Do not promote extern statics

compiler/rustc_mir_transform/src/promote_consts.rs

@@ -317,6 +317,8 @@ impl<'tcx> Validator<'_, 'tcx> {
                     // can only promote static accesses inside statics.
                     && let Some(hir::ConstContext::Static(..)) = self.const_kind
                     && !self.tcx.is_thread_local_static(did)
+                    // Extern statics can never be read by CTFE, even inside a static.
+                    && !self.tcx.is_foreign_item(did)
                 {
                     // Recurse.
                 } else {

tests/mir-opt/const_promotion_extern_static.rs

@@ -1,18 +1,11 @@
 //@ skip-filecheck
 //@ ignore-endian-big
-extern "C" {
-    static X: i32;
-}
 static Y: i32 = 42;
 
 // EMIT_MIR const_promotion_extern_static.BAR.PromoteTemps.diff
 // EMIT_MIR const_promotion_extern_static.BAR-promoted[0].SimplifyCfg-pre-optimizations.after.mir
 static mut BAR: *const &i32 = [&Y].as_ptr();
 
-// EMIT_MIR const_promotion_extern_static.FOO.PromoteTemps.diff
-// EMIT_MIR const_promotion_extern_static.FOO-promoted[0].SimplifyCfg-pre-optimizations.after.mir
-static mut FOO: *const &i32 = [unsafe { &X }].as_ptr();
-
 // EMIT_MIR const_promotion_extern_static.BOP.built.after.mir
 static BOP: &i32 = &13;
 

tests/ui/statics/extern-static-in-static-ice-143174.rs

@@ -0,0 +1,15 @@
+// Regression test for #143174.
+
+#![crate_type = "lib"]
+
+type Fun = unsafe extern "C" fn();
+
+struct Foo(Fun);
+
+static FOO: &Foo = &Foo(BAR);
+//~^ ERROR cannot access extern static `BAR` [E0080]
+//~| ERROR use of extern static is unsafe and requires unsafe function or block [E0133]
+
+unsafe extern "C" {
+    static BAR: Fun;
+}

tests/ui/statics/extern-static-in-static-ice-143174.stderr

@@ -0,0 +1,18 @@
+error[E0080]: cannot access extern static `BAR`
+  --> $DIR/extern-static-in-static-ice-143174.rs:9:25
+   |
+LL | static FOO: &Foo = &Foo(BAR);
+   |                         ^^^ evaluation of `FOO` failed here
+
+error[E0133]: use of extern static is unsafe and requires unsafe function or block
+  --> $DIR/extern-static-in-static-ice-143174.rs:9:25
+   |
+LL | static FOO: &Foo = &Foo(BAR);
+   |                         ^^^ use of extern static
+   |
+   = note: extern statics are not controlled by the Rust type system: invalid data, aliasing violations or data races will cause undefined behavior
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0080, E0133.
+For more information about an error, try `rustc --explain E0080`.

tests/ui/statics/extern-static-promotion-rejected.rs

@@ -0,0 +1,11 @@
+// previously part of tests/mir-opt/const_promotion_extern_static.rs
+// promotion of extern statics is now rejected entirely, even if we're not trying to read its value
+
+unsafe extern "C" {
+    static X: i32;
+}
+
+static mut FOO: *const &i32 = [unsafe { &X }].as_ptr();
+//~^ ERROR dangling pointer
+
+fn main() {}

tests/ui/statics/extern-static-promotion-rejected.stderr

@@ -0,0 +1,8 @@
+error: encountered dangling pointer in final value of mutable static
+  --> $DIR/extern-static-promotion-rejected.rs:8:1
+   |
+LL | static mut FOO: *const &i32 = [unsafe { &X }].as_ptr();
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+