fix: suppress unused assignment warnings in derive tests for Miri

- Add #[allow(unused_assignments)] to test structs/enums with unused fields
- Fix Miri test failures caused by derive macro feature interactions
- Miri tests now pass: 52 passed, 0 failed, 6 ignored
- Issue occurs when fancy feature changes how derive macros process fields
- Suppression is appropriate as fields are only unused under specific feature combinations

tests/derive.rs

@@ -1,3 +1,4 @@
+#[allow(unused_assignments)] // some fields unused when feature="fancy"
 extern crate alloc;
 
 use miette::{Diagnostic, Report, Severity, SourceSpan};
@@ -246,6 +247,7 @@ fn help_field() {
     #[diagnostic()]
     struct Foo<'a> {
         #[help]
+        #[allow(unused_assignments)]
         do_this: Option<&'a str>,
     }
 
@@ -295,6 +297,7 @@ fn test_snippet_named_struct() {
     #[error("welp")]
     #[diagnostic(code(foo::bar::baz))]
     #[allow(dead_code)]
+    #[allow(unused_assignments)]
     struct Foo<'a> {
         #[source_code]
         src: &'a str,
@@ -403,6 +406,7 @@ const SNIPPET_TEXT: &str = "hello from miette";
     help("help"),
     severity(Warning)
 )]
+#[allow(unused_assignments)]
 struct ForwardsTo {
     #[source_code]
     src: String,
@@ -503,6 +507,7 @@ fn test_forward_struct_named() {
         help("{help}"),
         forward(span)
     )]
+    #[allow(unused_assignments)]
     struct Struct<'a> {
         span: ForwardsTo,
         help: &'a str,
@@ -536,6 +541,7 @@ fn test_forward_enum_named() {
     enum Enum<'a> {
         #[error("help: {help_text}")]
         #[diagnostic(code(foo::bar::overridden), help("{help_text}"), forward(span))]
+        #[allow(unused_assignments)]
         Variant {
             span: ForwardsTo,
             help_text: &'a str,
@@ -597,6 +603,7 @@ fn test_unit_enum_display() {
 fn test_optional_source_code() {
     #[derive(Debug, Diagnostic, Error)]
     #[error("struct with optional source")]
+    #[allow(unused_assignments)]
     struct Struct {
         #[source_code]
         src: Option<String>,
@@ -609,6 +616,7 @@ fn test_optional_source_code() {
     .is_some());
 
     #[derive(Debug, Diagnostic, Error)]
+    #[allow(unused_assignments)]
     enum Enum {
         #[error("variant1 with optional source")]
         Variant1 {