implement builtin function unique-id

Cargo.toml

@@ -34,7 +34,7 @@ default = ["commandline", "random"]
 commandline = ["clap"]
 # Option: enable nightly-only features (for right now, only the `track_caller` attribute) 
 nightly = []
-# Option (enabled by default): enable the builtin function `random([$limit])`
+# Option (enabled by default): enable the builtin functions `random([$limit])` and `unique-id()`
 random = ["rand"]
 
 [dev-dependencies]

README.md

@@ -17,7 +17,7 @@ The large features remaining are
 ```
 special case certain functions (min, max, calc, element, expression, progid, url)
 all builtin selector functions (274 tests)
-content-exists, unique-id, min, min
+content-exists, min, min
 @extend (~600 tests)
 indented syntax (27 tests)
 a special parser for plain css

src/builtin/string.rs

@@ -3,6 +3,9 @@ use std::collections::HashMap;
 use num_bigint::BigInt;
 use num_traits::{Signed, ToPrimitive, Zero};
 
+#[cfg(feature = "random")]
+use rand::{distributions::Alphanumeric, thread_rng, Rng};
+
 use super::Builtin;
 use crate::common::QuoteKind;
 use crate::unit::Unit;
@@ -217,4 +220,17 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
             Ok(Value::Ident(string, quotes))
         }),
     );
+    #[cfg(feature = "random")]
+    f.insert(
+        "unique-id".to_owned(),
+        Builtin::new(|args, _, _| {
+            max_args!(args, 0);
+            let mut rng = thread_rng();
+            let string = std::iter::repeat(())
+                .map(|()| rng.sample(Alphanumeric))
+                .take(7)
+                .collect();
+            Ok(Value::Ident(string, QuoteKind::None))
+        }),
+    );
 }