fix: prevent panic when formatting complex generics

Handle cases where rewrite_generics fails due to long generic constraints
by implementing a fallback to infinite width and preserving original
formatting when necessary.

Author: avrabe

src/items.rs

@@ -3325,7 +3325,20 @@ fn format_generics(
     used_width: usize,
 ) -> Option<String> {
     let shape = Shape::legacy(context.budget(used_width + offset.width()), offset);
-    let mut result = rewrite_generics(context, "", generics, shape).ok()?;
+    let mut result = match rewrite_generics(context, "", generics, shape) {
+        Ok(r) => r,
+        Err(_) => {
+            // First attempt failed, try with infinite width for complex generics
+            let wide_shape = shape.infinite_width();
+            match rewrite_generics(context, "", generics, wide_shape) {
+                Ok(r) => r,
+                Err(_) => {
+                    // If even that fails, preserve original formatting from source
+                    context.snippet(generics.span).to_string()
+                }
+            }
+        }
+    };
 
     // If the generics are not parameterized then generics.span.hi() == 0,
     // so we use span.lo(), which is the position after `struct Foo`.

tests/source/issue-6571.rs

@@ -0,0 +1,16 @@
+// Regression test for panic when formatting very long generic constraints
+// This used to cause rustfmt to panic at src/items.rs:556 with "Option::unwrap() on a None value"
+pub enum TestEnum<
+    T: std::collections::HashMap<String, Vec<Box<dyn std::fmt::Debug + Send + Sync + 'static>>> + Clone + Default + PartialEq + Eq + std::fmt::Debug + serde::Serialize + serde::Deserialize<'static> + Send + Sync + 'static = std::collections::HashMap<String, Vec<Box<dyn std::fmt::Debug + Send + Sync + 'static>>>,
+> {
+    Variant1(T),
+    Variant2 { field: T },
+}
+
+// More realistic example from real codebase
+pub enum ElementInit<
+    P: wrt_foundation::MemoryProvider + Clone + Default + PartialEq + Eq = wrt_foundation::NoStdProvider<1024>,
+> {
+    FuncIndices(crate::WasmVec<u32, P>),
+    Expressions(crate::WasmVec<crate::WasmVec<u8, P>, P>),
+}

tests/target/issue-6571.rs

@@ -0,0 +1,16 @@
+// Regression test for panic when formatting very long generic constraints
+// This used to cause rustfmt to panic at src/items.rs:556 with "Option::unwrap() on a None value"
+pub enum TestEnum<
+    T: std::collections::HashMap<String, Vec<Box<dyn std::fmt::Debug + Send + Sync + 'static>>> + Clone + Default + PartialEq + Eq + std::fmt::Debug + serde::Serialize + serde::Deserialize<'static> + Send + Sync + 'static = std::collections::HashMap<String, Vec<Box<dyn std::fmt::Debug + Send + Sync + 'static>>>,
+> {
+    Variant1(T),
+    Variant2 { field: T },
+}
+
+// More realistic example from real codebase
+pub enum ElementInit<
+    P: wrt_foundation::MemoryProvider + Clone + Default + PartialEq + Eq = wrt_foundation::NoStdProvider<1024>,
+> {
+    FuncIndices(crate::WasmVec<u32, P>),
+    Expressions(crate::WasmVec<crate::WasmVec<u8, P>, P>),
+}