Fix zero-sized Array GEPs

Previously GEP on zero-sized arrays, would fail. This change fixes arrays to instead emit
runtime arrays. I do not know if this will lead to any runtime cost, but it fixes all the
compile errors.

crates/rustc_codegen_spirv/src/abi.rs

@@ -649,9 +649,6 @@ fn trans_aggregate<'tcx>(cx: &CodegenCx<'tcx>, span: Span, ty: TyAndLayout<'tcx>
                     element: element_type,
                 }
                 .def(span, cx)
-            } else if count == 0 {
-                // spir-v doesn't support zero-sized arrays
-                create_zst(cx, span, ty)
             } else {
                 let count_const = cx.constant_u32(span, count as u32);
                 let element_spv = cx.lookup_type(element_type);

crates/rustc_codegen_spirv/src/builder/builder_methods.rs

@@ -406,7 +406,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         let ptr = ptr.strip_ptrcasts();
         let mut leaf_ty = match self.lookup_type(ptr.ty) {
             SpirvType::Pointer { pointee } => pointee,
-            other => self.fatal(format!("non-pointer type: {other:?}")),
+            SpirvType::Adt { .. } => return None,
+            other => self.fatal(format!("adjust_pointer for non-pointer type: {other:?}")),
         };
 
         // FIXME(eddyb) this isn't efficient, `recover_access_chain_from_offset`
@@ -528,8 +529,13 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     let stride = ty_kind.sizeof(self)?;
                     ty_size = MaybeSized::Sized(stride);
 
-                    indices.push((offset.bytes() / stride.bytes()).try_into().ok()?);
-                    offset = Size::from_bytes(offset.bytes() % stride.bytes());
+                    if stride.bytes() == 0 {
+                        indices.push(0);
+                        offset = Size::from_bytes(0);
+                    } else {
+                        indices.push((offset.bytes() / stride.bytes()).try_into().ok()?);
+                        offset = Size::from_bytes(offset.bytes() % stride.bytes());
+                    }
                 }
                 _ => return None,
             }
@@ -566,6 +572,14 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             .iter()
             .map(|index| {
                 result_pointee_type = match self.lookup_type(result_pointee_type) {
+                    SpirvType::Array { count, element }
+                        if self.builder.lookup_const_u64(count) == Some(0) =>
+                    {
+                        self.fatal(format!(
+                            "evaluation of constant value failed: cannot index into [{}; 0]",
+                            self.debug_type(element)
+                        ))
+                    }
                     SpirvType::Array { element, .. } | SpirvType::RuntimeArray { element } => {
                         element
                     }

crates/rustc_codegen_spirv/src/codegen_cx/constant.rs

@@ -549,6 +549,9 @@ impl<'tcx> CodegenCx<'tcx> {
                 }
                 self.constant_composite(ty, values.into_iter())
             }
+            SpirvType::Array { count, .. } if self.builder.lookup_const_u64(count) == Some(0) => {
+                self.undef(ty)
+            }
             SpirvType::Array { element, count } => {
                 let count = self.builder.lookup_const_u64(count).unwrap() as usize;
                 let values = (0..count).map(|_| {

crates/rustc_codegen_spirv/src/spirv_type.rs

@@ -368,6 +368,9 @@ impl SpirvType<'_> {
                     .bytes(),
             )
             .expect("alignof: Vectors must have power-of-2 size"),
+            Self::Array { count, .. } if cx.builder.lookup_const_u64(count) == Some(0) => {
+                Align::from_bytes(0).unwrap()
+            }
             Self::Array { element, .. }
             | Self::RuntimeArray { element }
             | Self::Matrix { element, .. } => cx.lookup_type(element).alignof(cx),

tests/ui/lang/core/array/array_0.rs

@@ -0,0 +1,11 @@
+#![allow(unconditional_panic)]
+
+// build-fail
+#![cfg_attr(target_arch = "spirv", no_std)]
+use spirv_std::spirv;
+
+#[spirv(compute(threads(1, 1, 1)))]
+pub fn compute() {
+    let array = [0; 0];
+    let x = array[0];
+}

tests/ui/lang/core/array/array_0.stderr

@@ -0,0 +1,8 @@
+error: evaluation of constant value failed: cannot index into [i32; 0]
+  --> $DIR/array_0.rs:10:13
+   |
+10 |     let x = array[0];
+   |             ^^^^^^^^
+
+error: aborting due to 1 previous error
+

tests/ui/lang/core/array/array_zst_0.rs

@@ -0,0 +1,12 @@
+// build-pass
+#![cfg_attr(target_arch = "spirv", no_std)]
+use spirv_std::spirv;
+
+#[spirv(compute(threads(1, 1, 1)))]
+pub fn compute() {
+    let mut array = [(); 0];
+    for i in 0..array.len() {
+      array[i] = ();
+    }
+    let () = array[0];
+}

tests/ui/lang/core/array/array_zst_0.stderr

@@ -0,0 +1,10 @@
+error: this operation will panic at runtime
+  --> $DIR/array_zst_0.rs:11:14
+   |
+11 |     let () = array[0];
+   |              ^^^^^^^^ index out of bounds: the length is 0 but the index is 0
+   |
+   = note: `#[deny(unconditional_panic)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/lang/core/array/gep0.rs

@@ -0,0 +1,16 @@
+// build-fail
+
+#![cfg_attr(target_arch = "spirv", no_std)]
+use spirv_std::spirv;
+
+fn example<const N: usize>() {
+    let mut array = [0; N];
+    for i in 0..N {
+        array[i] += i;
+    }
+}
+
+#[spirv(compute(threads(1, 1, 1)))]
+pub fn compute() {
+    example::<0>();
+}

tests/ui/lang/core/array/gep0.stderr

@@ -0,0 +1,8 @@
+error: evaluation of constant value failed: cannot index into [u32; 0]
+ --> $DIR/gep0.rs:9:9
+  |
+9 |         array[i] += i;
+  |         ^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+

tests/ui/lang/core/array/oob_0_array.rs

@@ -0,0 +1,11 @@
+#![allow(unconditional_panic)]
+
+#![cfg_attr(target_arch = "spirv", no_std)]
+use spirv_std::spirv;
+
+#[spirv(compute(threads(1, 1, 1)))]
+pub fn compute() {
+    let mut array = [0; 0];
+    // for some reason this now compiles?
+    array[0] = 1;
+}

tests/ui/lang/core/array/unused_array_0.rs

@@ -0,0 +1,7 @@
+#![cfg_attr(target_arch = "spirv", no_std)]
+use spirv_std::spirv;
+
+#[spirv(compute(threads(1, 1, 1)))]
+pub fn compute() {
+    let array = [0; 0];
+}