When CSS exists between two tailwind directives, then the CSS will be
wrapped in an `@layer` to ensure it all ends up in the correct location
in the final CSS file.
However, if the only thing between two tailwind directives is a comment,
then the comment will also be wrapped in an `@layer` directive.
E.g.:
```css
@tailwind base;
/* This is a comment */
@tailwind components;
/* This is another comment */
@tailwind utilities;
```
Results in:
```css
@import "tailwindcss";
@layer base {
/* This is a comment */
}
@layer components {
/* This is another comment */
}
```
In this case, the layers don't matter, so this can be simplified to:
```css
@import "tailwindcss";
/* This is a comment */
/* This is another comment */
```
This PR fixes an issue where some `@import` at-rules had an empty
`layer()` attached at the end of the `@import` string.
We should only add that if a Tailwind directive or Tailwind import such
as `@tailwind base` or `@import "tailwindcss/base"` preceded the current
`@import` at-rule. If there was no Tailwind directive, the `lastLayer`
will be empty and we don't need to attach it to the `@import` string.
This PR improves the missing layers codemod where everything after the
last Tailwind directive can stay as-is without wrapping it in a `@layer`
directive.
The `@layer` at-rules are only important for CSS that exists between
Tailwind directives.
E.g.:
```css
@tailwind base;
html {}
@tailwind components;
.btn {}
@tailwind utilities;
.foo {}
.bar {}
```
Was transformed into:
```css
@import "tailwindcss";
@layer base {
html {}
}
@layer components {
.btn {}
}
@layer utilities {
.foo {}
.bar {}
}
```
But the last `@layer utilities` is already in the correct spot, so we
can simplify this to just this instead:
```css
@import "tailwindcss";
@layer base {
html {}
}
@layer components {
.btn {}
}
.foo {}
.bar {}
```
This PR adds a codemod that ensures that some parts of your stylesheet
are wrapped in an `@layer`.
This is a follow-up PR of #14411, in that PR we migrate `@tailwind`
directives to imports.
As a quick summary, that will turn this:
```css
@tailwind base;
@tailwind components;
@tailwind utilities;
```
Into:
```css
@import 'tailwindcss';
```
But there are a few issues with that _if_ we have additional CSS on the
page. For example let's imagine we had this:
```css
@tailwind base;
body {
background-color: red;
}
@tailwind components;
.btn {}
@tailwind utilities;
```
This will now be turned into:
```css
@import 'tailwindcss';
body {
background-color: red;
}
.btn {}
```
But in v4 we use real layers, in v3 we used to replace the directive
with the result of that layer. This means that now the `body` and `.btn`
styles are in the incorrect spot.
To solve this, we have to wrap them in a layer. The `body` should go in
an `@layer base`, and the `.btn` should be in an `@layer components` to
make sure it's in the same spot as it was before.
That's what this PR does, the original input will now be turned into:
```css
@import 'tailwindcss';
@layer base {
body {
background-color: red;
}
}
@layer components {
.btn {
}
}
```
There are a few internal refactors going on as well, but those are less
important.
