Auto merge of #63831 - eddyb:proc-macro-statecheck, r=<try>

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

Along the lines of #63809, this PR attempts to get rid of the main (or only?) place `proc_macro` handles could be leaked to, *and* further disallow/discourage sharing (other) state between invocations.

The approach of banning (interior-)mutable `static`s was most recently mentioned in #63804 (comment), but it's likely been brought up several times, we just never tried it.
(Note that this is not foolproof: one would have to scan all dependencies for such `static`s, modulo `proc_macro`/`std`, and even then it's possible there would be a lot of false positives)

So this is mostly for a check-only crater run, to see what (if anything) breaks.

src/librustc_mir/transform/qualify_consts.rs

@@ -1723,6 +1723,19 @@ impl MirPass for QualifyAndPromoteConstants {
             }
         }
 
+        // 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");
+                }
+            }
+        }
+
         // Statics must be Sync.
         if mode == Mode::Static {
             // `#[thread_local]` statics don't have to be `Sync`.

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
+