Rollup merge of #113142 - the8472:opt-cstr-display, r=Mark-Simulacrum

optimize EscapeAscii's Display  and CStr's Debug

```
old:
    ascii::bench_ascii_escape_display_mixed      17.97µs/iter +/- 204.00ns
    ascii::bench_ascii_escape_display_no_escape 545.00ns/iter   +/- 6.00ns
new:
    ascii::bench_ascii_escape_display_mixed      4.99µs/iter +/- 56.00ns
    ascii::bench_ascii_escape_display_no_escape 91.00ns/iter  +/- 1.00ns
```

7 files changed, 105 insertions, 1 deletions

diff --git a/library/core/benches/ascii.rs b/library/core/benches/ascii.rs
index 64938745a4a..71ec9fed2fe 100644
--- a/library/core/benches/ascii.rs
+++ b/library/core/benches/ascii.rs
@@ -63,6 +63,7 @@ macro_rules! benches {
     }
 }
 
+use std::fmt::Write;
 use test::black_box;
 use test::Bencher;
 
@@ -351,3 +352,30 @@ static ASCII_CHARACTER_CLASS: [AsciiCharacterClass; 256] = [
     N, N, N, N, N, N, N, N, N, N, N, N, N, N, N, N,
     N, N, N, N, N, N, N, N, N, N, N, N, N, N, N, N,
 ];
+
+const ASCII_PATH: &[u8] = b"home/kyubey/rust/build/x86_64-unknown-linux-gnu/stage0/lib:/home/kyubey/workspace/rust/build/x86_64-unknown-linux-gnu/stage0-tools/release/deps";
+const RUST_INCANTATION: &[u8] = br#"AR_x86_64_unknown_linux_gnu="ar" CARGO_INCREMENTAL="0" CARGO_PROFILE_RELEASE_DEBUG="1" CARGO_PROFILE_RELEASE_DEBUG_ASSERTIONS="false" CARGO_PROFILE_RELEASE_OVERFLOW_CHECKS="false" CARGO_TARGET_DIR="/home/kyubey/workspace/rust/build/x86_64-unknown-linux-gnu/stage0-std" CC_x86_64_unknown_linux_gnu="cc" CFG_COMPILER_HOST_TRIPLE="x86_64-unknown-linux-gnu" CFG_RELEASE_CHANNEL="dev" CFLAGS_x86_64_unknown_linux_gnu="-ffunction-sections -fdata-sections -fPIC -m64" CXXFLAGS_x86_64_unknown_linux_gnu="-ffunction-sections -fdata-sections -fPIC -m64" CXX_x86_64_unknown_linux_gnu="c++" LD_LIBRARY_PATH="/home/kyubey/workspace/rust/build/x86_64-unknown-linux-gnu/stage0-sysroot/lib/rustlib/x86_64-unknown-linux-gnu/lib" LIBC_CHECK_CFG="1" RANLIB_x86_64_unknown_linux_gnu="ar s" REAL_LIBRARY_PATH_VAR="LD_LIBRARY_PATH" RUSTBUILD_NATIVE_DIR="/home/kyubey/workspace/rust/build/x86_64-unknown-linux-gnu/native" RUSTC="/home/kyubey/workspace/rust/build/bootstrap/debug/rustc" RUSTC_BOOTSTRAP="1" RUSTC_BREAK_ON_ICE="1" RUSTC_ERROR_METADATA_DST="/home/kyubey/workspace/rust/build/tmp/extended-error-metadata" RUSTC_FORCE_UNSTABLE="1" RUSTC_HOST_FUSE_LD_LLD="1" RUSTC_INSTALL_BINDIR="bin" RUSTC_LIBDIR="/home/kyubey/workspace/rust/build/x86_64-unknown-linux-gnu/stage0/lib" RUSTC_LINT_FLAGS="-Wrust_2018_idioms -Wunused_lifetimes -Wsemicolon_in_expressions_from_macros" RUSTC_REAL="/home/kyubey/workspace/rust/build/x86_64-unknown-linux-gnu/stage0/bin/rustc" RUSTC_SNAPSHOT="/home/kyubey/workspace/rust/build/x86_64-unknown-linux-gnu/stage0/bin/rustc" RUSTC_SNAPSHOT_LIBDIR="/home/kyubey/workspace/rust/build/x86_64-unknown-linux-gnu/stage0/lib" RUSTC_STAGE="0" RUSTC_SYSROOT="/home/kyubey/workspace/rust/build/x86_64-unknown-linux-gnu/stage0-sysroot" RUSTC_VERBOSE="0" RUSTDOC="/home/kyubey/workspace/rust/build/bootstrap/debug/rustdoc" RUSTDOCFLAGS="-C target-cpu=native --cfg=bootstrap -Csymbol-mangling-version=legacy -Zunstable-options -Zunstable-options --check-cfg=values(bootstrap) --check-cfg=values(stdarch_intel_sde) --check-cfg=values(no_fp_fmt_parse) --check-cfg=values(no_global_oom_handling) --check-cfg=values(no_rc) --check-cfg=values(no_sync) --check-cfg=values(freebsd12) --check-cfg=values(freebsd13) --check-cfg=values(backtrace_in_libstd) --check-cfg=values(target_env,\"libnx\") --check-cfg=values(target_arch,\"asmjs\",\"spirv\",\"nvptx\",\"xtensa\") -Clink-arg=-fuse-ld=lld -Clink-arg=-Wl,--threads=1 -Wrustdoc::invalid_codeblock_attributes --crate-version 1.72.0-dev -Zcrate-attr=doc(html_root_url=\"https://doc.rust-lang.org/nightly/\") -Zcrate-attr=warn(rust_2018_idioms)" RUSTDOC_FUSE_LD_LLD="1" RUSTDOC_LIBDIR="/home/kyubey/workspace/rust/build/x86_64-unknown-linux-gnu/stage0/lib" RUSTDOC_REAL="/path/to/nowhere/rustdoc/not/required" RUSTFLAGS="-C target-cpu=native --cfg=bootstrap -Csymbol-mangling-version=legacy -Zunstable-options -Zunstable-options --check-cfg=values(bootstrap) --check-cfg=values(stdarch_intel_sde) --check-cfg=values(no_fp_fmt_parse) --check-cfg=values(no_global_oom_handling) --check-cfg=values(no_rc) --check-cfg=values(no_sync) --check-cfg=values(freebsd12) --check-cfg=values(freebsd13) --check-cfg=values(backtrace_in_libstd) --check-cfg=values(target_env,\"libnx\") --check-cfg=values(target_arch,\"asmjs\",\"spirv\",\"nvptx\",\"xtensa\") -Zmacro-backtrace -Clink-args=-Wl,-z,origin -Clink-args=-Wl,-rpath,$ORIGIN/../lib -Clink-args=-fuse-ld=lld -Csplit-debuginfo=off -Cprefer-dynamic -Zinline-mir -Clto=off -Zcrate-attr=doc(html_root_url=\"https://doc.rust-lang.org/nightly/\")" RUST_COMPILER_RT_ROOT="/home/kyubey/workspace/rust/src/llvm-project/compiler-rt" RUST_TEST_THREADS="48" WINAPI_NO_BUNDLED_LIBRARIES="1" __CARGO_DEFAULT_LIB_METADATA="bootstrapstd" "/home/kyubey/workspace/rust/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "bench" "--target" "x86_64-unknown-linux-gnu" "-Zcheck-cfg=names,values,output" "-Zbinary-dep-depinfo" "-j" "48" "--features" " panic-unwind backtrace compiler-builtins-c" "--manifest-path" "/home/kyubey/workspace/rust/library/sysroot/Cargo.toml" "-p" "core" "--" "bench_ascii_escape_display" "--quiet" "-Z" "unstable-options" "--format" "json""#;
+
+#[bench]
+fn bench_ascii_escape_display_no_escape(b: &mut Bencher) {
+    let mut writer = String::with_capacity(8 * 1024);
+
+    b.iter(move || {
+        writer.clear();
+        let iter = ASCII_PATH.escape_ascii();
+        write!(writer, "{}", iter).unwrap();
+        writer.len()
+    })
+}
+
+#[bench]
+fn bench_ascii_escape_display_mixed(b: &mut Bencher) {
+    let mut writer = String::with_capacity(8 * 1024);
+
+    b.iter(move || {
+        writer.clear();
+        let iter = RUST_INCANTATION.escape_ascii();
+        write!(writer, "{}", iter).unwrap();
+        writer.len()
+    })
+}
diff --git a/library/core/src/ascii.rs b/library/core/src/ascii.rs
index ef8e4d098ed..02867789b79 100644
--- a/library/core/src/ascii.rs
+++ b/library/core/src/ascii.rs
@@ -96,6 +96,17 @@ pub fn escape_default(c: u8) -> EscapeDefault {
     EscapeDefault(escape::EscapeIterInner::new(data, range))
 }
 
