Porting Documentation - At-Rules - warn

source/documentation/at-rules/warn.liquid

@@ -0,0 +1,66 @@
+---
+title: "@warn"
+introduction: >
+  When writing [mixins](/documentation/at-rules/mixin) and [functions](/documentation/at-rules/function), you may want to
+  discourage users from passing certain arguments or certain values. They may be
+  passing legacy arguments that are now deprecated, or they may be calling your
+  API in a way that’s not quite optimal.
+---
+
+{% markdown %}
+The `@warn` rule is designed just for that. It's written `@warn <expression>`
+and it prints the value of the [expression][] (usually a string) for the user,
+along with a stack trace indicating how the current mixin or function was
+called. Unlike the [`@error` rule][], though, it doesn't stop Sass entirely.
+
+[expression]: /documentation/syntax/structure#expressions
+[`@error` rule]: /documentation/at-rules/error
+{% endmarkdown %}
+
+{% codeExample 'warn' %}
+$known-prefixes: webkit, moz, ms, o;
+
+@mixin prefix($property, $value, $prefixes) {
+  @each $prefix in $prefixes {
+    @if not index($known-prefixes, $prefix) {
+      @warn "Unknown prefix #{$prefix}.";
+    }
+
+    -#{$prefix}-#{$property}: $value;
+  }
+  #{$property}: $value;
+}
+
+.tilt {
+  // Oops, we typo'd "webkit" as "wekbit"!
+  @include prefix(transform, rotate(15deg), wekbit ms);
+}
+===
+$known-prefixes: webkit, moz, ms, o
+
+@mixin prefix($property, $value, $prefixes)
+  @each $prefix in $prefixes
+    @if not index($known-prefixes, $prefix)
+      @warn "Unknown prefix #{$prefix}."
+
+
+    -#{$prefix}-#{$property}: $value
+
+  #{$property}: $value
+
+
+.tilt
+  // Oops, we typo'd "webkit" as "wekbit"!
+  @include prefix(transform, rotate(15deg), wekbit ms)
+{% endcodeExample %}
+
+{% markdown %}
+The exact format of the warning and stack trace varies from implementation to
+implementation. This is what it looks like in Dart Sass:
+
+```
+Warning: Unknown prefix wekbit.
+    example.scss 6:7   prefix()
+    example.scss 16:3  root stylesheet
+```
+{% endmarkdown %}