Remove dependency on slice_internals feature in rustc_ast

Cargo.lock

@@ -3633,6 +3633,7 @@ name = "rustc_ast"
 version = "0.0.0"
 dependencies = [
  "bitflags",
+ "memchr",
  "rustc_data_structures",
  "rustc_index",
  "rustc_lexer",

compiler/rustc_ast/Cargo.toml

@@ -7,6 +7,7 @@ edition = "2021"
 
 [dependencies]
 bitflags = "1.2.1"
+memchr = "2.5.0"
 rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_index = { path = "../rustc_index" }
 rustc_lexer = { path = "../rustc_lexer" }

compiler/rustc_ast/src/lib.rs

@@ -16,7 +16,6 @@
 #![feature(let_chains)]
 #![feature(min_specialization)]
 #![feature(negative_impls)]
-#![feature(slice_internals)]
 #![feature(stmt_expr_attributes)]
 #![recursion_limit = "256"]
 #![deny(rustc::untranslatable_diagnostic)]

compiler/rustc_ast/src/util/unicode.rs

@@ -17,7 +17,7 @@ pub fn contains_text_flow_control_chars(s: &str) -> bool {
     // U+2069 - E2 81 A9
     let mut bytes = s.as_bytes();
     loop {
-        match core::slice::memchr::memchr(0xE2, bytes) {
+        match memchr::memchr(0xE2, bytes) {
             Some(idx) => {
                 // bytes are valid UTF-8 -> E2 must be followed by two bytes
                 let ch = &bytes[idx..idx + 3];