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

@@ -650,8 +650,11 @@ fn trans_aggregate<'tcx>(cx: &CodegenCx<'tcx>, span: Span, ty: TyAndLayout<'tcx>
                 }
                 .def(span, cx)
             } else if count == 0 {
-                // spir-v doesn't support zero-sized arrays
-                create_zst(cx, span, ty)
+                // create a run-time array instead of ZST ADT
+                SpirvType::RuntimeArray {
+                    element: element_type,
+                }
+                .def(span, cx)
             } 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

@@ -528,8 +528,13 @@ 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,
             }

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 +1,9 @@
+// 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 +1,10 @@
+error: this operation will panic at runtime
+ --> $DIR/array_0.rs:8:13
+  |
+8 |     let x = 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/array_zst_0.rs

@@ -0,0 +1,11 @@
+// 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] = ();
+    }
+}

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

@@ -0,0 +1,14 @@
+#![cfg_attr(target_arch = "spirv", no_std)]
+use spirv_std::spirv;
+
+fn example<const LENGTH: usize>() {
+    let mut array = [0; LENGTH];
+    for i in 0..array.len() {
+        array[i] += i;
+    }
+}
+
+#[spirv(compute(threads(1, 1, 1)))]
+pub fn compute() {
+    example::<0>();
+}

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

@@ -0,0 +1,10 @@
+// build-fail
+
+#![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];
+    array[0] = 1;
+}

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

@@ -0,0 +1,10 @@
+error: this operation will panic at runtime
+ --> $DIR/oob_0_array.rs:9:5
+  |
+9 |     array[0] = 1;
+  |     ^^^^^^^^ 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
+