[WIP] rustc_mir: disallow global mutable state in proc macros.

src/librustc_mir/transform/qualify_consts.rs

@@ -1648,6 +1648,19 @@ impl<'tcx> MirPass<'tcx> for QualifyAndPromoteConstants<'tcx> {
             remove_drop_and_storage_dead_on_promoted_locals(body, promoted_temps);
         }
 
+        // HACK(eddyb) try to prevent global mutable state in proc macros.
+        // (this is not perfect and could also have false positives)
+        if mode == Mode::Static || mode == Mode::StaticMut {
+            use rustc::session::config::CrateType;
+            if tcx.sess.crate_types.borrow().contains(&CrateType::ProcMacro) {
+                let ty = body.return_ty();
+                let param_env = ty::ParamEnv::empty();
+                if mode == Mode::StaticMut || !ty.is_freeze(tcx, param_env, DUMMY_SP) {
+                    tcx.sess.span_err(body.span, "mutable global state in a proc-macro");
+                }
+            }
+        }
+
         if mode == Mode::Static && !tcx.has_attr(def_id, sym::thread_local) {
             // `static`s (not `static mut`s) which are not `#[thread_local]` must be `Sync`.
             check_static_is_sync(tcx, body, hir_id);

src/test/ui/proc-macro/global-mut-state.rs

@@ -0,0 +1,19 @@
+// force-host
+// no-prefer-dynamic
+
+#![crate_type = "proc-macro"]
+#![allow(warnings)]
+
+use std::cell::Cell;
+use std::sync::atomic::AtomicBool;
+
+static mut FOO: u8 = 0;
+//~^ ERROR mutable global state in a proc-macro
+
+static BAR: AtomicBool = AtomicBool::new(false);
+//~^ ERROR mutable global state in a proc-macro
+
+thread_local!(static BAZ: Cell<String> = Cell::new(String::new()));
+//~^ ERROR mutable global state in a proc-macro
+
+static FROZEN: &str = "snow";

src/test/ui/proc-macro/global-mut-state.stderr

@@ -0,0 +1,22 @@
+error: mutable global state in a proc-macro
+  --> $DIR/global-mut-state.rs:10:1
+   |
+LL | static mut FOO: u8 = 0;
+   | ^^^^^^^^^^^^^^^^^^^^^^^
+
+error: mutable global state in a proc-macro
+  --> $DIR/global-mut-state.rs:13:1
+   |
+LL | static BAR: AtomicBool = AtomicBool::new(false);
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: mutable global state in a proc-macro
+  --> $DIR/global-mut-state.rs:16:1
+   |
+LL | thread_local!(static BAZ: Cell<String> = Cell::new(String::new()));
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
+
+error: aborting due to 3 previous errors
+