The problem lies in the second line. The first argument to \inline{push} is an \inline{&mut self}, and so the compiler implicitly borrows from \inline{v}. But then another (shared) reference is created for the call to \inline{len}. This is not allowed, and a reborrow doesn’t help either since the \inline{&mut self} is currently used. On the other hand, it is clear that the call to \inline{len} will definitely return before the unique reference is ever accessed. In fact, we can work around this problem if we save the result of \inline{v.len()} in a temporary variable, as shown in \autoref{lst:two-phase-borrow-workaround}.
\begin{lstlisting}[language=Rust, caption={Borrow twice in one method call}, label={lst:two-phase-borrow-workaround}]
\begin{lstlisting}[language=Rust, caption={Workaround for \autoref{lst:two-phase-borrow}}, label={lst:two-phase-borrow-workaround}]
let mut v = Vec::new();
let vlen = v.len();
v.push(vlen);
...
...
@@ -469,7 +469,7 @@ This pattern—calling a method with a unique reference but also using shared re
Right now, two-phase borrowing works only for method calls where the first argument is \inline{&mut self}, and it is not resolved if generalized \ac{TPB} should even be supported. \todo{Point to Issue \#49434 for generalized TPB.}
