add tests for first class mixins

spec/core_functions/meta/accepts_content.hrx

@@ -0,0 +1,77 @@
+<===> options.yml
+---
+:todo:
+- sass/libsass#2807
+
+<===>
+================================================================================
+<===> accepts/direct-child/input.scss
+@use "sass:meta";
+
+@mixin foo() {
+    @content;
+}
+
+a {b: meta.accepts-content(meta.get-mixin("foo"))}
+
+<===> accepts/direct-child/output.css
+a {
+  b: true;
+}
+
+<===>
+================================================================================
+<===> accepts/nested-child/input.scss
+@use "sass:meta";
+
+@mixin foo() {
+    @if false {
+        @content;
+    }
+}
+
+a {b: meta.accepts-content(meta.get-mixin("foo"))}
+
+<===> accepts/nested-child/output.css
+a {
+  b: true;
+}
+
+<===>
+================================================================================
+<===> accepts/builtin/input.scss
+@use "sass:meta";
+@mixin foo() {}
+
+a {b: meta.accepts-content(meta.get-mixin(apply, meta))}
+
+<===> accepts/builtin/output.css
+a {
+  b: true;
+}
+
+<===>
+================================================================================
+<===> doesnt-accept/empty/input.scss
+@use "sass:meta";
+@mixin foo() {}
+
+a {b: meta.accepts-content(meta.get-mixin("foo"))}
+
+<===> doesnt-accept/empty/output.css
+a {
+  b: false;
+}
+
+<===>
+================================================================================
+<===> doesnt-accept/builtin/input.scss
+@use "sass:meta";
+@mixin foo() {}
+
+a {b: meta.accepts-content(meta.get-mixin(load-css, meta))}
+
+<===> doesnt-accept/builtin/output.css
+a {
+  b: false;
+}

spec/core_functions/meta/apply.hrx

@@ -0,0 +1,122 @@
+<===> options.yml
+---
+:todo:
+- sass/libsass#2807
+
+<===>
+================================================================================
+<===> use-as-function/input.scss
+@use "sass:meta";
+
+@mixin foo {}
+$foo: meta.get-mixin("foo");
+
+a {b: meta.apply($foo)}
+
+<===> use-as-function/error
+Error: Undefined function.
+  ,
+6 | a {b: meta.apply($foo)}
+  |       ^^^^^^^^^^^^^^^^
+  '
+  input.scss 6:7  root stylesheet
+
+<===>
+================================================================================
+<===> no-args/input.scss
+@use "sass:meta";
+
+a {@include meta.apply()}
+
+<===> no-args/error
+Error: Missing argument $mixin.
+  ,--> input.scss
+3 | a {@include meta.apply()}
+  |    ^^^^^^^^^^^^^^^^^^^^^ invocation
+  '
+  ,--> sass:meta
+1 | @mixin apply($mixin, $args...) {
+  |        ======================= declaration
+  '
+  input.scss 3:4  root stylesheet
+
+<===>
+================================================================================
+<===> wrong-arg-type/input.scss
+@use "sass:meta";
+
+a {@include meta.apply(2px)}
+
+<===> wrong-arg-type/error
+Error: $mixin: 2px is not a mixin reference.
+  ,
+3 | a {@include meta.apply(2px)}
+  |    ^^^^^^^^^^^^^^^^^^^^^^^^
+  '
+  input.scss 3:4  root stylesheet
+
+<===>
+================================================================================
+<===> too-many-args/input.scss
+@use "sass:meta";
+
+@mixin foo {}
+$foo: meta.get-mixin("foo");
+
+a {@include meta.apply($foo, 2px)}
+
+<===> too-many-args/error
+Error: Only 0 arguments allowed, but 1 was passed.
+    ,
+3   | @mixin foo {}
+    |        === declaration
+... |
+6   | a {@include meta.apply($foo, 2px)}
+    |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ invocation
+    '
+  input.scss 6:4  foo()
+  input.scss 6:4  root stylesheet
+
+<===>
+================================================================================
+<===> missing-mixin-args/input.scss
+@use "sass:meta";
+
+@mixin foo($a) {}
+$foo: meta.get-mixin("foo");
+
+a {@include meta.apply($foo)}
+
+<===> missing-mixin-args/error
+Error: Missing argument $a.
+    ,
+3   | @mixin foo($a) {}
+    |        ======= declaration
+... |
+6   | a {@include meta.apply($foo)}
+    |    ^^^^^^^^^^^^^^^^^^^^^^^^^ invocation
+    '
+  input.scss 6:4  foo()
+  input.scss 6:4  root stylesheet
+
+<===>
+================================================================================
+<===> too-many-args-mixin-accepts-args/input.scss
+@use "sass:meta";
+
+@mixin foo($a) {}
+$foo: meta.get-mixin("foo");
+
+a {@include meta.apply($foo, 2px, 3px)}
+
+<===> too-many-args-mixin-accepts-args/error
+Error: Only 1 argument allowed, but 2 were passed.
+    ,
+3   | @mixin foo($a) {}
+    |        ======= declaration
+... |
+6   | a {@include meta.apply($foo, 2px, 3px)}
+    |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ invocation
+    '
+  input.scss 6:4  foo()
+  input.scss 6:4  root stylesheet

