[bug][rust][real time diagnostics] the diagnostic is not real time

[binaryoak]

Issue Prelude

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• I have read and understood YCM's [CONTRIBUTING][cont] document.
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• If filing a bug report, I have included the output of vim --version.
• If filing a bug report, I have included the output of :YcmDebugInfo.
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  version) I am using.
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Issue Details

Provide a clear description of the problem, including the following key
questions:
When I start a new workspace with cargo, and coding inside, the diagnostic info is not real time, it only works when a file is open, during the coding, the diagnostic info is not updated automatically, even I ran :ycmDiags manually, the info would not change.

• What did you do?

Include steps to reproduce here.

1. cargo new <your project name>
2. vim src/main.rs
3. Enter insert mode and type some nonsense inside the main function or your own function

Include description of a minimal test case, including any actual code required
to reproduce the issue.
for example:
I typed a p inside a function, but no diagnostic info displayed

Then I ran `:ycmDiags` , still nothing changed

I closed the current file, and opened it again, diagnostic was there

If you made changes to vimrc_ycm_minimal, pase them here:

<contents of ycm_vimrc_minimal>

• What did you expect to happen?
  The diagnostic info should be updated/refreshed automatically, at least, when :ycmDiags is ran manually, it should be updated.

Include description of the expected behaviour.

• What actually happened?
  The diagnostic info only got updated when a file was opened, after that, no matter what changes were made, the diagnostic info wound't change until the file was closed and reopen again.

Include description of the observed behaviour, including actual output,
screenshots, etc.

Diagnostic data

Output of vim --version

VIM - Vi IMproved 8.2 (2019 Dec 12, compiled Jan 24 2021 12:49:30)
macOS version - x86_64
Included patches: 1-2400
Compiled by Homebrew
Huge version without GUI.  Features included (+) or not (-):
+acl               -farsi             +mouse_sgr         +tag_binary
+arabic            +file_in_path      -mouse_sysmouse    -tag_old_static
+autocmd           +find_in_path      +mouse_urxvt       -tag_any_white
+autochdir         +float             +mouse_xterm       -tcl
-autoservername    +folding           +multi_byte        +termguicolors
-balloon_eval      -footer            +multi_lang        +terminal
+balloon_eval_term +fork()            -mzscheme          +terminfo
-browse            +gettext           +netbeans_intg     +termresponse
++builtin_terms    -hangul_input      +num64             +textobjects
+byte_offset       +iconv             +packages          +textprop
+channel           +insert_expand     +path_extra        +timers
+cindent           +ipv6              +perl              +title
-clientserver      +job               +persistent_undo   -toolbar
+clipboard         +jumplist          +popupwin          +user_commands
+cmdline_compl     +keymap            +postscript        +vartabs
+cmdline_hist      +lambda            +printer           +vertsplit
+cmdline_info      +langmap           +profile           +virtualedit
+comments          +libcall           -python            +visual
+conceal           +linebreak         +python3           +visualextra
+cryptv            +lispindent        +quickfix          +viminfo
+cscope            +listcmds          +reltime           +vreplace
+cursorbind        +localmap          +rightleft         +wildignore
+cursorshape       +lua               +ruby              +wildmenu
+dialog_con        +menu              +scrollbind        +windows
+diff              +mksession         +signs             +writebackup
+digraphs          +modify_fname      +smartindent       -X11
-dnd               +mouse             -sound             -xfontset
-ebcdic            -mouseshape        +spell             -xim
+emacs_tags        +mouse_dec         +startuptime       -xpm
+eval              -mouse_gpm         +statusline        -xsmp
+ex_extra          -mouse_jsbterm     -sun_workshop      -xterm_clipboard
+extra_search      +mouse_netterm     +syntax            -xterm_save
   system vimrc file: "$VIM/vimrc"
     user vimrc file: "$HOME/.vimrc"
 2nd user vimrc file: "~/.vim/vimrc"
      user exrc file: "$HOME/.exrc"
       defaults file: "$VIMRUNTIME/defaults.vim"
  fall-back for $VIM: "/usr/local/share/vim"
Compilation: clang -c -I. -Iproto -DHAVE_CONFIG_H -DMACOS_X -DMACOS_X_DARWIN -g -O2 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=1
Linking: clang -L. -fstack-protector-strong -L/usr/local/lib -L/usr/local/opt/libyaml/lib -L/usr/local/opt/[email protected]/lib -L/usr/local/opt/readline/lib -L/usr/local/lib -o vim -lm -lncurses -liconv -lintl -framework AppKit -L/usr/local/opt/lua/lib -llua5.4 -mmacosx-version-min=11.1 -fstack-protector-strong -L/usr/local/lib -L/usr/local/Cellar/perl/5.32.1/lib/perl5/5.32.1/darwin-thread-multi-2level/CORE -lperl -L/usr/local/opt/[email protected]/Frameworks/Python.framework/Versions/3.9/lib/python3.9/config-3.9-darwin -lpython3.9 -framework CoreFoundation -lruby.3.0 -L/usr/local/Cellar/ruby/3.0.0_1/lib

Output of YcmDebugInfo

Printing YouCompleteMe debug information...
-- Resolve completions: Up front
-- Client logfile: /var/folders/0t/9yws28hx1rg576_2nvtwhxjm0000gn/T/ycm_379p09a3.log
-- Server Python interpreter: /usr/local/opt/[email protected]/bin/python3.9
-- Server Python version: 3.9.1
-- Server has Clang support compiled in: True
-- Clang version: clang version 11.0.0 (https://github.com/llvm/llvm-project.git 176249bd6732a8044d457092ed932768724a6f06)
-- No extra configuration file found
-- Rust completer debug information:
--   Rust Language Server running
--   Rust Language Server process ID: 6850
--   Rust Language Server executable: ['/Users/chengcan.he/.vim/bundle/youcompleteme/third_party/ycmd/third_party/rust-analyzer/bin/rust-analyzer']
--   Rust Language Server logfiles:
--     /var/folders/0t/9yws28hx1rg576_2nvtwhxjm0000gn/T/rust_language_server_stderrml9abgcr.log
--   Rust Language Server Server State: Initialized
--   Rust Language Server Project Directory: /Users/chengcan.he/rust_prog/practice
--   Rust Language Server Settings: {}
--   Rust Language Server Project State: ready
--   Rust Language Server Version: 0d03fe6
--   Rust Language Server Rust Root: /Users/chengcan.he/.vim/bundle/youcompleteme/third_party/ycmd/third_party/rust-analyzer
-- Server running at: http://127.0.0.1:62521
-- Server process ID: 6845
-- Server logfiles:
--   /var/folders/0t/9yws28hx1rg576_2nvtwhxjm0000gn/T/ycmd_62521_stdout_n7zfw261.log
--   /var/folders/0t/9yws28hx1rg576_2nvtwhxjm0000gn/T/ycmd_62521_stderr_uzo8riur.log

Output of YcmDiags

Paste output here

Output of git rev-parse HEAD in YouCompleteMe installation directory

ed423e8a1d2a5842a126d33b824ad3b65f85f3ba

OS version, distribution, etc.

Include system information here.
macOs big sur, 11.2.3

[puremourning]

rust-lang/rust-analyzer#4185