Warn if importing core, overrides w/o dependencies

Add deprecation warnings if users import files from the core and overrides layers without first importing the settings, helpers and tools layers.

We'll remove the imports of "base" from files in these layers in 4.0[1], so this will no longer work.

[1]: #1800

src/govuk/core/_global-styles.scss

@@ -1,3 +1,7 @@
+@if not mixin-exists("govuk-exports") {
+  @warn "Importing items from the core layer without first importing `base` is deprecated, and will no longer work as of GOV.UK Frontend v4.0.";
+}
+
 @import "../base";
 
 @import "links";

src/govuk/core/_links.scss

@@ -1,3 +1,7 @@
+@if not mixin-exists("govuk-exports") {
+  @warn "Importing items from the core layer without first importing `base` is deprecated, and will no longer work as of GOV.UK Frontend v4.0.";
+}
+
 @import "../base";
 
 @include govuk-exports("govuk/core/links") {

src/govuk/core/_lists.scss

@@ -1,3 +1,7 @@
+@if not mixin-exists("govuk-exports") {
+  @warn "Importing items from the core layer without first importing `base` is deprecated, and will no longer work as of GOV.UK Frontend v4.0.";
+}
+
 @import "../base";
 
 @include govuk-exports("govuk/core/lists") {

src/govuk/core/_section-break.scss

@@ -1,3 +1,7 @@
+@if not mixin-exists("govuk-exports") {
+  @warn "Importing items from the core layer without first importing `base` is deprecated, and will no longer work as of GOV.UK Frontend v4.0.";
+}
+
 @import "../base";
 
 @include govuk-exports("govuk/core/section-break") {

src/govuk/core/_template.scss

@@ -1,3 +1,7 @@
+@if not mixin-exists("govuk-exports") {
+  @warn "Importing items from the core layer without first importing `base` is deprecated, and will no longer work as of GOV.UK Frontend v4.0.";
+}
+
 @import "../base";
 
 @include govuk-exports("govuk/core/template") {

src/govuk/core/_typography.scss

@@ -1,3 +1,7 @@
+@if not mixin-exists("govuk-exports") {
+  @warn "Importing items from the core layer without first importing `base` is deprecated, and will no longer work as of GOV.UK Frontend v4.0.";
+}
+
 @import "../base";
 
 @include govuk-exports("govuk/core/typography") {

src/govuk/core/core.test.js

@@ -1,11 +1,31 @@
 /* eslint-env jest */
 
+const sass = require('node-sass')
+
 const glob = require('glob')
 const { renderSass } = require('../../../lib/jest-helpers')
 const configPaths = require('../../../config/paths.json')
 
 const sassFiles = glob.sync(`${configPaths.src}/core/**/*.scss`)
 
-it.each(sassFiles)('%s renders to CSS without errors', (file) => {
-  return renderSass({ file: file })
+it.each(sassFiles)('%s renders with a deprecation warning', (file) => {
+  // Create a mock warn function that we can use to override the native @warn
+  // function, that we can make assertions about post-render.
+  const mockWarnFunction = jest.fn()
+    .mockReturnValue(sass.NULL)
+
+  return renderSass({
+    file: file,
+    functions: {
+      '@warn': mockWarnFunction
+    }
+  }).then(() => {
+    // Expect our mocked @warn function to have been called once with a single
+    // argument, which should be the deprecation notice
+    return expect(mockWarnFunction.mock.calls[0][0].getValue())
+      .toEqual(
+        'Importing items from the core layer without first importing `base` ' +
+        'is deprecated, and will no longer work as of GOV.UK Frontend v4.0.'
+      )
+  })
 })

src/govuk/overrides/_display.scss

@@ -1,3 +1,7 @@
+@if not mixin-exists("govuk-exports") {
+  @warn "Importing items from the overrides layer without first importing `base` is deprecated, and will no longer work as of GOV.UK Frontend v4.0.";
+}
+
 @import "../base";
 
 @include govuk-exports("govuk/overrides/display") {

src/govuk/overrides/_spacing.scss

@@ -1,3 +1,7 @@
+@if not mixin-exists("govuk-exports") {
+  @warn "Importing items from the overrides layer without first importing `base` is deprecated, and will no longer work as of GOV.UK Frontend v4.0.";
+}
+
 @import "../base";
 
 ////

src/govuk/overrides/_typography.scss

@@ -1,3 +1,7 @@
+@if not mixin-exists("govuk-exports") {
+  @warn "Importing items from the overrides layer without first importing `base` is deprecated, and will no longer work as of GOV.UK Frontend v4.0.";
+}
+
 @import "../base";
 
 @include govuk-exports("govuk/overrides/typography") {

src/govuk/overrides/_width.scss

@@ -1,3 +1,7 @@
+@if not mixin-exists("govuk-exports") {
+  @warn "Importing items from the overrides layer without first importing `base` is deprecated, and will no longer work as of GOV.UK Frontend v4.0.";
+}
+
 @import "../base";
 
 @include govuk-exports("govuk/overrides/width") {

src/govuk/overrides/overrides.test.js

@@ -1,11 +1,31 @@
 /* eslint-env jest */
 
+const sass = require('node-sass')
+
 const glob = require('glob')
 const { renderSass } = require('../../../lib/jest-helpers')
 const configPaths = require('../../../config/paths.json')
 
 const sassFiles = glob.sync(`${configPaths.src}/overrides/**/*.scss`)
 
-it.each(sassFiles)('%s renders to CSS without errors', (file) => {
-  return renderSass({ file: file })
+it.each(sassFiles)('%s renders with a deprecation warning', (file) => {
+  // Create a mock warn function that we can use to override the native @warn
+  // function, that we can make assertions about post-render.
+  const mockWarnFunction = jest.fn()
+    .mockReturnValue(sass.NULL)
+
+  return renderSass({
+    file: file,
+    functions: {
+      '@warn': mockWarnFunction
+    }
+  }).then(() => {
+    // Expect our mocked @warn function to have been called once with a single
+    // argument, which should be the deprecation notice
+    return expect(mockWarnFunction.mock.calls[0][0].getValue())
+      .toEqual(
+        'Importing items from the overrides layer without first importing ' +
+        '`base` is deprecated, and will no longer work as of GOV.UK Frontend v4.0.'
+      )
+  })
 })