Fix #1031 - Make the rust flags configurable

test/command_callback/test_rustc_command_callback.vader

@@ -0,0 +1,37 @@
+Before:
+  Save g:ale_rust_rustc_options
+
+  unlet! g:ale_rust_rustc_options
+
+  runtime ale_linters/rust/rustc.vim
+  call ale#test#SetDirectory('/testplugin/test/command_callback')
+
+After:
+  Restore
+
+  unlet! b:ale_rust_rustc_options
+
+  call ale#test#RestoreDirectory()
+  call ale#linter#Reset()
+
+Execute(The default command should be correct):
+  AssertEqual
+  \ 'rustc --error-format=json -Z no-trans -',
+  \ ale_linters#rust#rustc#RustcCommand(bufnr(''))
+
+Execute(The options should be configurable):
+  let b:ale_rust_rustc_options = '--foo'
+
+  AssertEqual
+  \ 'rustc --error-format=json --foo -',
+  \ ale_linters#rust#rustc#RustcCommand(bufnr(''))
+
+Execute(Some default paths should be included when the project is a Cargo project):
+  call ale#test#SetFilename('cargo_paths/test.rs')
+
+  AssertEqual
+  \ 'rustc --error-format=json -Z no-trans'
+  \   . ' -L ' . ale#Escape(ale#path#GetAbsPath(g:dir,  'cargo_paths/target/debug/deps'))
+  \   . ' -L ' . ale#Escape(ale#path#GetAbsPath(g:dir, 'cargo_paths/target/release/deps'))
+  \   . ' -',
+  \ ale_linters#rust#rustc#RustcCommand(bufnr(''))