Arena: Copy cold_path and remove rustc_data_structures dependency

Cargo.lock

@@ -3350,7 +3350,6 @@ dependencies = [
 name = "rustc_arena"
 version = "0.0.0"
 dependencies = [
- "rustc_data_structures",
  "smallvec 1.4.2",
 ]
 

compiler/rustc_arena/Cargo.toml

@@ -5,5 +5,4 @@ version = "0.0.0"
 edition = "2018"
 
 [dependencies]
-rustc_data_structures = { path = "../rustc_data_structures" }
 smallvec = { version = "1.0", features = ["union", "may_dangle"] }

compiler/rustc_arena/src/lib.rs

@@ -16,7 +16,6 @@
 #![feature(maybe_uninit_slice)]
 #![cfg_attr(test, feature(test))]
 
-use rustc_data_structures::cold_path;
 use smallvec::SmallVec;
 
 use std::alloc::Layout;
@@ -27,6 +26,12 @@
 use std::ptr;
 use std::slice;
 
+#[inline(never)]
+#[cold]
+pub fn cold_path<F: FnOnce() -> R, R>(f: F) -> R {
+    f()
+}
+
 /// An arena that can hold objects of only one type.
 pub struct TypedArena<T> {
     /// A pointer to the next object to be allocated.