`racer-complete-at-point` fails even when `$ racer complete use std::io::B` works from terminal

I feel like there is a missed opportunity at emacs-racer working automagically without extra install steps.

From the terminal $ racer complete use std::io::B works just fine, even though I did not explicitly install Rust sources using $ rustup component add rust-src. However, from Emacs I get the following error when trying to complete:

completion--some: No such directory: /usr/local/Cellar/rust/1.34.0/lib/rustlib/src/rust/src. Please set ‘racer-rust-src-path’ or ‘RUST_SRC_PATH’.

My feeling that there is a missed opportunity stems from the fact that, when I remove the check causing this error from racer--call, I get the right completions.

I changed:

(defun racer--call (command &rest args)
  "Call racer command COMMAND with args ARGS.
Return stdout if COMMAND exits normally, otherwise show an
error."
  (let ((rust-src-path (or (when racer-rust-src-path (expand-file-name racer-rust-src-path))
                           (getenv "RUST_SRC_PATH")))
        (cargo-home (or (when racer-cargo-home (expand-file-name racer-cargo-home))
                        (getenv "CARGO_HOME"))))
    (when (null rust-src-path)
      (user-error "You need to set `racer-rust-src-path' or `RUST_SRC_PATH'"))
    (unless (file-exists-p rust-src-path)
      (user-error "No such directory: %s. Please set `racer-rust-src-path' or `RUST_SRC_PATH'"
                  rust-src-path))
    (let ((default-directory (or (racer--cargo-project-root) default-directory))
          (process-environment (append (list
                                        (format "RUST_SRC_PATH=%s" rust-src-path)
                                        (format "CARGO_HOME=%s" cargo-home))
                                       process-environment)))
      (-let [(exit-code stdout _stderr)
             (racer--shell-command racer-cmd (cons command args))]
        ;; Use `equal' instead of `zero' as exit-code can be a string
        ;; "Aborted" if racer crashes.
        (unless (equal 0 exit-code)
          (user-error "%s exited with %s. `M-x racer-debug' for more info"
                      racer-cmd exit-code))
        stdout))))

to:

(defun racer--call (command &rest args)
  "Call racer command COMMAND with args ARGS.
Return stdout if COMMAND exits normally, otherwise show an
error."
  (let ((rust-src-path (or (when racer-rust-src-path (expand-file-name racer-rust-src-path))
                           (getenv "RUST_SRC_PATH")))
        (cargo-home (or (when racer-cargo-home (expand-file-name racer-cargo-home))
                        (getenv "CARGO_HOME"))))
    (let ((default-directory (or (racer--cargo-project-root) default-directory)))
      (-let [(exit-code stdout _stderr)
             (racer--shell-command racer-cmd (cons command args))]
        ;; Use `equal' instead of `zero' as exit-code can be a string
        ;; "Aborted" if racer crashes.
        (unless (equal 0 exit-code)
          (user-error "%s exited with %s. `M-x racer-debug' for more info"
                      racer-cmd exit-code))
        stdout))))

and started getting the right completions when pressing after use std::io::B.

I am not sure what to think about it as I don't have global knowledge about Racer, Emacs-racer, and even about Rust as I am a beginner.

I need insight from someone more experienced with this codebase. Am I right in thinking that there is room for improvement ? If so, how, and would there be tradeoffs ?

racer-rust / emacs-racer

`racer-complete-at-point` fails even when `$ racer complete use std::io::B` works from terminal #134