[naga wgsl-out] Consolidate unsupported Naga IR feature errors.

Introduce a new variant of `naga::back::wgsl::Error`, `Unsupported`,
and let it subsume `UnsupportedMathFunction` and various uses of
`Custom`.

Introduce a helper function, `Error::unsupported`, for building these
errors.

naga/src/back/wgsl/mod.rs

@@ -7,6 +7,7 @@ Backend for [WGSL][wgsl] (WebGPU Shading Language).
 mod polyfill;
 mod writer;
 
+use alloc::format;
 use alloc::string::String;
 
 use thiserror::Error;
@@ -21,10 +22,28 @@ pub enum Error {
     Custom(String),
     #[error("{0}")]
     Unimplemented(String), // TODO: Error used only during development
-    #[error("Unsupported math function: {0:?}")]
-    UnsupportedMathFunction(crate::MathFunction),
     #[error("Unsupported relational function: {0:?}")]
     UnsupportedRelationalFunction(crate::RelationalFunction),
+    #[error("Unsupported {kind}: {value}")]
+    Unsupported {
+        /// What kind of unsupported thing this is: interpolation, builtin, etc.
+        kind: &'static str,
+
+        /// The debug form of the Naga IR value that this backend can't express.
+        value: String,
+    },
+}
+
+impl Error {
+    /// Produce an [`Unsupported`] error for `value`.
+    ///
+    /// [`Unsupported`]: Error::Unsupported
+    fn unsupported<T: core::fmt::Debug>(kind: &'static str, value: T) -> Error {
+        Error::Unsupported {
+            kind,
+            value: format!("{value:?}"),
+        }
+    }
 }
 
 pub fn write_string(

naga/src/back/wgsl/writer.rs

@@ -1765,12 +1765,12 @@ impl<W: Write> Writer<W> {
                         let ty = func_ctx.resolve_type(arg, &module.types);
 
                         let Some(overload) = InversePolyfill::find_overload(ty) else {
-                            return Err(Error::UnsupportedMathFunction(fun));
+                            return Err(Error::unsupported("math function", fun));
                         };
 
                         Function::InversePolyfill(overload)
                     }
-                    Mf::Outer => return Err(Error::UnsupportedMathFunction(fun)),
+                    Mf::Outer => return Err(Error::unsupported("math function", fun)),
                 };
 
                 match function {
@@ -1981,7 +1981,7 @@ fn builtin_str(built_in: crate::BuiltIn) -> Result<&'static str, Error> {
         | Bi::PointSize
         | Bi::PointCoord
         | Bi::WorkGroupSize
-        | Bi::DrawID => return Err(Error::Custom(format!("Unsupported builtin {built_in:?}"))),
+        | Bi::DrawID => return Err(Error::unsupported("builtin", built_in)),
     })
 }