Fix #[trace] macro suppressing dead_code warnings

[Copilot]

The #[trace] attribute macro was preventing the compiler from emitting dead_code warnings for unused functions.

Root Cause

The macro used quote!() to generate output, which doesn't preserve span information needed for lint attribution. Without proper spans, the compiler's dead_code analysis couldn't correctly identify unused functions.

Changes

• fastrace-macro/src/lib.rs: Use quote_spanned!() with the function identifier's span instead of quote!()
• tests/macros/tests/ui/ok/preserves-dead-code-lint.rs: Add test case documenting the expected behavior

Example

Before this change, no warning was emitted:

struct Foo;

impl Foo {
    #[fastrace::trace]
    async fn unused_function(&self) -> i32 {
        42
    }
}

After this change, the compiler correctly warns:

warning: method `unused_function` is never used

Original prompt

---

This section details on the original issue you should resolve

<issue_title>#[trace] attribute suppresses dead_code warnings for unused functions</issue_title>
<issue_description>## Summary

The #[fastrace::trace] attribute suppresses Rust's dead_code lint, causing the compiler to not warn about unused functions.

Reproduction

struct Foo;

impl Foo {
    // No warning - even though this function is never called
    #[fastrace::trace]
    async fn unused_function(&self) -> i32 {
        42
    }
}

Expected behavior

The compiler should warn: method 'unused_function' is never used.

Actual behavior

No warning. Removing #[fastrace::trace] causes the warning to appear.

Environment

• fastrace 0.7.14 with enable feature enabled
• rustc 1.94.0-nightly (f52090008 2025-12-10)
  </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes #[trace] attribute suppresses dead_code warnings for unused functions #156

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