<===> variable_use/input.scss
@use "midstream";

a {b: midstream.$c}

<===> variable_use/_midstream.scss
@forward "upstream";

$c: midstream;

<===> variable_use/_upstream.scss
$c: upstream;

<===> variable_use/output.css
a {
  b: midstream;
}

<===>
================================================================================
<===> variable_assignment/top_level/input.scss
@use "midstream";

midstream.$a: new value;

b {
  midstream: midstream.get-midstream-a();
  upstream: midstream.get-upstream-a();
};

<===> variable_assignment/top_level/_midstream.scss
@forward "upstream";

$a: midstream value;

@function get-midstream-a() {@return $a}

<===> variable_assignment/top_level/_upstream.scss
$a: upstream value;

@function get-upstream-a() {@return $a}

<===> variable_assignment/top_level/output.css
b {
  midstream: midstream value;
  upstream: new value;
}

<===>
================================================================================
<===> function/input.scss
@use "midstream";

a {b: midstream.c()}

<===> function/_midstream.scss
@forward "upstream";

@function c() {@return midstream}

<===> function/_upstream.scss
@function c() {@return upstream}

<===> function/output.css
a {
  b: midstream;
}

<===>
================================================================================
<===> mixin/input.scss
@use "midstream";

@include midstream.a;

<===> mixin/_midstream.scss
@forward "upstream";

@mixin a() {b {c: midstream}}

<===> mixin/_upstream.scss
@mixin a() {b {c: upstream}}

<===> mixin/output.css
b {
  c: midstream;
}