Add AsciiSet::EMPTY and impl ops::Add for AsciiSet

In RFCs, the sets of characters to percent-encode are often defined as
the union of multiple sets. This change adds an `EMPTY` constant to
`AsciiSet` and implements the `Add` trait for `AsciiSet` so that sets
can be combined with the `+` operator.

AsciiSet now derives `Debug`, `PartialEq`, and `Eq` so that it can be
used in tests.

percent_encoding/src/lib.rs

@@ -51,7 +51,7 @@ use alloc::{
     string::String,
     vec::Vec,
 };
-use core::{fmt, mem, slice, str};
+use core::{fmt, mem, ops, slice, str};
 
 /// Represents a set of characters or bytes in the ASCII range.
 ///
@@ -66,6 +66,7 @@ use core::{fmt, mem, slice, str};
 /// /// https://url.spec.whatwg.org/#fragment-percent-encode-set
 /// const FRAGMENT: &AsciiSet = &CONTROLS.add(b' ').add(b'"').add(b'<').add(b'>').add(b'`');
 /// ```
+#[derive(Debug, PartialEq, Eq)]
 pub struct AsciiSet {
     mask: [Chunk; ASCII_RANGE_LEN / BITS_PER_CHUNK],
 }
@@ -77,6 +78,11 @@ const ASCII_RANGE_LEN: usize = 0x80;
 const BITS_PER_CHUNK: usize = 8 * mem::size_of::<Chunk>();
 
 impl AsciiSet {
+    /// An empty set.
+    pub const EMPTY: AsciiSet = AsciiSet {
+        mask: [0; ASCII_RANGE_LEN / BITS_PER_CHUNK],
+    };
+
     /// Called with UTF-8 bytes rather than code points.
     /// Not used for non-ASCII bytes.
     const fn contains(&self, byte: u8) -> bool {
@@ -102,6 +108,18 @@ impl AsciiSet {
     }
 }
 
+impl ops::Add for AsciiSet {
+    type Output = Self;
+
+    fn add(self, other: Self) -> Self {
+        let mut mask = self.mask.clone();
+        for i in 0..mask.len() {
+            mask[i] |= other.mask[i];
+        }
+        AsciiSet { mask }
+    }
+}
+
 /// The set of 0x00 to 0x1F (C0 controls), and 0x7F (DEL).
 ///
 /// Note that this includes the newline and tab characters, but not the space 0x20.
@@ -478,3 +496,16 @@ fn decode_utf8_lossy(input: Cow<'_, [u8]>) -> Cow<'_, str> {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn add() {
+        let left = AsciiSet::EMPTY.add(b'A');
+        let right = AsciiSet::EMPTY.add(b'B');
+        let expected = AsciiSet::EMPTY.add(b'A').add(b'B');
+        assert_eq!(left + right, expected);
+    }
+}