spec/core_functions/meta/get_mixin/content.hrx

@@ -0,0 +1,172 @@
+<===> passes-content/input.scss
+@use "sass:meta";
+
+@mixin foo {
+    @content;
+}
+
+a {
+    @include meta.apply(meta.get-mixin("foo")) {
+        b: red;
+    }
+}
+
+<===> passes-content/output.css
+a {
+  b: red;
+}
+
+<===>
+================================================================================
+<===> passes-empty-content/input.scss
+@use "sass:meta";
+
+@mixin foo {
+    @content;
+}
+
+a {
+    @include meta.apply(meta.get-mixin("foo")) {}
+}
+
+<===> passes-empty-content/output.css
+
+<===>
+================================================================================
+<===> denies-content/user-defined/input.scss
+@use "sass:meta";
+@mixin foo {}
+
+a {
+    @include meta.apply(meta.get-mixin("foo")) {}
+}
+
+<===> denies-content/user-defined/error
+Error: Mixin doesn't accept a content block.
+    ,
+2   | @mixin foo {}
+    |        === declaration
+... |
+5   |     @include meta.apply(meta.get-mixin("foo")) {}
+    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ invocation
+    '
+  input.scss 5:5  root stylesheet
+
+<===>
+================================================================================
+<===> denies-content/builtin/input.scss
+@use "sass:meta";
+
+a {
+    @include meta.apply(meta.get-mixin(load-css, meta), "foo") {
+        a: b;
+    }
+}
+
+<===> denies-content/builtin/error
+Error: Mixin doesn't accept a content block.
+  ,--> input.scss
+4 |     @include meta.apply(meta.get-mixin(load-css, meta), "foo") {
+  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ invocation
+  '
+  ,--> sass:meta
+1 | @mixin load-css($url, $with: null) {
+  |        =========================== declaration
+  '
+  input.scss 4:5  root stylesheet
+
+<===>
+================================================================================
+<===> scope/redeclare-vars/input.scss
+@use "sass:meta";
+@mixin foo($a: a) {
+    $b: b;
+    @content;
+    a: $a;
+    b: $b;
+}
+
+$c: c;
+
+a {
+    $d: d;
+    @include meta.apply(meta.get-mixin("foo")) {
+        $a: x;
+        $b: x;
+        $c: x;
+        $d: x;
+    }
+    c: $c;
+    d: $d;
+}
+
+<===> scope/redeclare-vars/output.css
+a {
+  a: a;
+  b: b;
+  c: c;
+  d: x;
+}
+
+<===>
+================================================================================
+<===> scope/redeclare/using/input.scss
+@use "sass:meta";
+@mixin foo($a: x) {
+    @content(a);
+}
+
+$a: y;
+
+a {
+    $a: z;
+    @include meta.apply(meta.get-mixin("foo")) using ($a) {
+        a: $a;
+        $a: a;
+    }
+    b: $a;
+}
+
+<===> scope/redeclare/using/output.css
+a {
+  a: a;
+  b: z;
+}
+
+<===>
+================================================================================
+<===> scope/fall-through/input.scss
+@use "sass:meta";
+$b: x;
+
+@mixin foo {
+    @content(x);
+    b: $b;
+}
+
+@mixin bar {
+    $b: a;
+    @include meta.apply(meta.get-mixin(foo)) using ($a) {
+        @content($a);
+    }
+}
+
+@mixin baz {
+    $b: b;
+    @include meta.apply(meta.get-mixin(bar)) using ($a) {
+        @content($a);
+    }
+}
+
+a {
+    $b: c;
+    @include meta.apply(meta.get-mixin(baz)) using ($a) {
+        a: $a;
+    }
+}
+
+<===> scope/fall-through/output.css
+a {
+  a: x;
+  b: x;
+}