+impl EscapeDefault {
+    pub(crate) fn empty() -> Self {
+        let data = [Char::Null; 4];
+        EscapeDefault(escape::EscapeIterInner::new(data, 0..0))
+    }
+
+    pub(crate) fn as_str(&self) -> &str {
+        self.0.as_str()
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Iterator for EscapeDefault {
     type Item = u8;
diff --git a/library/core/src/iter/adapters/flatten.rs b/library/core/src/iter/adapters/flatten.rs
index 6122332da0d..7d6f746845e 100644
--- a/library/core/src/iter/adapters/flatten.rs
+++ b/library/core/src/iter/adapters/flatten.rs
@@ -24,6 +24,14 @@ impl<I: Iterator, U: IntoIterator, F: FnMut(I::Item) -> U> FlatMap<I, U, F> {
     pub(in crate::iter) fn new(iter: I, f: F) -> FlatMap<I, U, F> {
         FlatMap { inner: FlattenCompat::new(iter.map(f)) }
     }
+
+    pub(crate) fn into_parts(self) -> (Option<U::IntoIter>, Option<I>, Option<U::IntoIter>) {
+        (
+            self.inner.frontiter,
+            self.inner.iter.into_inner().map(Map::into_inner),
+            self.inner.backiter,
+        )
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
diff --git a/library/core/src/iter/adapters/fuse.rs b/library/core/src/iter/adapters/fuse.rs
index 462a7e87733..7781ed088b7 100644
--- a/library/core/src/iter/adapters/fuse.rs
+++ b/library/core/src/iter/adapters/fuse.rs
@@ -24,6 +24,10 @@ impl<I> Fuse<I> {
     pub(in crate::iter) fn new(iter: I) -> Fuse<I> {
         Fuse { iter: Some(iter) }
     }
+
+    pub(crate) fn into_inner(self) -> Option<I> {
+        self.iter
+    }
 }
 
