Rollup merge of #114014 - davidtwco:issue-114010-env-rawstr, r=cjgillot

builtin_macros: expect raw strings too

Fixes #114010.

`expr_to_string` allows raw strings through so this code should be expected to handle those.

compiler/rustc_errors/src/diagnostic_impls.rs

@@ -164,6 +164,12 @@ impl IntoDiagnosticArg for hir::ConstContext {
     }
 }
 
+impl IntoDiagnosticArg for ast::Expr {
+    fn into_diagnostic_arg(self) -> DiagnosticArgValue<'static> {
+        DiagnosticArgValue::Str(Cow::Owned(pprust::expr_to_string(&self)))
+    }
+}
+
 impl IntoDiagnosticArg for ast::Path {
     fn into_diagnostic_arg(self) -> DiagnosticArgValue<'static> {
         DiagnosticArgValue::Str(Cow::Owned(pprust::path_to_string(&self)))