Remove useless #[global_allocator] from rustc and rustdoc.

This was added in #83152, which has several errors in its comments.

This commit also fix up the comments, which are quite wrong and
misleading.

Author: nnethercote

compiler/rustc/src/main.rs

@@ -1,25 +1,32 @@
-// Configure jemalloc as the `global_allocator` when configured. This is
-// so that we use the sized deallocation apis jemalloc provides
-// (namely `sdallocx`).
+// A note about jemalloc: rustc uses jemalloc when built for CI and
+// distribution. The obvious way to do this is with the `#[global_allocator]`
+// mechanism. However, for complicated reasons (see
+// https://github.com/rust-lang/rust/pull/81782#issuecomment-784438001 for some
+// details) that mechanism doesn't work here. Also, we must use a consistent
+// allocator across the rustc <-> llvm boundary, and `#[global_allocator]`
+// wouldn't provide that.
 //
-// The symbol overrides documented below are also performed so that we can
-// ensure that we use a consistent allocator across the rustc <-> llvm boundary
-#[cfg(feature = "jemalloc")]
-#[global_allocator]
-static ALLOC: tikv_jemallocator::Jemalloc = tikv_jemallocator::Jemalloc;
-
+// Instead, we use a lower-level mechanism. rustc is linked with jemalloc in a
+// way such that jemalloc's implementation of `malloc`, `free`, etc., override
+// the libc allocator's implementation. This means that Rust's `System`
+// allocator, which calls `libc::malloc()` et al., is actually calling into
+// jemalloc.
+//
+// A consequence of not using `GlobalAlloc` (and the `tikv-jemallocator` crate
+// provides an impl of that trait, which is called `Jemalloc`) is that we
+// cannot use the sized deallocation APIs (`sdallocx`) that jemalloc provides.
+// It's unclear how much performance is lost because of this.
+//
+// As for the symbol overrides in `main` below: we're pulling in a static copy
+// of jemalloc. We need to actually reference its symbols for it to get linked.
+// The two crates we link to here, `std` and `rustc_driver`, are both dynamic
+// libraries. So we must reference jemalloc symbols one way or another, because
+// this file is the only object code in the rustc executable.
 #[cfg(feature = "tikv-jemalloc-sys")]
 use tikv_jemalloc_sys as jemalloc_sys;
 
 fn main() {
-    // Pull in jemalloc when enabled.
-    //
-    // Note that we're pulling in a static copy of jemalloc which means that to
-    // pull it in we need to actually reference its symbols for it to get
-    // linked. The two crates we link to here, std and rustc_driver, are both
-    // dynamic libraries. That means to pull in jemalloc we actually need to
-    // reference allocation symbols one way or another (as this file is the only
-    // object code in the rustc executable).
+    // See the comment at the top of this file for an explanation of this.
     #[cfg(feature = "tikv-jemalloc-sys")]
     {
         use std::os::raw::{c_int, c_void};

src/librustdoc/lib.rs

@@ -63,14 +63,12 @@ extern crate rustc_trait_selection;
 extern crate rustc_typeck;
 extern crate test;
 
+// See docs in https://github.com/rust-lang/rust/blob/master/compiler/rustc/src/main.rs
+// about jemalloc.
 #[cfg(feature = "jemalloc")]
 extern crate tikv_jemalloc_sys;
 #[cfg(feature = "jemalloc")]
 use tikv_jemalloc_sys as jemalloc_sys;
-#[cfg(feature = "jemalloc")]
-extern crate tikv_jemallocator;
-#[cfg(feature = "jemalloc")]
-use tikv_jemallocator as jemallocator;
 
 use std::default::Default;
 use std::env;
@@ -125,15 +123,9 @@ mod visit;
 mod visit_ast;
 mod visit_lib;
 
-// See docs in https://github.com/rust-lang/rust/blob/master/compiler/rustc/src/main.rs
-// about jemallocator
-#[cfg(feature = "jemalloc")]
-#[global_allocator]
-static ALLOC: jemallocator::Jemalloc = jemallocator::Jemalloc;
-
 pub fn main() {
     // See docs in https://github.com/rust-lang/rust/blob/master/compiler/rustc/src/main.rs
-    // about jemalloc-sys
+    // about jemalloc.
     #[cfg(feature = "jemalloc")]
     {
         use std::os::raw::{c_int, c_void};
@@ -152,10 +144,6 @@ pub fn main() {
         #[used]
         static _F6: unsafe extern "C" fn(*mut c_void) = jemalloc_sys::free;
 
-        // On OSX, jemalloc doesn't directly override malloc/free, but instead
-        // registers itself with the allocator's zone APIs in a ctor. However,
-        // the linker doesn't seem to consider ctors as "used" when statically
-        // linking, so we need to explicitly depend on the function.
         #[cfg(target_os = "macos")]
         {
             extern "C" {