 #[stable(feature = "fused", since = "1.26.0")]
diff --git a/library/core/src/iter/adapters/map.rs b/library/core/src/iter/adapters/map.rs
index e27fc7257f6..c882c9e7f3f 100644
--- a/library/core/src/iter/adapters/map.rs
+++ b/library/core/src/iter/adapters/map.rs
@@ -69,6 +69,10 @@ impl<I, F> Map<I, F> {
     pub(in crate::iter) fn new(iter: I, f: F) -> Map<I, F> {
         Map { iter, f }
     }
+
+    pub(crate) fn into_inner(self) -> I {
+        self.iter
+    }
 }
 
 #[stable(feature = "core_impl_debug", since = "1.9.0")]
diff --git a/library/core/src/slice/ascii.rs b/library/core/src/slice/ascii.rs
index ce04a9f4089..5c4f0bf9b2b 100644
--- a/library/core/src/slice/ascii.rs
+++ b/library/core/src/slice/ascii.rs
@@ -5,6 +5,7 @@ use crate::fmt::{self, Write};
 use crate::iter;
 use crate::mem;
 use crate::ops;
+use core::ascii::EscapeDefault;
 
 #[cfg(not(test))]
 impl [u8] {
@@ -253,7 +254,45 @@ impl<'a> iter::FusedIterator for EscapeAscii<'a> {}
 #[stable(feature = "inherent_ascii_escape", since = "1.60.0")]
 impl<'a> fmt::Display for EscapeAscii<'a> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        self.clone().try_for_each(|b| f.write_char(b as char))
+        // disassemble iterator, including front/back parts of flatmap in case it has been partially consumed
+        let (front, slice, back) = self.clone().inner.into_parts();
+        let front = front.unwrap_or(EscapeDefault::empty());
+        let mut bytes = slice.unwrap_or_default().as_slice();
+        let back = back.unwrap_or(EscapeDefault::empty());
+
+        // usually empty, so the formatter won't have to do any work
+        for byte in front {
+            f.write_char(byte as char)?;
+        }
+
+        fn needs_escape(b: u8) -> bool {
+            b > 0x7E || b < 0x20 || b == b'\\' || b == b'\'' || b == b'"'
+        }
+
+        while bytes.len() > 0 {
+            // fast path for the printable, non-escaped subset of ascii
+            let prefix = bytes.iter().take_while(|&&b| !needs_escape(b)).count();
+            // SAFETY: prefix length was derived by counting bytes in the same splice, so it's in-bounds
+            let (prefix, remainder) = unsafe { bytes.split_at_unchecked(prefix) };
+            // SAFETY: prefix is a valid utf8 sequence, as it's a subset of ASCII
+            let prefix = unsafe { crate::str::from_utf8_unchecked(prefix) };
+
+            f.write_str(prefix)?; // the fast part
+
+            bytes = remainder;
+
+            if let Some(&b) = bytes.first() {
+                // guaranteed to be non-empty, better to write it as a str
+                f.write_str(ascii::escape_default(b).as_str())?;
+                bytes = &bytes[1..];
+            }
+        }
+
+        // also usually empty
+        for byte in back {
+            f.write_char(byte as char)?;
+        }
+        Ok(())
     }
 }
 #[stable(feature = "inherent_ascii_escape", since = "1.60.0")]
diff --git a/library/core/tests/ascii.rs b/library/core/tests/ascii.rs
index f5f2dd04778..3d3f8ac10c6 100644
--- a/library/core/tests/ascii.rs
+++ b/library/core/tests/ascii.rs
@@ -479,3 +479,13 @@ fn ascii_ctype_const() {
         is_ascii_control      => [false, false, false, false, false];
     }
 }
+
+#[test]
+fn test_ascii_display() {
+    assert_eq!(b"foo'bar".escape_ascii().to_string(), r#"foo\'bar"#);
+    assert_eq!(b"\0\xff".escape_ascii().to_string(), r#"\x00\xff"#);
+    let mut it = b"\0fastpath\xffremainder\xff".escape_ascii();
+    let _ = it.advance_by(4);
+    let _ = it.advance_back_by(4);
+    assert_eq!(it.to_string(), r#"fastpath\xffremainder"#);
+}