Auto merge of #56813 - oli-obk:main_🧶, r=pnkfelix

Always run rustc in a thread

cc @ishitatsuyuki @eddyb

r? @pnkfelix

[Previously](https://github.com/rust-lang/rust/pull/48575) we moved to only producing threads when absolutely necessary. Even before we opted to only create threads in some cases, which [is unsound](https://github.com/rust-lang/rust/pull/48575#issuecomment-380635967) due to the way we use thread local storage.

src/librustc_driver/lib.rs

@@ -25,7 +25,6 @@
 #![feature(rustc_diagnostic_macros)]
 #![feature(slice_sort_by_cached_key)]
 #![feature(set_stdio)]
-#![feature(rustc_stack_internals)]
 #![feature(no_debug)]
 
 #![recursion_limit="256"]
@@ -1481,69 +1480,13 @@ pub fn in_named_rustc_thread<F, R>(name: String, f: F) -> Result<R, Box<dyn Any
     where F: FnOnce() -> R + Send + 'static,
           R: Send + 'static,
 {
-    #[cfg(all(unix, not(target_os = "haiku")))]
-    let spawn_thread = unsafe {
-        // Fetch the current resource limits
-        let mut rlim = libc::rlimit {
-            rlim_cur: 0,
-            rlim_max: 0,
-        };
-        if libc::getrlimit(libc::RLIMIT_STACK, &mut rlim) != 0 {
-            let err = io::Error::last_os_error();
-            error!("in_rustc_thread: error calling getrlimit: {}", err);
-            true
-        } else if rlim.rlim_max < STACK_SIZE as libc::rlim_t {
-            true
-        } else if rlim.rlim_cur < STACK_SIZE as libc::rlim_t {
-            std::rt::deinit_stack_guard();
-            rlim.rlim_cur = STACK_SIZE as libc::rlim_t;
-            if libc::setrlimit(libc::RLIMIT_STACK, &mut rlim) != 0 {
-                let err = io::Error::last_os_error();
-                error!("in_rustc_thread: error calling setrlimit: {}", err);
-                std::rt::update_stack_guard();
-                true
-            } else {
-                std::rt::update_stack_guard();
-                false
-            }
-        } else {
-            false
-        }
-    };
-
-    // We set the stack size at link time. See src/rustc/rustc.rs.
-    #[cfg(windows)]
-    let spawn_thread = false;
-
-    #[cfg(target_os = "haiku")]
-    let spawn_thread = unsafe {
-        // Haiku does not have setrlimit implemented for the stack size.
-        // By default it does have the 16 MB stack limit, but we check this in
-        // case the minimum STACK_SIZE changes or Haiku's defaults change.
-        let mut rlim = libc::rlimit {
-            rlim_cur: 0,
-            rlim_max: 0,
-        };
-        if libc::getrlimit(libc::RLIMIT_STACK, &mut rlim) != 0 {
-            let err = io::Error::last_os_error();
-            error!("in_rustc_thread: error calling getrlimit: {}", err);
-            true
-        } else if rlim.rlim_cur >= STACK_SIZE {
-            false
-        } else {
-            true
-        }
-    };
-
-    #[cfg(not(any(windows, unix)))]
-    let spawn_thread = true;
-
-    // The or condition is added from backward compatibility.
-    if spawn_thread || env::var_os("RUST_MIN_STACK").is_some() {
+    // We need a thread for soundness of thread local storage in rustc. For debugging purposes
+    // we allow an escape hatch where everything runs on the main thread.
+    if env::var_os("RUSTC_UNSTABLE_NO_MAIN_THREAD").is_none() {
         let mut cfg = thread::Builder::new().name(name);
 
