[naga] Move back::wgsl::address_space_str to common::wgsl.

Move `back::wgsl::address_space_str` to `common::wgsl`, so that the
front end can use it too. This commit is code motion and `use`
adjustment only; there should be no change in behavior.

Author: jimblandy

naga/src/back/wgsl/writer.rs

@@ -13,7 +13,7 @@ use crate::{
     back::{self, Baked},
     common::{
         self,
-        wgsl::{ToWgsl, TryToWgsl},
+        wgsl::{address_space_str, ToWgsl, TryToWgsl},
     },
     proc::{self, ExpressionKindTracker, NameKey},
     valid, Handle, Module, ShaderStage, TypeInner,
@@ -1877,33 +1877,6 @@ impl<W: Write> Writer<W> {
     }
 }
 
-const fn address_space_str(
-    space: crate::AddressSpace,
-) -> (Option<&'static str>, Option<&'static str>) {
-    use crate::AddressSpace as As;
-
-    (
-        Some(match space {
-            As::Private => "private",
-            As::Uniform => "uniform",
-            As::Storage { access } => {
-                if access.contains(crate::StorageAccess::ATOMIC) {
-                    return (Some("storage"), Some("atomic"));
-                } else if access.contains(crate::StorageAccess::STORE) {
-                    return (Some("storage"), Some("read_write"));
-                } else {
-                    "storage"
-                }
-            }
-            As::PushConstant => "push_constant",
-            As::WorkGroup => "workgroup",
-            As::Handle => return (None, None),
-            As::Function => "function",
-        }),
-        None,
-    )
-}
-
 fn map_binding_to_attribute(binding: &crate::Binding) -> Vec<Attribute> {
     match *binding {
         crate::Binding::BuiltIn(built_in) => {

naga/src/common/wgsl.rs

@@ -337,3 +337,48 @@ impl ToWgsl for crate::ImageDimension {
         }
     }
 }
+
+/// Return the WGSL address space and access mode strings for `space`.
+///
+/// Why don't we implement [`ToWgsl`] for [`AddressSpace`]?
+///
+/// In WGSL, the full form of a pointer type is `ptr<AS, T, AM>`, where:
+/// - `AS` is the address space,
+/// - `T` is the store type, and
+/// - `AM` is the access mode.
+///
+/// Since the type `T` intervenes between the address space and the
+/// access mode, there isn't really any individual WGSL grammar
+/// production that corresponds to an [`AddressSpace`], so [`ToWgsl`]
+/// is too simple-minded for this case.
+///
+/// Furthermore, we want to write `var<AS[, AM]>` for most address
+/// spaces, but we want to just write `var foo: T` for handle types.
+///
+/// [`AddressSpace`]: crate::AddressSpace
+pub const fn address_space_str(
+    space: crate::AddressSpace,
+) -> (Option<&'static str>, Option<&'static str>) {
+    use crate::AddressSpace as As;
+
+    (
+        Some(match space {
+            As::Private => "private",
+            As::Uniform => "uniform",
+            As::Storage { access } => {
+                if access.contains(crate::StorageAccess::ATOMIC) {
+                    return (Some("storage"), Some("atomic"));
+                } else if access.contains(crate::StorageAccess::STORE) {
+                    return (Some("storage"), Some("read_write"));
+                } else {
+                    "storage"
+                }
+            }
+            As::PushConstant => "push_constant",
+            As::WorkGroup => "workgroup",
+            As::Handle => return (None, None),
+            As::Function => "function",
+        }),
+        None,
+    )
+}