Auto merge of #62094 - oli-obk:zst_intern, r=eddyb

 Don't ICE on mutable zst slices

fixes #62045

Author: bors

src/librustc_mir/interpret/intern.rs

@@ -3,7 +3,6 @@
 //! After a const evaluation has computed a value, before we destroy the const evaluator's session
 //! memory, we need to extract all memory allocations to the global memory pool so they stay around.
 
-use rustc::ty::layout::LayoutOf;
 use rustc::ty::{Ty, TyCtxt, ParamEnv, self};
 use rustc::mir::interpret::{
     InterpResult, ErrorHandled,
@@ -143,18 +142,15 @@ for
         // Handle Reference types, as these are the only relocations supported by const eval.
         // Raw pointers (and boxes) are handled by the `leftover_relocations` logic.
         let ty = mplace.layout.ty;
-        if let ty::Ref(_, _, mutability) = ty.sty {
+        if let ty::Ref(_, referenced_ty, mutability) = ty.sty {
             let value = self.ecx.read_immediate(mplace.into())?;
             // Handle trait object vtables
             if let Ok(meta) = value.to_meta() {
-                let layout = self.ecx.layout_of(ty.builtin_deref(true).unwrap().ty)?;
-                if layout.is_unsized() {
-                    if let ty::Dynamic(..) = self.ecx.tcx.struct_tail(layout.ty).sty {
-                        if let Ok(vtable) = meta.unwrap().to_ptr() {
-                            // explitly choose `Immutable` here, since vtables are immutable, even
-                            // if the reference of the fat pointer is mutable
-                            self.intern_shallow(vtable, Mutability::Immutable)?;
-                        }
+                if let ty::Dynamic(..) = self.ecx.tcx.struct_tail(referenced_ty).sty {
+                    if let Ok(vtable) = meta.unwrap().to_ptr() {
+                        // explitly choose `Immutable` here, since vtables are immutable, even
+                        // if the reference of the fat pointer is mutable
+                        self.intern_shallow(vtable, Mutability::Immutable)?;
                     }
                 }
             }
@@ -178,8 +174,14 @@ for
                     (InternMode::Static, hir::Mutability::MutMutable) => {},
                     // we statically prevent `&mut T` via `const_qualif` and double check this here
                     (InternMode::ConstBase, hir::Mutability::MutMutable) |
-                    (InternMode::Const, hir::Mutability::MutMutable) =>
-                        bug!("const qualif failed to prevent mutable references"),
+                    (InternMode::Const, hir::Mutability::MutMutable) => {
+                        match referenced_ty.sty {
+                            ty::Array(_, n) if n.unwrap_usize(self.ecx.tcx.tcx) == 0 => {}
+                            ty::Slice(_)
+                                if value.to_meta().unwrap().unwrap().to_usize(self.ecx)? == 0 => {}
+                            _ => bug!("const qualif failed to prevent mutable references"),
+                        }
+                    },
                 }
                 // Compute the mutability with which we'll start visiting the allocation. This is
                 // what gets changed when we encounter an `UnsafeCell`

src/test/ui/consts/issue-62045.rs

@@ -0,0 +1,5 @@
+// compile-pass
+
+fn main() {
+    assert_eq!(&mut [0; 1][..], &mut []);
+}

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

@@ -3,6 +3,7 @@
 // rustc-env:RUST_BACKTRACE=0
 // normalize-stderr-test "note: rustc 1.* running on .*" -> "note: rustc VERSION running on TARGET"
 // normalize-stderr-test "note: compiler flags: .*" -> "note: compiler flags: FLAGS"
+// normalize-stderr-test "interpret/intern.rs:[0-9]*:[0-9]*" -> "interpret/intern.rs:LL:CC"
 
 #![allow(const_err)]
 

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

@@ -1,12 +1,12 @@
 warning: skipping const checks
-  --> $DIR/mutable_references_ice.rs:26:9
+  --> $DIR/mutable_references_ice.rs:27:9
    |
 LL |         *MUH.x.get() = 99;
    |         ^^^^^^^^^^^^^^^^^
 
 thread 'rustc' panicked at 'assertion failed: `(left != right)`
   left: `Const`,
- right: `Const`: UnsafeCells are not allowed behind references in constants. This should have been prevented statically by const qualification. If this were allowed one would be able to change a constant at one use site and other use sites may arbitrarily decide to change, too.', src/librustc_mir/interpret/intern.rs:127:17
+ right: `Const`: UnsafeCells are not allowed behind references in constants. This should have been prevented statically by const qualification. If this were allowed one would be able to change a constant at one use site and other use sites may arbitrarily decide to change, too.', src/librustc_mir/interpret/intern.rs:LL:CC
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
 
 error: internal compiler error: unexpected panic