Miri interning: replace ICEs by proper errors, make intern_shallow type signature more precise

Author: RalfJung

src/librustc_mir/interpret/eval_context.rs

@@ -871,6 +871,9 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         // Our result will later be validated anyway, and there seems no good reason
         // to have to fail early here.  This is also more consistent with
         // `Memory::get_static_alloc` which has to use `const_eval_raw` to avoid cycles.
+        // FIXME: We can hit delay_span_bug if this is an invalid const, interning finds
+        // that problem, but we never run validation to show an error. Can we ensure
+        // this does not happen?
         let val = self.tcx.const_eval_raw(param_env.and(gid))?;
         self.raw_const_to_mplace(val)
     }

src/librustc_mir/interpret/intern.rs

@@ -17,12 +17,11 @@ struct Foo<T>(T);
 // this is fine for the same reason as `BAR`.
 static BOO: &mut Foo<()> = &mut Foo(());
 
+// interior mutability is fine
 struct Meh {
     x: &'static UnsafeCell<i32>,
 }
-
 unsafe impl Sync for Meh {}
-
 static MEH: Meh = Meh {
     x: &UnsafeCell::new(42),
 };

src/test/ui/consts/miri_unleashed/mutable_const.rs

@@ -1,5 +1,5 @@
 error[E0594]: cannot assign to `*OH_YES`, as `OH_YES` is an immutable static item
-  --> $DIR/mutable_references.rs:37:5
+  --> $DIR/mutable_references.rs:36:5
    |
 LL |     *OH_YES = 99;
    |     ^^^^^^^^^^^^ cannot assign
@@ -22,12 +22,12 @@ help: skipping check for `const_mut_refs` feature
 LL | static BOO: &mut Foo<()> = &mut Foo(());
    |                            ^^^^^^^^^^^^
 help: skipping check that does not even have a feature gate
-  --> $DIR/mutable_references.rs:27:8
+  --> $DIR/mutable_references.rs:26:8
    |
 LL |     x: &UnsafeCell::new(42),
    |        ^^^^^^^^^^^^^^^^^^^^
 help: skipping check for `const_mut_refs` feature
-  --> $DIR/mutable_references.rs:31:27
+  --> $DIR/mutable_references.rs:30:27
    |
 LL | static OH_YES: &mut i32 = &mut 42;
    |                           ^^^^^^^

src/test/ui/consts/miri_unleashed/mutable_const.stderr

@@ -0,0 +1,37 @@
+// compile-flags: -Zunleash-the-miri-inside-of-you
+
+#![allow(const_err)]
+
+use std::cell::UnsafeCell;
+
+// this test ICEs to ensure that our mutability story is sound
+
+struct Meh {
+    x: &'static UnsafeCell<i32>,
+}
+unsafe impl Sync for Meh {}
+
+// the following will never be ok! no interior mut behind consts, because
+// all allocs interned here will be marked immutable.
+const MUH: Meh = Meh { //~ ERROR: mutable memory (`UnsafeCell`) is not allowed in constant
+    x: &UnsafeCell::new(42),
+};
+
+struct Synced {
+    x: UnsafeCell<i32>,
+}
+unsafe impl Sync for Synced {}
+
+// Make sure we also catch this behind a type-erased `dyn Trait` reference.
+const SNEAKY: &dyn Sync = &Synced { x: UnsafeCell::new(42) };
+//~^ ERROR: mutable memory (`UnsafeCell`) is not allowed in constant
+
+// Make sure we also catch mutable references.
+const BLUNT: &mut i32 = &mut 42;
+//~^ ERROR: mutable memory (`&mut`) is not allowed in constant
+
+fn main() {
+    unsafe {
+        *MUH.x.get() = 99;
+    }
+}

src/test/ui/consts/miri_unleashed/mutable_const2.rs

@@ -0,0 +1,40 @@
+error: mutable memory (`UnsafeCell`) is not allowed in constant
+  --> $DIR/mutable_references_err.rs:16:1
+   |
+LL | / const MUH: Meh = Meh {
+LL | |     x: &UnsafeCell::new(42),
+LL | | };
+   | |__^
+
+error: mutable memory (`UnsafeCell`) is not allowed in constant
+  --> $DIR/mutable_references_err.rs:26:1
+   |
+LL | const SNEAKY: &dyn Sync = &Synced { x: UnsafeCell::new(42) };
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: mutable memory (`&mut`) is not allowed in constant
+  --> $DIR/mutable_references_err.rs:30:1
+   |
+LL | const BLUNT: &mut i32 = &mut 42;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+warning: skipping const checks
+   |
+help: skipping check that does not even have a feature gate
+  --> $DIR/mutable_references_err.rs:17:8
+   |
+LL |     x: &UnsafeCell::new(42),
+   |        ^^^^^^^^^^^^^^^^^^^^
+help: skipping check that does not even have a feature gate
+  --> $DIR/mutable_references_err.rs:26:27
+   |
+LL | const SNEAKY: &dyn Sync = &Synced { x: UnsafeCell::new(42) };
+   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+help: skipping check for `const_mut_refs` feature
+  --> $DIR/mutable_references_err.rs:30:25
+   |
+LL | const BLUNT: &mut i32 = &mut 42;
+   |                         ^^^^^^^
+
+error: aborting due to 3 previous errors; 1 warning emitted
+