_font-face.scss

@import "shared";

// Cross-browser support for @font-face. Supports IE, Gecko, Webkit, Opera.
//
// $name     : (string or list, required) The name and optional SVG-ID to use.
//              - string "simpleName" => name: simpleName, ID: #simpleName
//              - list "name #id" => name: name, ID: #id
// $location : (string) The location of the font files - try to find in fonts_dir if none provided.
//              - Locations include the file name, without extensions:
//                'simple' => 'simple.eot', 'simple.ttf', 'simple.woff'
//                'folder/nested-font' => 'folder/nested-font.eot' etc...
//              - Locations are relative to the set fonts_dir:
//                'simple' => 'project/fonts_dir/simple'
// $weight   : (string | number) any accepted font-weight value.
// $style    : (string) any accepted font-style value.
// $inline   : (boolean | string ) true/false or the version of the font to inline (e.g. "woff").
//
// * For android 2.2 Compatiblity, please ensure that your web page has
//   a meta viewport tag.
// * To support iOS < 4.2, an SVG file must be provided
//
// Possible File Structure
//
// - fonts_dir/ [config setting]
//    - simple.eot
//    - simple.svg
//    - simple.ttf
//    - simple.woff
//    - folder/ (recommanded)
//      - nested-font.eot
//      - nested-font.otf
//      - nested-font.woff
//
// Formats are be determined based on extension:
// '.ttf'  => 'truetype'
// '.otf'  => 'opentype'
// '.woff' => 'woff'
// '.svg'  => 'svg'
// '.svgz' => 'svg'
// '.eot'  => 'eot'
//
// Fonts to be loaded in the same font-face mixin (same variation, different format)
// should share the same filename, not counting the extension.
//
// If you need to generate other formats check out the Font Squirrel
// [font generator](http://www.fontsquirrel.com/fontface/generator)
//
// In order to refer to a specific style of the font in your stylesheets as
// e.g. "font-style: italic;",  you may add a couple of @font-face includes
// containing the respective font files for each style and specying
// respective the $style parameter.

// Order of the includes matters, and it is: normal, bold, italic, bold+italic.

$font-face-inline: false !default;
$font-face-files-type-discovered: eot woff ttf svg !default;
$font-face-apply-tricks: eot svg !default;

@mixin font-face(
  $name,
  $location: $name,
  $weight: null,
  $style: null,
  $inline: $font-face-inline,
  $files-type: $font-face-files-type-discovered,
  $apply-tricks: $font-face-apply-tricks
) {
  // if no id specified, suppose id is the name
  $id: $name;

  // but we try to explod $name: name #fontId;
  @if length($name) > 1 {
    $name: nth($name, 1);
    $id: nth($name, 2);
  }

  // get all available font files in the location
  $files: discover_font_files($location, $files-type, $apply-tricks, $id);

  @font-face {
    font-family: quote($name);
    src: font-files($font-files);
    font: {
      weight: $weight;
      style: $style;
    }
  }
}

I like it. Thanks for working on this! A few comments:

1. It looks like $id may or may not have a preceding "#" when it is passed to your discover_font_files function. Does the function take care of that?
2. What does $font-face-apply-tricks do exactly? (Also, I think it should be file-types rather than files-type).
3. I wonder if we could use lists to get this even simpler. Something like:

@mixin font-face(
  $name, // <name> [<id> <font-weight> <font-style>]
  $location: $name,
  $inline: $font-face-inline,
  $files-type: $font-face-files-type-discovered,
  $apply-tricks: $font-face-apply-tricks
) { ... }
// "myFont normal" == font-weight: normal; font-style: normal;
// "myFont italic" == font-style: italic;
// "myFont normal italic" == font-weight: normal; font-style: italic;
// etc.

Now that I look more closely, there is a strong chance already that $name is a list, in which case we need to take only the 1st value of that list for our default $location

If everything else is working as needed, I'm happy to keep exploring the Sass side of things. Thanks again for the great contribution!