Close #2281 - Separate cwd commands from commands

Working directories are now set seperately from the commands so they
can later be swapped out when running linters over projects is
supported, and also better support filename mapping for running linters
on other machines in future.

ale_linters/rust/cargo.vim

@@ -23,6 +23,19 @@ function! ale_linters#rust#cargo#GetCargoExecutable(bufnr) abort
     endif
 endfunction
 
+function! ale_linters#rust#cargo#GetCwd(buffer) abort
+    if ale#Var(a:buffer, 'rust_cargo_avoid_whole_workspace')
+        let l:nearest_cargo = ale#path#FindNearestFile(a:buffer, 'Cargo.toml')
+        let l:nearest_cargo_dir = fnamemodify(l:nearest_cargo, ':h')
+
+        if l:nearest_cargo_dir isnot# '.'
+            return l:nearest_cargo_dir
+        endif
+    endif
+
+    return ''
+endfunction
+
 function! ale_linters#rust#cargo#GetCommand(buffer, version) abort
     let l:use_check = ale#Var(a:buffer, 'rust_cargo_use_check')
     \   && ale#semver#GTE(a:version, [0, 17, 0])
@@ -42,18 +55,6 @@ function! ale_linters#rust#cargo#GetCommand(buffer, version) abort
         let l:include_features = ' --features ' . ale#Escape(l:include_features)
     endif
 
-    let l:avoid_whole_workspace = ale#Var(a:buffer, 'rust_cargo_avoid_whole_workspace')
-    let l:nearest_cargo_prefix = ''
-
-    if l:avoid_whole_workspace
-        let l:nearest_cargo = ale#path#FindNearestFile(a:buffer, 'Cargo.toml')
-        let l:nearest_cargo_dir = fnamemodify(l:nearest_cargo, ':h')
-
-        if l:nearest_cargo_dir isnot# '.'
-            let l:nearest_cargo_prefix = 'cd '. ale#Escape(l:nearest_cargo_dir) .' && '
-        endif
-    endif
-
     let l:default_feature_behavior = ale#Var(a:buffer, 'rust_cargo_default_feature_behavior')
 
     if l:default_feature_behavior is# 'all'
@@ -81,7 +82,7 @@ function! ale_linters#rust#cargo#GetCommand(buffer, version) abort
         endif
     endif
 
-    return l:nearest_cargo_prefix . 'cargo '
+    return 'cargo '
     \   . l:subcommand
     \   . (l:use_all_targets ? ' --all-targets' : '')
     \   . (l:use_examples ? ' --examples' : '')
@@ -96,6 +97,7 @@ endfunction
 call ale#linter#Define('rust', {
 \   'name': 'cargo',
 \   'executable': function('ale_linters#rust#cargo#GetCargoExecutable'),
+\   'cwd': function('ale_linters#rust#cargo#GetCwd'),
 \   'command': {buffer -> ale#semver#RunWithVersionCheck(
 \       buffer,
 \       ale_linters#rust#cargo#GetCargoExecutable(buffer),