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',
+\})

ale_linters/rust/rustc.vim

@@ -0,0 +1,99 @@
+" Author: Daniel Schemala <istjanichtzufassen@gmail.com>
+" Description: rustc for rust files
+
+if !exists('g:ale_rust_ignore_error_codes')
+    let g:ale_rust_ignore_error_codes = []
+endif
+
+
+function! ale_linters#rust#rustc#handle_rustc_errors(buffer_number, errorlines)
+    let l:file_name = fnamemodify(bufname(a:buffer_number), ':t')
+    let l:output = []
+
+    for l:errorline in a:errorlines
+        " ignore everything that is not Json
+        if l:errorline !~# '^{'
+            continue
+        endif
+
+        let l:error = json_decode(l:errorline)
+
+        if !empty(l:error.code) && index(g:ale_rust_ignore_error_codes, l:error.code.code) > -1
+            continue
+        endif
+
+        for l:span in l:error.spans
+            if l:span.is_primary &&
+                \   (l:span.file_name ==# l:file_name || l:span.file_name ==# '<anon>')
+                call add(l:output, {
+                \   'bufnr': a:buffer_number,
+                \   'lnum': l:span.line_start,
+                \   'vcol': 0,
+                \   'col': l:span.byte_start,
+                \   'nr': -1,
+                \   'text': l:error.message,
+                \   'type': toupper(l:error.level[0]),
+                \})
+            else
+                " when the error is caused in the expansion of a macro, we have
+                " to bury deeper
+                let l:root_cause = s:find_error_in_expansion(l:span, l:file_name)
+
+                if !empty(l:root_cause)
+                    call add(l:output, {
+                    \   'bufnr': a:buffer_number,
+                    \   'lnum': l:root_cause[0],
+                    \   'vcol': 0,
+                    \   'col': l:root_cause[1],
+                    \   'nr': -1,
+                    \   'text': l:error.message,
+                    \   'type': toupper(l:error.level[0]),
+                    \})
+                endif
+            endif
+        endfor
+    endfor
+
+    return l:output
+endfunction
+
+
+" returns: a list [lnum, col] with the location of the error or []
+function! s:find_error_in_expansion(span, file_name)
+    if a:span.file_name ==# a:file_name
+        return [a:span.line_start, a:span.byte_start]
+    endif
+
+    if !empty(a:span.expansion)
+        return s:find_error_in_expansion(a:span.expansion.span, a:file_name)
+    endif
+
+    return []
+endfunction
+
+
+function! ale_linters#rust#rustc#rustc_command(buffer_number)
+    " Try to guess the library search path. If the project is managed by cargo,
+    " it's usually <project root>/target/debug/deps/ or
+    " <project root>/target/release/deps/
+    let l:cargo_file = ale#util#FindNearestFile(a:buffer_number, 'Cargo.toml')
+
+    if l:cargo_file !=# ''
+        let l:project_root = fnamemodify(l:cargo_file, ':h')
+        let l:dependencies = '-L ' . l:project_root . '/target/debug/deps -L ' .
+        \   l:project_root . '/target/release/deps'
+    else
+        let l:dependencies = ''
+    endif
+
+    return 'rustc --error-format=json -Z no-trans ' . l:dependencies . ' -'
+endfunction
+
+
+call ale#linter#Define('rust', {
+\   'name': 'rustc',
+\   'executable': 'rustc',
+\   'command_callback': 'ale_linters#rust#rustc#rustc_command',
+\   'callback': 'ale_linters#rust#rustc#handle_rustc_errors',
+\   'output_stream': 'stderr',
+\})