const validation: fix ICE on dangling ZST reference

compiler/rustc_const_eval/src/interpret/validity.rs

@@ -29,7 +29,7 @@ use rustc_target::abi::{
 use std::hash::Hash;
 
 use super::{
-    err_ub, format_interp_error, machine::AllocMap, throw_ub, AllocId, CheckInAllocMsg,
+    err_ub, format_interp_error, machine::AllocMap, throw_ub, AllocId, AllocKind, CheckInAllocMsg,
     GlobalAlloc, ImmTy, Immediate, InterpCx, InterpResult, MPlaceTy, Machine, MemPlaceMeta, OpTy,
     Pointer, Projectable, Scalar, ValueVisitor,
 };
@@ -413,8 +413,6 @@ impl<'rt, 'tcx, M: Machine<'tcx>> ValidityVisitor<'rt, 'tcx, M> {
             Ub(PointerOutOfBounds { .. }) => DanglingPtrOutOfBounds {
                 ptr_kind
             },
-            // This cannot happen during const-eval (because interning already detects
-            // dangling pointers), but it can happen in Miri.
             Ub(PointerUseAfterFree(..)) => DanglingPtrUseAfterFree {
                 ptr_kind,
             },
@@ -493,9 +491,17 @@ impl<'rt, 'tcx, M: Machine<'tcx>> ValidityVisitor<'rt, 'tcx, M> {
                     }
                 }
 
-                // Mutability check.
+                // Dangling and Mutability check.
+                let (size, _align, alloc_kind) = self.ecx.get_alloc_info(alloc_id);
+                if alloc_kind == AllocKind::Dead {
+                    // This can happen for zero-sized references. We can't have *any* references to non-existing
+                    // allocations though, interning rejects them all as the rest of rustc isn't happy with them...
+                    // so we throw an error, even though this isn't really UB.
+                    // A potential future alternative would be to resurrect this as a zero-sized allocation
+                    // (which codegen will then compile to an aligned dummy pointer anyway).
+                    throw_validation_failure!(self.path, DanglingPtrUseAfterFree { ptr_kind });
+                }
                 // If this allocation has size zero, there is no actual mutability here.
-                let (size, _align, _alloc_kind) = self.ecx.get_alloc_info(alloc_id);
                 if size != Size::ZERO {
                     let alloc_actual_mutbl = mutability(self.ecx, alloc_id);
                     // Mutable pointer to immutable memory is no good.

tests/ui/consts/dangling-alloc-id-ice.rs

@@ -10,7 +10,7 @@ union Foo<'a> {
 }
 
 const FOO: &() = {
-    //~^ ERROR encountered dangling pointer
+    //~^ ERROR it is undefined behavior to use this value
     let y = ();
     unsafe { Foo { y: &y }.long_live_the_unit }
 };

tests/ui/consts/dangling-alloc-id-ice.stderr

@@ -1,8 +1,14 @@
-error: encountered dangling pointer in final value of constant
+error[E0080]: it is undefined behavior to use this value
   --> $DIR/dangling-alloc-id-ice.rs:12:1
    |
 LL | const FOO: &() = {
-   | ^^^^^^^^^^^^^^
+   | ^^^^^^^^^^^^^^ constructing invalid value: encountered a dangling reference (use-after-free)
+   |
+   = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
+   = note: the raw bytes of the constant (size: $SIZE, align: $ALIGN) {
+               HEX_DUMP
+           }
 
 error: aborting due to 1 previous error
 
+For more information about this error, try `rustc --explain E0080`.

tests/ui/consts/dangling-zst-ice-issue-126393.rs

@@ -0,0 +1,15 @@
+// Strip out raw byte dumps to make comparison platform-independent:
+//@ normalize-stderr-test "(the raw bytes of the constant) \(size: [0-9]*, align: [0-9]*\)" -> "$1 (size: $$SIZE, align: $$ALIGN)"
+//@ normalize-stderr-test "([0-9a-f][0-9a-f] |╾─*A(LLOC)?[0-9]+(\+[a-z0-9]+)?(<imm>)?─*╼ )+ *│.*" -> "HEX_DUMP"
+//@ normalize-stderr-test "HEX_DUMP\s*\n\s*HEX_DUMP" -> "HEX_DUMP"
+
+pub struct Wrapper;
+pub static MAGIC_FFI_REF: &'static Wrapper = unsafe {
+//~^ERROR: it is undefined behavior to use this value
+    std::mem::transmute(&{
+        let y = 42;
+        y
+    })
+};
+
+fn main() {}

tests/ui/consts/dangling-zst-ice-issue-126393.stderr

@@ -0,0 +1,14 @@
+error[E0080]: it is undefined behavior to use this value
+  --> $DIR/dangling-zst-ice-issue-126393.rs:7:1
+   |
+LL | pub static MAGIC_FFI_REF: &'static Wrapper = unsafe {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered a dangling reference (use-after-free)
+   |
+   = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
+   = note: the raw bytes of the constant (size: $SIZE, align: $ALIGN) {
+               HEX_DUMP
+           }
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0080`.