Make cargo linter lighter: (#408)

- Use `cargo check` instead of `cargo build` when
  `g:ale_rust_cargo_use_check` is truish.
- Use `--frozen` to avoid locking the project.

ale_linters/rust/cargo.vim

@@ -1,6 +1,8 @@
 " Author: Daniel Schemala <istjanichtzufassen@gmail.com>
 " Description: rustc invoked by cargo for rust files
 
+let g:ale_rust_cargo_use_check = get(g:, 'ale_rust_cargo_use_check', 0)
+
 function! ale_linters#rust#cargo#GetCargoExecutable(bufnr) abort
     if ale#util#FindNearestFile(a:bufnr, 'Cargo.toml') !=# ''
         return 'cargo'
@@ -11,10 +13,18 @@ function! ale_linters#rust#cargo#GetCargoExecutable(bufnr) abort
     endif
 endfunction
 
+function! ale_linters#rust#cargo#GetCommand(buffer) abort
+    let l:command = g:ale_rust_cargo_use_check
+    \   ? 'check'
+    \   : 'build'
+
+    return 'cargo ' . l:command . ' --frozen --message-format=json -q'
+endfunction
+
 call ale#linter#Define('rust', {
 \   'name': 'cargo',
 \   'executable_callback': 'ale_linters#rust#cargo#GetCargoExecutable',
-\   'command': 'cargo build --message-format=json -q',
+\   'command_callback': 'ale_linters#rust#cargo#GetCommand',
 \   'callback': 'ale#handlers#rust#HandleRustErrors',
 \   'output_stream': 'stdout',
 \})