[naga wgsl] Move Scalar diagnostic formatting to common code.

Add function `naga::common::wgsl::write_scalar_for_diagnostics`
for writing a WGSL representation of a `Scalar` appropriate for
diagnostic purposes.

Change `naga::front::wgsl::to_wgsl`'s implementation of `TypeContext`
to use that new function.

Author: jimblandy

naga/src/common/wgsl/mod.rs

@@ -6,4 +6,4 @@ mod types;
 
 pub use diagnostics::DisplayFilterableTriggeringRule;
 pub use to_wgsl::{address_space_str, ToWgsl, TryToWgsl};
-pub use types::TypeContext;
+pub use types::{write_scalar_for_diagnostics, TypeContext};

naga/src/common/wgsl/types.rs

@@ -299,3 +299,30 @@ impl From<core::fmt::Error> for WriteTypeError {
         Self::Format(err)
     }
 }
+
+/// Write `scalar` in WGSL as appropriate for diagnostic messages.
+///
+/// If `scalar` can be expressed in WGSL, write its WGSL form to `out`.
+///
+/// If `scalar` is an abstract type, write something suggestive and
+/// human-readable to `out`.
+///
+/// Otherwise, write something human-readable that includes the
+/// [`Debug`] representation of `scalar`.
+///
+/// [`Debug`]: core::fmt::Debug
+pub fn write_scalar_for_diagnostics<W: Write>(scalar: Scalar, out: &mut W) -> core::fmt::Result {
+    match scalar.kind {
+        crate::ScalarKind::Sint
+        | crate::ScalarKind::Uint
+        | crate::ScalarKind::Float
+        | crate::ScalarKind::Bool => write!(out, "{{non-WGSL Naga scalar {scalar:?}}}"),
+
+        // The abstract types are kind of an odd pseudo-WGSL category:
+        // they are definitely part of the spec, but they are not expressible
+        // in WGSL itself. So we want to call them out by name in error messages,
+        // but the WGSL backend should never generate these.
+        crate::ScalarKind::AbstractInt => out.write_str("{AbstractInt}"),
+        crate::ScalarKind::AbstractFloat => out.write_str("{AbstractFloat}"),
+    }
+}

naga/src/front/wgsl/to_wgsl.rs

@@ -86,19 +86,7 @@ impl<W: fmt::Write> TypeContext<W> for GlobalCtx<'_> {
     }
 
     fn write_non_wgsl_scalar(&self, scalar: Scalar, out: &mut W) -> fmt::Result {
-        match scalar.kind {
-            crate::ScalarKind::Sint
-            | crate::ScalarKind::Uint
-            | crate::ScalarKind::Float
-            | crate::ScalarKind::Bool => write!(out, "{{non-WGSL Naga scalar {scalar:?}}}"),
-
-            // The abstract types are kind of an odd pseudo-WGSL category:
-            // they are definitely part of the spec, but they are not expressible
-            // in WGSL itself. So we want to call them out by name in error messages,
-            // but the WGSL backend should never generate these.
-            crate::ScalarKind::AbstractInt => out.write_str("{AbstractInt}"),
-            crate::ScalarKind::AbstractFloat => out.write_str("{AbstractFloat}"),
-        }
+        crate::common::wgsl::write_scalar_for_diagnostics(scalar, out)
     }
 }