Add support for Rust using rustc and cargo (#230)

* Add rustc checker for rust files

* Add documentation for rustc

* Use a nice helper function

* Add cargo as linter

* Complete the doc for rust linters

* Put l: in front of every local variable

* Apply the requested stylistic changes

ale_linters/rust/cargo.vim

@@ -0,0 +1,21 @@
+" Author: Daniel Schemala <istjanichtzufassen@gmail.com>
+" Description: rustc invoked by cargo for rust files
+
+
+function! ale_linters#rust#cargo#cargo_or_not_cargo(bufnr)
+    if ale#util#FindNearestFile(a:bufnr, 'Cargo.toml') !=# ''
+        return 'cargo'
+    else
+        " if there is no Cargo.toml file, we don't use cargo even if it exists,
+        " so we return '', because executable('') apparently always fails
+        return ''
+    endif
+endfunction
+
+call ale#linter#Define('rust', {
+\   'name': 'cargo',
+\   'executable_callback': 'ale_linters#rust#cargo#cargo_or_not_cargo',
+\   'command': 'cargo rustc -- --error-format=json -Z no-trans',
+\   'callback': 'ale_linters#rust#rustc#handle_rustc_errors',
+\   'output_stream': 'stderr',
+\})