-        // FIXME: Hacks on hacks. If the env is trying to override the stack size
-        // then *don't* set it explicitly.
+        // If the env is trying to override the stack size then *don't* set it explicitly.
+        // The libstd thread impl will fetch the `RUST_MIN_STACK` env var itself.
         if env::var_os("RUST_MIN_STACK").is_none() {
             cfg = cfg.stack_size(STACK_SIZE);
         }

src/libstd/rt.rs

@@ -73,18 +73,3 @@ fn lang_start<T: ::process::Termination + 'static>
 {
     lang_start_internal(&move || main().report(), argc, argv)
 }
-
-/// Function used for reverting changes to the main stack before setrlimit().
-/// This is POSIX (non-Linux) specific and unlikely to be directly stabilized.
-#[unstable(feature = "rustc_stack_internals", issue = "0")]
-pub unsafe fn deinit_stack_guard() {
-    ::sys::thread::guard::deinit();
-}
-
-/// Function used for resetting the main stack guard address after setrlimit().
-/// This is POSIX specific and unlikely to be directly stabilized.
-#[unstable(feature = "rustc_stack_internals", issue = "0")]
-pub unsafe fn update_stack_guard() {
-    let main_guard = ::sys::thread::guard::init();
-    ::sys_common::thread_info::reset_guard(main_guard);
-}

src/libstd/sys/cloudabi/thread.rs

@@ -121,7 +121,6 @@ pub mod guard {
     pub unsafe fn init() -> Option<Guard> {
         None
     }
-    pub unsafe fn deinit() {}
 }
 
 fn min_stack_size(_: *const libc::pthread_attr_t) -> usize {

src/libstd/sys/redox/thread.rs

@@ -92,5 +92,4 @@ pub mod guard {
     pub type Guard = !;
     pub unsafe fn current() -> Option<Guard> { None }
     pub unsafe fn init() -> Option<Guard> { None }
-    pub unsafe fn deinit() {}
 }

src/libstd/sys/sgx/thread.rs

@@ -97,5 +97,4 @@ pub mod guard {
     pub type Guard = !;
     pub unsafe fn current() -> Option<Guard> { None }
     pub unsafe fn init() -> Option<Guard> { None }
-    pub unsafe fn deinit() {}
 }

src/libstd/sys/unix/thread.rs

@@ -211,7 +211,6 @@ pub mod guard {
     pub type Guard = Range<usize>;
     pub unsafe fn current() -> Option<Guard> { None }
     pub unsafe fn init() -> Option<Guard> { None }
-    pub unsafe fn deinit() {}
 }
 
 
@@ -355,26 +354,6 @@ pub mod guard {
         }
     }
 
-    pub unsafe fn deinit() {
-        if !cfg!(target_os = "linux") {
-            if let Some(stackaddr) = get_stack_start_aligned() {
-                // Remove the protection on the guard page.
-                // FIXME: we cannot unmap the page, because when we mmap()
-                // above it may be already mapped by the OS, which we can't
-                // detect from mmap()'s return value. If we unmap this page,
-                // it will lead to failure growing stack size on platforms like
-                // macOS. Instead, just restore the page to a writable state.
-                // This ain't Linux, so we probably don't need to care about
-                // execstack.
-                let result = mprotect(stackaddr, PAGE_SIZE, PROT_READ | PROT_WRITE);
-
-                if result != 0 {
-                    panic!("unable to reset the guard page");
-                }
-            }
-        }
-    }
-
     #[cfg(any(target_os = "macos",
               target_os = "bitrig",
               target_os = "openbsd",

src/libstd/sys/wasm/thread.rs

@@ -68,7 +68,6 @@ pub mod guard {
     pub type Guard = !;
     pub unsafe fn current() -> Option<Guard> { None }
     pub unsafe fn init() -> Option<Guard> { None }
-    pub unsafe fn deinit() {}
 }
 
 cfg_if! {

src/libstd/sys/windows/thread.rs

@@ -98,5 +98,4 @@ pub mod guard {
     pub type Guard = !;
     pub unsafe fn current() -> Option<Guard> { None }
     pub unsafe fn init() -> Option<Guard> { None }
-    pub unsafe fn deinit() {}
 }