code polish

[loongs-zhang]

// Describe your PR here; eg. Fixes #issueNo

Make sure that:

• You submit test cases (unit or integration tests) that back your changes.
• Your local test passed.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 76.64%. Comparing base (35884fa) to head (66a06b6).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
core/src/coroutine/mod.rs	0.00%	7 Missing ⚠️
open-coroutine/src/lib.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #401      +/-   ##
==========================================
- Coverage   76.83%   76.64%   -0.19%     
==========================================
  Files          45       45              
  Lines        5296     5301       +5     
==========================================
- Hits         4069     4063       -6     
- Misses       1227     1238      +11     

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