Add support for _index files (#2456)

Closes #690

doc-src/SASS_CHANGELOG.md

@@ -3,6 +3,11 @@
 * Table of contents
 {:toc}
 
+## 3.6.0 (Unreleased)
+
+* Add support for importing an `_index.scss` or `_index.sass` file when
+  importing a directory.
+
 ## 3.5.6 (Unreleased)
 
 * `var()` may now be passed in place of multiple arguments to `rgb()`, `rgba()`,

doc-src/SASS_REFERENCE.md

@@ -1349,6 +1349,14 @@ Note that you may not include a partial and a non-partial with the same name in
 the same directory. For example, `_colors.scss` may not exist alongside
 `colors.scss`.
 
+#### Index Files
+
+If you write a file with the special name `_index.scss` or `_index.sass`, it
+will be loaded if you import the directory that contains it. For example, if you
+have `dir/_index.scss`, you can write `@import "dir";` and it will load your
+file. However, if you have a file named `_dir.scss` *and* a file named
+`dir/_index.scss`, `_dir.scss` will take precedence.
+
 #### Nested `@import`
 
 Although most of the time it's most useful to just have `@import`s

lib/sass/importers/filesystem.rb

@@ -154,7 +154,9 @@ def find_real_file(dir, name, options)
             [Sass::Util.cleanpath(full_path).to_s, s]
           end
         end.flatten(1)
-        return if found.empty?
+        if found.empty? && split(name)[2].nil? && File.directory?("#{dir}/#{name}")
+          return find_real_file("#{dir}/#{name}", "index", options)
+        end
 
         if found.size > 1 && !@same_name_warnings.include?(found.first.first)
           found.each {|(f, _)| @same_name_warnings << f}
@@ -202,7 +204,7 @@ def split(name)
 
       def _find(dir, name, options)
         full_filename, syntax = Sass::Util.destructure(find_real_file(dir, name, options))
-        return unless full_filename && File.readable?(full_filename)
+        return unless full_filename && File.file?(full_filename) && File.readable?(full_filename)
 
         # TODO: this preserves historical behavior, but it's possible
         # :filename should be either normalized to the native format