Add regression test for issue 1718

    error[E0502]: cannot borrow `generics.params` as mutable because it is also borrowed as immutable
      --> tests/test_iterators.rs:81:25
       |
    77 |       let _ = generics
       |               --------
       |               |
       |  _____________immutable borrow occurs here
       | |
    78 | |         .lifetimes()
       | |____________________- a temporary with access to the immutable borrow is created here ...
    ...
    81 |           .unwrap_or_else(|| {
       |                           ^^ mutable borrow occurs here
    82 |               let lifetime: Lifetime = parse_quote!('a);
    83 |               generics.params.insert(
       |               --------------- second borrow occurs due to use of `generics.params` in closure
    ...
    88 |           });
       |             - ... and the immutable borrow might be used here, when that temporary is dropped and runs the destructor for type `impl Iterator<Item = &LifetimeParam>`

tests/test_iterators.rs

@@ -1,7 +1,7 @@
-#![allow(clippy::uninlined_format_args)]
+#![allow(clippy::map_unwrap_or, clippy::uninlined_format_args)]
 
 use syn::punctuated::{Pair, Punctuated};
-use syn::Token;
+use syn::{parse_quote, GenericParam, Generics, Lifetime, LifetimeParam, Token};
 
 #[macro_use]
 mod macros;
@@ -68,3 +68,22 @@ fn may_dangle() {
         }
     }
 }
+
+// Regression test for https://github.com/dtolnay/syn/issues/1718
+#[test]
+fn no_opaque_drop() {
+    let mut generics = Generics::default();
+
+    let _ = generics
+        .lifetimes()
+        .next()
+        .map(|param| param.lifetime.clone())
+        .unwrap_or_else(|| {
+            let lifetime: Lifetime = parse_quote!('a);
+            generics.params.insert(
+                0,
+                GenericParam::Lifetime(LifetimeParam::new(lifetime.clone())),
+            );
+            lifetime
+        });
+}