Add a display type for JsString to allow formatting without allocations (#3951)

Co-authored-by: Haled Odat <8566042+HalidOdat@users.noreply.github.com>

core/string/src/display.rs

@@ -0,0 +1,44 @@
+//! Display implementations for [`crate::JsString`].
+use crate::{CodePoint, JsStr, JsStrVariant};
+use std::fmt;
+use std::fmt::Write;
+
+/// Display implementation for [`crate::JsString`] that escapes unicode characters.
+#[derive(Debug)]
+pub struct JsStrDisplayEscaped<'a> {
+    inner: JsStr<'a>,
+}
+
+impl fmt::Display for JsStrDisplayEscaped<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self.inner.variant() {
+            // SAFETY: `JsStrVariant::Latin1` does not contain any unpaired surrogates, so need to check.
+            JsStrVariant::Latin1(v) => v
+                .iter()
+                .copied()
+                .map(char::from)
+                .try_for_each(|c| f.write_char(c)),
+            JsStrVariant::Utf16(_) => self.inner.code_points().try_for_each(|r| match r {
+                CodePoint::Unicode(c) => f.write_char(c),
+                CodePoint::UnpairedSurrogate(c) => {
+                    write!(f, "\\u{c:04X}")
+                }
+            }),
+        }
+    }
+}
+
+impl<'a> From<JsStr<'a>> for JsStrDisplayEscaped<'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).
+    let s = JsStr::latin1(b"Hello \xE9 world!");
+
+    let rust_str = format!("{}", JsStrDisplayEscaped { inner: s });
+    assert_eq!(rust_str, "Hello é world!");
+}

core/string/src/lib.rs

@@ -17,6 +17,7 @@
 #![allow(clippy::module_name_repetitions)]
 
 mod common;
+mod display;
 mod iter;
 mod str;
 mod tagged;
@@ -25,6 +26,7 @@ mod tagged;
 mod tests;
 
 use self::{iter::Windows, str::JsSliceIndex};
+use crate::display::JsStrDisplayEscaped;
 use crate::tagged::{Tagged, UnwrappedTagged};
 #[doc(inline)]
 pub use crate::{
@@ -32,6 +34,7 @@ pub use crate::{
     iter::Iter,
     str::{JsStr, JsStrVariant},
 };
+use std::fmt::Write;
 use std::{
     alloc::{alloc, dealloc, Layout},
     cell::Cell,
@@ -150,6 +153,17 @@ impl CodePoint {
     }
 }
 
+impl std::fmt::Display for CodePoint {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            CodePoint::Unicode(c) => f.write_char(*c),
+            CodePoint::UnpairedSurrogate(c) => {
+                write!(f, "\\u{c:04X}")
+            }
+        }
+    }
+}
+
 /// A `usize` contains a flag and the length of Latin1/UTF-16 .
 /// ```text
 /// ┌────────────────────────────────────┐
@@ -528,10 +542,7 @@ impl JsString {
     /// Gets an iterator of all the Unicode codepoints of a [`JsString`].
     #[inline]
     pub fn code_points(&self) -> impl Iterator<Item = CodePoint> + Clone + '_ {
-        char::decode_utf16(self.iter()).map(|res| match res {
-            Ok(c) => CodePoint::Unicode(c),
-            Err(e) => CodePoint::UnpairedSurrogate(e.unpaired_surrogate()),
-        })
+        self.as_str().code_points()
     }
 
     /// Abstract operation `StringIndexOf ( string, searchValue, fromIndex )`
@@ -935,6 +946,15 @@ impl JsString {
             UnwrappedTagged::Tag(_inner) => None,
         }
     }
+
+    /// Gets a displayable escaped string. This may be faster and has less
+    /// allocations than `format!("{}", str.to_string_escaped())` when
+    /// displaying.
+    #[inline]
+    #[must_use]
+    pub fn display_escaped(&self) -> JsStrDisplayEscaped<'_> {
+        JsStrDisplayEscaped::from(self.as_str())
+    }
 }
 
 impl Clone for JsString {
@@ -1036,10 +1056,7 @@ impl Drop for JsString {
 impl ToStringEscaped for JsString {
     #[inline]
     fn to_string_escaped(&self) -> String {
-        match self.as_str().variant() {
-            JsStrVariant::Latin1(v) => v.iter().copied().map(char::from).collect(),
-            JsStrVariant::Utf16(v) => v.to_string_escaped(),
-        }
+        format!("{}", self.display_escaped())
     }
 }
 
@@ -1232,11 +1249,6 @@ pub(crate) trait ToStringEscaped {
 impl ToStringEscaped for [u16] {
     #[inline]
     fn to_string_escaped(&self) -> String {
-        char::decode_utf16(self.iter().copied())
-            .map(|r| match r {
-                Ok(c) => String::from(c),
-                Err(e) => format!("\\u{:04X}", e.unpaired_surrogate()),
-            })
-            .collect()
+        JsString::from(self).to_string_escaped()
     }
 }

core/string/src/str.rs

@@ -1,4 +1,4 @@
-use crate::{is_trimmable_whitespace, is_trimmable_whitespace_latin1, Iter};
+use crate::{is_trimmable_whitespace, is_trimmable_whitespace_latin1, CodePoint, Iter};
 use std::{
     hash::{Hash, Hasher},
     slice::SliceIndex,
@@ -234,6 +234,16 @@ impl<'a> JsStr<'a> {
         let (m, n) = (self.len(), needle.len());
         m >= n && needle == self.get(m - n..).expect("already checked size")
     }
+
+    /// Gets an iterator of all the Unicode codepoints of a [`JsStr`].
+    /// This is not optimized for Latin1 strings.
+    #[inline]
+    pub(crate) fn code_points(self) -> impl Iterator<Item = CodePoint> + Clone + 'a {
+        char::decode_utf16(self.iter()).map(|res| match res {
+            Ok(c) => CodePoint::Unicode(c),
+            Err(e) => CodePoint::UnpairedSurrogate(e.unpaired_surrogate()),
+        })
+    }
 }
 
 impl Hash for JsStr<'_> {

core/string/src/tests.rs

@@ -2,7 +2,7 @@
 
 use std::hash::{BuildHasher, BuildHasherDefault, Hash};
 
-use crate::{JsStr, JsString, StaticJsString, StaticJsStrings};
+use crate::{JsStr, JsString, StaticJsString, StaticJsStrings, ToStringEscaped};
 
 use rustc_hash::FxHasher;
 
@@ -174,6 +174,26 @@ fn conversion_to_known_static_js_string() {
     assert!(string.unwrap().as_str().is_latin1());
 }
 
+#[test]
+fn to_string_escaped() {
+    assert_eq!(
+        JsString::from("Hello, \u{1D49E} world!").to_string_escaped(),
+        "Hello, \u{1D49E} world!"
+    );
+
+    assert_eq!(
+        JsString::from("Hello, world!").to_string_escaped(),
+        "Hello, world!"
+    );
+
+    // 15 should not be escaped.
+    let unpaired_surrogates: [u16; 3] = [0xDC58, 0xD83C, 0x0015];
+    assert_eq!(
+        JsString::from(&unpaired_surrogates).to_string_escaped(),
+        "\\uDC58\\uD83C\u{15}"
+    );
+}
+
 #[test]
 fn from_static_js_string() {
     static STATIC_HELLO_WORLD: StaticJsString =