Implement Group derive proc macro

cove-macro/src/util.rs

@@ -0,0 +1,33 @@
+use syn::{Expr, ExprLit, Field, Lit, LitStr};
+
+pub fn strlit(expr: &Expr) -> Option<&LitStr> {
+    match expr {
+        Expr::Lit(ExprLit {
+            lit: Lit::Str(lit), ..
+        }) => Some(lit),
+        _ => None,
+    }
+}
+
+/// Given a struct field, this finds all attributes like `#[doc = "bla"]`,
+/// unindents, concatenates and returns them.
+pub fn docstring(field: &Field) -> syn::Result<String> {
+    let mut lines = vec![];
+
+    for attr in field
+        .attrs
+        .iter()
+        .filter(|attr| attr.path().is_ident("doc"))
+    {
+        if let Some(lit) = strlit(&attr.meta.require_name_value()?.value) {
+            let value = lit.value();
+            let value = value
+                .strip_prefix(' ')
+                .map(|value| value.to_string())
+                .unwrap_or(value);
+            lines.push(value);
+        }
+    }
+
+    Ok(lines.join("\n"))
+}