Make import paths work right

tools/sass/defs.bzl

@@ -33,12 +33,12 @@ https://github.com/dart-lang/rules_dart).
 SassInfo = provider(
     doc = "Collects files from sass_library for use in downstream sass_binary",
     fields = {
+        "transitive_includes": "Directories to be treated as @use/@import search paths",
         "transitive_sources": "Sass sources for this target and its dependencies",
     },
 )
 
 def _collect_transitive_sources(srcs, deps):
-    "Sass compilation requires all transitive .sass source files"
     return depset(
         srcs,
         transitive = [dep[SassInfo].transitive_sources for dep in deps],
@@ -46,6 +46,13 @@ def _collect_transitive_sources(srcs, deps):
         order = "postorder",
     )
 
+def _collect_transitive_imports(imports, deps):
+    return depset(
+        imports,
+        transitive = [dep[SassInfo].transitive_includes for dep in deps],
+        order = "postorder",
+    )
+
 def _sass_library_impl(ctx):
     """sass_library collects all transitive sources for given srcs and deps.
 
@@ -61,8 +68,16 @@ def _sass_library_impl(ctx):
         ctx.files.srcs,
         ctx.attr.deps,
     )
+    transitive_includes = _collect_transitive_imports(
+        [ctx.label.package] + ctx.attr.imports,
+        ctx.attr.deps,
+    )
+
     return [
-        SassInfo(transitive_sources = transitive_sources),
+        SassInfo(
+            transitive_sources = transitive_sources,
+            transitive_includes = transitive_includes,
+        ),
         DefaultInfo(
             files = transitive_sources,
             runfiles = ctx.runfiles(transitive_files = transitive_sources),
@@ -84,11 +99,8 @@ def _run_sass(ctx, input, css_output, map_output = None):
     if ctx.attr.sourcemap_embed_sources:
         args.add("--sourcemap-contents")
 
-    # Sources for compilation may exist in the source tree, in bazel-bin, or bazel-genfiles.
-    for prefix in [".", ctx.var["BINDIR"], ctx.var["GENDIR"]]:
-        args.add("--include-path=%s/" % prefix)
-        for include_path in ctx.attr.include_paths:
-            args.add("--include-path=%s/%s" % (prefix, include_path))
+    for include_path in _collect_transitive_imports([ctx.label.package], ctx.attr.deps).to_list():
+        args.add("--include-path=%s/" % include_path)
 
     # Last arguments are input and output paths
     # Note that the sourcemap is implicitly written to a path the same as the
@@ -163,6 +175,9 @@ _sass_library_attrs = {
         allow_empty = False,
         mandatory = True,
     ),
+    "imports": attr.string_list(
+        doc = "Import prefixes",
+    ),
     "deps": sass_deps_attr,
 }
 
@@ -179,6 +194,9 @@ _sass_binary_attrs = {
         mandatory = True,
         allow_single_file = _ALLOWED_SRC_FILE_EXTENSIONS,
     ),
+    "imports": attr.string_list(
+        doc = "Import prefixes",
+    ),
     "sourcemap": attr.bool(
         default = True,
         doc = "Whether source maps should be emitted.",