When talking about Emacs configuration, the rust-analyzer manual mentions

[...] this excellent guide from @rksm.

Which I followed to good effect, in particular:

  (use-package lsp-mode
    :commands lsp
    ;; what to use when checking on-save. "check" is default,
    ;; I prefer clippy.
    (lsp-rust-analyzer-cargo-watch-command "clippy")
    (lsp-eldoc-render-all t)
    (lsp-idle-delay 0.6)
    (lsp-rust-analyzer-server-display-inlay-hints t)
    (lsp-rust-analyzer-inlay-face ((t (:foreground "dim gray"))))
    (add-hook 'lsp-mode-hook 'lsp-ui-mode))

  (use-package lsp-ui
    :commands lsp-ui-mode
    (lsp-ui-peek-always-show t)
    (lsp-ui-sideline-show-hover nil) ; I found this confusing
    (lsp-ui-doc-enable nil))

However when having an async fn in my code, for example as suggested by the rocket.rs guide,

Rust analyzer is complaining about:

error[E0670]: `async fn` is not permitted in Rust 2015
  --> .../hello-rocket/src/main.rs:16:28
16 | #[get("/delay/<seconds>")] async fn delay(seconds: u64) -> String {
   |                            ^^^^^ to use `async fn`, switch to Rust 2018 or later
   = help: set `edition = "2018"` in `Cargo.toml`
   = note: for more on editions, read https://doc.rust-lang.org/edition-guide

despite the fact that I already have in my Cargo.toml:

edition = "2018"

My code compiles just fine, and rust-analyzer diagnostics $PWD seems to complete without errors.

So I guess my question is: How should I tell rust-analyzer via lsp-rust-analyzer to consider my Cargo.toml for the value of edition?

  • Try to use the equivalent raw identifier syntax: r#async - from doc.rust-lang.org/stable/edition-guide/editions/index.html.
    – Ian
    Commented Oct 6, 2021 at 13:46
  • @Ian, this doesn't makes it any better. In fact after adding r#async this no longer compiles. As noted above the original code compiles without errors (or warnings), but lsp-rust-analyzer still complains. So it seems like an Emacs configuration issue, not a Rust code issue.
    – Chen Levy
    Commented Oct 6, 2021 at 16:11

1 Answer 1


It took me some time to realize that rust-analyzer was not the one to complain, but the buffer with the error message mentioned above was *rustfmt*, and indeed running rustfmt on my source file gave me the same error.

As suggested in this (inexplicably closed) issue, one can convince rustfmt to behave by adding edition = "2018" to .rustfmt.toml.

Since I am not expecting to hack on 2015 edition code any time soon, I chosen a different (and more Emacsy) workaround:

  (use-package rustic
    ;; ...
    (rustic-rustfmt-config-alist '((edition . "2018")))
    ;; ...

Hopefully, I will find a better solution by the time the 2021 edition will come to fruition.

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