Ensure -- is allowed inside candidates (tailwindlabs#16972)

This PR fixes an issue where named utilities that contain double dashes
`--` are not extracted correctly.

Some people use `--` in the middle of the utility to create some form of
namespaced utility.

Given this input:
```js
let x = 'foo--bar'
```

The extracted candidates before this change:
```js
[ "let", "x", "--bar" ]
```

The extracted candidates after this change:
```js
[ "let", "x", "foo--bar", "--bar" ]
```

The reason `--bar` is still extracted in both cases is because of the
CSS variable machine. We could improve its extraction by checking its
boundary characters but that's a different issue.

For now, the important thing is that `foo--bar` was extracted.

# Test plan

1. Added new test
2. Existing tests pass

Author: RobinMalfait

CHANGELOG.md

@@ -17,6 +17,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - _Experimental_: Add `user-valid` and `user-invalid` variants ([#12370](https://github.com/tailwindlabs/tailwindcss/pull/12370))
 - _Experimental_: Add `wrap-anywhere`, `wrap-break-word`, and `wrap-normal` utilities ([#12128](https://github.com/tailwindlabs/tailwindcss/pull/12128))
 
+### Fixed
+
+- Ensure classes containing `--` are extracted correctly ([#16972](https://github.com/tailwindlabs/tailwindcss/pull/16972))
+
 ## [4.0.10] - 2025-03-05
 
 ### Added
@@ -41,7 +45,6 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Removed `max-w-auto` and `max-h-auto` utilities as they generate invalid CSS ([#16917](https://github.com/tailwindlabs/tailwindcss/pull/16917))
 - Replaced the existing candidate extractor with a brand new extractor to improve maintainability, correctness, and performance ([#16306](https://github.com/tailwindlabs/tailwindcss/pull/16306))
 
-
 ## [4.0.9] - 2025-02-25
 
 ### Fixed

crates/oxide/src/extractor/mod.rs

@@ -297,6 +297,7 @@ mod tests {
             ("flex block", vec!["flex", "block"]),
             // Simple utility with dashes
             ("items-center", vec!["items-center"]),
+            ("items--center", vec!["items--center"]),
             // Simple utility with numbers
             ("px-2.5", vec!["px-2.5"]),
             // Arbitrary properties

crates/oxide/src/extractor/named_utility_machine.rs

@@ -171,7 +171,9 @@ impl Machine for NamedUtilityMachine {
                             //            ^    Invalid
                             // E.g.: `flex-2`
                             //            ^    Valid
-                            Class::AlphaLower | Class::AlphaUpper | Class::Number => {
+                            // E.g.: `foo--bar`
+                            //            ^    Valid
+                            Class::AlphaLower | Class::AlphaUpper | Class::Number | Class::Dash => {
                                 cursor.advance();
                             }
 
@@ -388,6 +390,8 @@ mod tests {
             ("a", vec!["a"]),
             // With dashes
             ("items-center", vec!["items-center"]),
+            // With double dashes
+            ("items--center", vec!["items--center"]),
             // With numbers
             ("px-5", vec!["px-5"]),
             ("px-2.5", vec!["px-2.5"]),

crates/oxide/src/extractor/utility_machine.rs

@@ -219,6 +219,7 @@ mod tests {
             ("flex! block", vec!["flex!", "block"]),
             // With dashes
             ("items-center", vec!["items-center"]),
+            ("items--center", vec!["items--center"]),
             // Inside a string
             ("'flex'", vec!["flex"]),
             // Multiple utilities