RustFmt: use '--edition 2018' if no rustfmt.toml available

References #368

[amended by alonid@gmail.com: due to lchdir inner function, we need to
provide absolute paths of the rustfmt.toml file]

Author: mhinz

autoload/rustfmt.vim

@@ -60,18 +60,19 @@ function! s:RustfmtWriteMode()
     endif
 endfunction
 
-function! s:RustfmtConfig()
+function! s:RustfmtConfigOptions()
     let l:rustfmt_toml = findfile('rustfmt.toml', expand('%:p:h') . ';')
     if l:rustfmt_toml !=# ''
-        return '--config-path '.l:rustfmt_toml
+        return '--config-path '.fnamemodify(l:rustfmt_toml, ":p")
     endif
 
     let l:_rustfmt_toml = findfile('.rustfmt.toml', expand('%:p:h') . ';')
     if l:_rustfmt_toml !=# ''
-        return '--config-path '.l:_rustfmt_toml
+        return '--config-path '.fnamemodify(l:_rustfmt_toml, ":p")
     endif
 
-    return ''
+    " Default to edition 2018 in case no rustfmt.toml was found.
+    return '--edition 2018'
 endfunction
 
 function! s:RustfmtCommandRange(filename, line1, line2)
@@ -82,7 +83,7 @@ function! s:RustfmtCommandRange(filename, line1, line2)
 
     let l:arg = {"file": shellescape(a:filename), "range": [a:line1, a:line2]}
     let l:write_mode = s:RustfmtWriteMode()
-    let l:rustfmt_config = s:RustfmtConfig()
+    let l:rustfmt_config = s:RustfmtConfigOptions()
 
     " FIXME: When --file-lines gets to be stable, add version range checking
     " accordingly.
@@ -96,14 +97,9 @@ function! s:RustfmtCommandRange(filename, line1, line2)
 endfunction
 
 function! s:RustfmtCommand()
-    if g:rustfmt_emit_files
-        let l:write_mode = "--emit=stdout"
-    else
-        let l:write_mode = "--write-mode=display"
-    endif
-    " rustfmt will pick on the right config on its own due to the
-    " current directory change.
-    return g:rustfmt_command . " ". l:write_mode . " " . g:rustfmt_options
+    let write_mode = g:rustfmt_emit_files ? '--emit=stdout' : '--write-mode=display'
+    let config = s:RustfmtConfigOptions()
+    return join([g:rustfmt_command, write_mode, config, g:rustfmt_options])
 endfunction
 
 function! s:DeleteLines(start, end) abort