Replace ESCAPE array with is_escape fn

This is not backed by benchmarks, but it seems reasonable that we'd be
more starved for cache than CPU in IO-bound tasks. It also simplifies
code a bit and frees up some memory, which is probably a good thing.

Author: purplesyringa

src/read.rs

@@ -222,7 +222,7 @@ where
     {
         loop {
             let ch = tri!(next_or_eof(self));
-            if !ESCAPE[ch as usize] {
+            if !is_escape(ch) {
                 scratch.push(ch);
                 continue;
             }
@@ -343,7 +343,7 @@ where
     fn ignore_str(&mut self) -> Result<()> {
         loop {
             let ch = tri!(next_or_eof(self));
-            if !ESCAPE[ch as usize] {
+            if !is_escape(ch) {
                 continue;
             }
             match ch {
@@ -819,33 +819,11 @@ pub trait Fused: private::Sealed {}
 impl<'a> Fused for SliceRead<'a> {}
 impl<'a> Fused for StrRead<'a> {}
 
-// Lookup table of bytes that must be escaped. A value of true at index i means
-// that byte i requires an escape sequence in the input.
-static ESCAPE: [bool; 256] = {
-    const CT: bool = true; // control character \x00..=\x1F
-    const QU: bool = true; // quote \x22
-    const BS: bool = true; // backslash \x5C
-    const __: bool = false; // allow unescaped
-    [
-        //   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F
-        CT, CT, CT, CT, CT, CT, CT, CT, CT, CT, CT, CT, CT, CT, CT, CT, // 0
-        CT, CT, CT, CT, CT, CT, CT, CT, CT, CT, CT, CT, CT, CT, CT, CT, // 1
-        __, __, QU, __, __, __, __, __, __, __, __, __, __, __, __, __, // 2
-        __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // 3
-        __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // 4
-        __, __, __, __, __, __, __, __, __, __, __, __, BS, __, __, __, // 5
-        __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // 6
-        __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // 7
-        __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // 8
-        __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // 9
-        __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // A
-        __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // B
-        __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // C
-        __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // D
-        __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // E
-        __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // F
-    ]
-};
+// This is only used in IoRead. SliceRead hardcodes the arguments to memchr.
+#[cfg(feature = "std")]
+fn is_escape(ch: u8) -> bool {
+    ch == b'"' || ch == b'\\' || ch < 0x20
+}
 
 fn next_or_eof<'de, R>(read: &mut R) -> Result<u8>
 where