Can't build with error "error[E0658]: `let` expressions in this position are unstable"

[mengzhuo]

Describe the bug
on master

rustc --version rustc 1.85.0 (4d91de4e4 2025-02-17) (built from a source tarball)

runs cargo build

   Compiling neli v0.7.1
   Compiling phantun v0.8.1 (/root/phantun/phantun)
error[E0658]: `let` expressions in this position are unstable
   --> phantun/src/bin/server.rs:223:48
    |
223 | ...                   && let Err(e) = udp_sock.send(&buf_tcp[..size]).await {
    |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information

error[E0658]: `let` expressions in this position are unstable
   --> phantun/src/bin/client.rs:275:48
    |
275 | ...                   && let Err(e) = udp_sock.send(&buf_tcp[..size]).await {
    |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information

For more information about this error, try `rustc --explain E0658`.
error: could not compile `phantun` (bin "server") due to 1 previous error
warning: build failed, waiting for other jobs to finish...
error: could not compile `phantun` (bin "client") due to 1 previous error

Operating system
Linux/Debian(13)