Introduce pmp-macro-core crate: implement attribute parsing, macro registration, and boilerplate generation for PmpMacro trait.

crates/pmp-core/src/attrs.rs

@@ -1,4 +1,4 @@
-use darling::{ FromMeta };
+use darling::FromMeta;
 
 /// Implemented by any item that carries a `Vec<syn::Attribute>`.
 ///

crates/pmp-core/src/types.rs

@@ -65,3 +65,21 @@ impl FromMeta for StrictPath
         Path::from_expr(expr).map(StrictPath)
     }
 }
+
+/// A Rust expression that must be written as `= <expr>`
+#[derive(Debug, Clone)]
+pub struct StrictExpr(pub syn::Expr);
+
+impl FromMeta for StrictExpr
+{
+    fn from_word() -> Result<Self>
+    {
+        Err(Error::custom(
+            "expected `= <expr>`, provide an expression, e.g. `target = Target::CLASS | Target::INTERFACE`"
+        ))
+    }
+
+    fn from_expr(expr: &syn::Expr) -> Result<Self> {
+        Ok(StrictExpr(expr.clone()))
+    }
+}