Add a display_lossy() to write a JsString lossily (#4023)

* Add a display_lossy() to write a JsString lossily

* cargo fmt

core/string/src/display.rs

@@ -34,6 +34,27 @@ impl<'a> From<JsStr<'a>> for JsStrDisplayEscaped<'a> {
     }
 }
 
+/// Display implementation for [`crate::JsString`] that escapes unicode characters.
+#[derive(Debug)]
+pub struct JsStrDisplayLossy<'a> {
+    inner: JsStr<'a>,
+}
+
+impl fmt::Display for JsStrDisplayLossy<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        // No need to optimize latin1.
+        self.inner
+            .code_points_lossy()
+            .try_for_each(|c| f.write_char(c))
+    }
+}
+
+impl<'a> From<JsStr<'a>> for JsStrDisplayLossy<'a> {
+    fn from(inner: JsStr<'a>) -> Self {
+        Self { inner }
+    }
+}
+
 #[test]
 fn latin1() {
     // 0xE9 is `é` in ISO-8859-1 (see https://www.ascii-code.com/ISO-8859-1).
@@ -41,4 +62,31 @@ fn latin1() {
 
     let rust_str = format!("{}", JsStrDisplayEscaped { inner: s });
     assert_eq!(rust_str, "Hello é world!");
+
+    let rust_str = format!("{}", JsStrDisplayLossy { inner: s });
+    assert_eq!(rust_str, "Hello é world!");
+}
+
+#[test]
+fn emoji() {
+    // 0x1F600 is `😀` (see https://www.fileformat.info/info/unicode/char/1f600/index.htm).
+    let s = JsStr::utf16(&[0xD83D, 0xDE00]);
+
+    let rust_str = format!("{}", JsStrDisplayEscaped { inner: s });
+    assert_eq!(rust_str, "😀");
+
+    let rust_str = format!("{}", JsStrDisplayLossy { inner: s });
+    assert_eq!(rust_str, "😀");
+}
+
+#[test]
+fn unpaired_surrogates() {
+    // 0xD800 is an unpaired surrogate (see https://www.fileformat.info/info/unicode/char/d800/index.htm).
+    let s = JsStr::utf16(&[0xD800]);
+
+    let rust_str = format!("{}", JsStrDisplayEscaped { inner: s });
+    assert_eq!(rust_str, "\\uD800");
+
+    let rust_str = format!("{}", JsStrDisplayLossy { inner: s });
+    assert_eq!(rust_str, "<EFBFBD>");
 }

core/string/src/lib.rs

@@ -26,7 +26,7 @@ mod tagged;
 mod tests;
 
 use self::{iter::Windows, str::JsSliceIndex};
-use crate::display::JsStrDisplayEscaped;
+use crate::display::{JsStrDisplayEscaped, JsStrDisplayLossy};
 use crate::tagged::{Tagged, UnwrappedTagged};
 #[doc(inline)]
 pub use crate::{
@@ -960,7 +960,7 @@ impl JsString {
         }
     }
 
-    /// Gets a displayable escaped string. This may be faster and has less
+    /// Gets a displayable escaped string. This may be faster and has fewer
     /// allocations than `format!("{}", str.to_string_escaped())` when
     /// displaying.
     #[inline]
@@ -968,6 +968,14 @@ impl JsString {
     pub fn display_escaped(&self) -> JsStrDisplayEscaped<'_> {
         JsStrDisplayEscaped::from(self.as_str())
     }
+
+    /// Gets a displayable lossy string. This may be faster and has fewer
+    /// allocations than `format!("{}", str.to_string_lossy())` when displaying.
+    #[inline]
+    #[must_use]
+    pub fn display_lossy(&self) -> JsStrDisplayLossy<'_> {
+        JsStrDisplayLossy::from(self.as_str())
+    }
 }
 
 impl Clone for JsString {

core/string/src/str.rs

@@ -235,6 +235,14 @@ impl<'a> JsStr<'a> {
         m >= n && needle == self.get(m - n..).expect("already checked size")
     }
 
+    /// Gets an iterator of all the Unicode codepoints of a [`JsStr`], replacing
+    /// unpaired surrogates with the replacement character. This is faster than
+    /// using [`Self::code_points`].
+    #[inline]
+    pub(crate) fn code_points_lossy(self) -> impl Iterator<Item = char> + 'a {
+        char::decode_utf16(self.iter()).map(|res| res.unwrap_or('\u{FFFD}'))
+    }
+
     /// Gets an iterator of all the Unicode codepoints of a [`JsStr`].
     /// This is not optimized for Latin1 strings.
     #[inline]