Method downcast_mut

crates/core/src/error/error.rs:1105–1138

Downcast this error into an exclusive `&mut E` borrow. If this error is an `E`, then `Some(&mut E)` is returned. Otherwise, `None` is returned. If there are multiple instances of `E` in this error's chain, then the first (as encountered by [`Error::chain`]'s iteration order) is returned. # Example ``` # use wasmtime_internal_core as wasmtime; use wasmtime::error::{Error, OutOfMemory}; let mut

(&mut self)

    /// );
    /// ```
    pub fn downcast_mut<E>(&mut self) -> Option<&mut E>
    where
        E: fmt::Display + fmt::Debug + Send + Sync + 'static,
    {
        self.error_ext_chain_mut(|e| {
            {
                // Safety: We are running into "problem case #3" of non-lexical
                // lifetimes: we are returning a mutable reference, which must
                // be live for the whole function's scope, but are accessing it
                // again below in the `anyhow` downcast. This is safe because
                // control flow ensures that the original borrow is not actually
                // active anymore if we didn't return.
                //
                // https://github.com/rust-lang/rfcs/blob/master/text/2094-nll.md#problem-case-3-conditional-control-flow-across-functions
                let e = unsafe { NonNull::from(&mut *e).as_mut() };
                if let Some(r) = e.inner.downcast_mut::<E>() {
                    return Some(r);
                }
            }

            #[cfg(feature = "anyhow")]
            {
                if let Some(result) = e
                    .inner
                    .downcast_mut::<anyhow::Error>()
                    .and_then(|anyhow| anyhow.downcast_mut::<E>())
                {
                    return Some(result);
                }
            }

            None
        })
    }

    /// Convert this error into a `Box<dyn core::error::Error>`.
    ///