Error Checking not working

Start by checking how rust.vim integrates with Syntastic and its Cargo checker; the report names g:syntastic_rust_checkers = ['cargo'], cargo, and rustc, but no relevant source files or tests. Reproduce the missing-const case with the reported setup and determine whether Syntastic runs the Cargo checker for that Rust file. Done means the compiler error appears in Vim, or the issue identifies why the configured checker does not report it.

• rust.vim version: the latest version as of today †

† I honestly couldn't find where the version was. I started by looking at the local vim plugin files, then grep'ing those files, then moving on to searching the github repo and nowhere was there a reported version. I looked under GitHub 'releases' for this project, nothing. I even checked the commit tags. You'll tell me where to find it and I'm likely going to kick myself 🙂

I'm using vim-plug:

call plug#begin('~/.vim/plugged')
Plug 'rust-lang/rust.vim'
call plug#end()

let g:rustfmt_autosave = 1
let g:syntastic_rust_checkers = ['cargo']

Steps to reproduce:

I was given a rust file that defined a const. I purposely deleted the const but left the reference to it in the code.

Expected vs. actual behavior:

I expected the vim plugin to display an error, such as "your code references something that no longer is being defined" (I'm new to rust so I would have expected something like that from the rust compiler). But the actual behaviour was that nothing happened.

Debugging Info:

lol :RustInfoToClipboard totally didn't work for me 🤷‍♂️ so I had to manually copy from shell.

rust.vim Global Variables:
let g:ftplugin_rust_source_path = v:null
let g:loaded_syntastic_rust_cargo_checker = v:null
let g:loaded_syntastic_rust_filetype = v:null
let g:loaded_syntastic_rust_rustc_checker = v:null
let g:rust_bang_comment_leader = v:null
let g:rust_cargo_avoid_whole_workspace = v:null
let g:rust_clip_command = v:null
let g:rust_conceal = v:null
let g:rust_conceal_mod_path = v:null
let g:rust_conceal_pub = v:null
let g:rust_fold = v:null
let g:rust_last_args = v:null
let g:rust_last_rustc_args = v:null
let g:rust_original_delimitMate_excluded_regions = v:null
let g:rust_playpen_url = v:null
let g:rust_prev_delimitMate_quotes = v:null
let g:rust_recent_nearest_cargo_tol = v:null
let g:rust_recent_root_cargo_toml = v:null
let g:rust_recommended_style = v:null
let g:rust_set_conceallevel = v:null
let g:rust_set_conceallevel=1 = v:null
let g:rust_set_foldmethod = v:null
let g:rust_set_foldmethod=1 = v:null
let g:rust_shortener_url = v:null
let g:rustc_makeprg_no_percent = v:null
let g:rustc_path = v:null
let g:rustfmt_autosave = 1
let g:rustfmt_autosave_if_config_present = v:null
let g:rustfmt_command = 'rustfmt'
let g:rustfmt_emit_files = 1
let g:rustfmt_fail_silently = 0
let g:rustfmt_options = ''
let g:syntastic_extra_filetypes = ['rust']
let g:syntastic_rust_cargo_fname = v:null
rustfmt 1.4.12-stable (a828ffea 2020-03-11)
rustc 1.43.0 (4fb7144ed 2020-04-20)
cargo 1.43.0 (3532cf738 2020-03-17)

VIM - Vi IMproved 8.2 (2019 Dec 12, compiled Jun 12 2020 17:04:06)
macOS version
Included patches: 1-960
Compiled by markmcdonnell@MarkMcdonnell-MBP
Huge version without GUI.  Features included (+) or not (-):
+acl               +clipboard         -dnd               +gettext           +listcmds          -mouse_sysmouse    +postscript        +spell             +textobjects       +wildignore
+arabic            +cmdline_compl     -ebcdic            -hangul_input      +localmap          +mouse_urxvt       +printer           +startuptime       +textprop          +wildmenu
+autocmd           +cmdline_hist      +emacs_tags        +iconv             -lua               +mouse_xterm       +profile           +statusline        +timers            +windows
+autochdir         +cmdline_info      +eval              +insert_expand     +menu              +multi_byte        -python            -sun_workshop      +title             +writebackup
-autoservername    +comments          +ex_extra          +ipv6              +mksession         +multi_lang        +python3           +syntax            -toolbar           -X11
-balloon_eval      +conceal           +extra_search      +job               +modify_fname      -mzscheme          +quickfix          +tag_binary        +user_commands     -xfontset
+balloon_eval_term +cryptv            -farsi             +jumplist          +mouse             +netbeans_intg     +reltime           -tag_old_static    +vartabs           -xim
-browse            +cscope            +file_in_path      +keymap            -mouseshape        +num64             +rightleft         -tag_any_white     +vertsplit         -xpm
++builtin_terms    +cursorbind        +find_in_path      +lambda            +mouse_dec         +packages          +ruby              -tcl               +virtualedit       -xsmp
+byte_offset       +cursorshape       +float             +langmap           -mouse_gpm         +path_extra        +scrollbind        +termguicolors     +visual            -xterm_clipboard