl<'a> StructMessage<'a> {
             method
         } else {
             if ty == MsgType::Instantiate {
-                emit_error!(source.span(), "No instantiation message");
+                emit_error!(
+                    source.span(), "Missing instantiation message.";
+                    note = source.span() => "`sylvia::contract` requires exactly one method marked with `#[sv::msg(instantiation)]` attribute."
+                );
             }
             return None;
         };

sylvia/Cargo.toml

@@ -46,6 +46,7 @@ cw-multi-test = { workspace = true }
 cw-storage-plus = { workspace = true }
 cw-utils = { workspace = true }
 thiserror = { workspace = true }
+trybuild = "1.0.91"
 
 [package.metadata.docs.rs]
 all-features = true

sylvia/tests/ui.rs

@@ -0,0 +1,5 @@
+#[test]
+fn ui() {
+    let t = trybuild::TestCases::new();
+    t.compile_fail("tests/ui/**/*.rs");
+}

sylvia/tests/ui/missing_method/instantiation.rs

@@ -0,0 +1,10 @@
+pub struct Contract {}
+
+#[sylvia::contract]
+impl Contract {
+    pub const fn new() -> Self {
+        Self {}
+    }
+}
+
+fn main() {}

sylvia/tests/ui/missing_method/instantiation.stderr

@@ -0,0 +1,8 @@
+error: Missing instantiation message.
+
+         = note: `sylvia::contract` requires exactly one method marked with `#[sv::msg(instantiation)]` attribute.
+
+ --> tests/ui/missing_method/instantiation.rs:4:1
+  |
+4 | impl Contract {
+  | ^^^^