fix some unused mutability & make some type private

[Guest0x0]

While improving MoonBit's unused warning, I found several struct field with unused mutability & some types that should be made private on core. This PR fixes these unused things

[coderabbitai]

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[peter-jerry-ye-code-review]

From the provided git diff output, here are the potential issues and suggestions:

1. Incorrect Implementation of Traits in builtin.mbti:

   • The diff shows changes in the implementation of traits Hash and Show for some types. Specifically, there are changes from impl Hash for $default_impl to impl Hash::hash and from impl Show for $default_impl to impl Show::to_string.
   • Issue: The impl Hash::hash and impl Show::to_string syntax seems incorrect based on the provided MoonBit context. The correct syntax should be impl Hash for $default_impl and impl Show for $default_impl.
   • Suggestion: Revert these changes to follow the correct syntax for implementing traits in MoonBit.

2. Mutable Field Declaration in algebraic_test.mbt:

   • The diff shows a change in the declaration of the T struct from mut x : Int to x : Int.
   • Issue: If x is intended to be mutable, removing the mut keyword will make it immutable, which might not be the intended behavior.
   • Suggestion: If x needs to be mutable, keep the mut keyword. Otherwise, ensure that the change is intentional and does not affect the functionality.

3. Mutable Field Declaration in random.mbt:

   • The diff shows a change in the RandomState struct from mut gamma : UInt64 to gamma : UInt64.
   • Issue: Similar to the previous issue, if gamma is intended to be mutable, removing the mut keyword will make it immutable.
   • Suggestion: If gamma needs to be mutable, keep the mut keyword. Otherwise, ensure that the change is intentional and does not affect the functionality.

These are the most noticeable issues based on the provided diff. Ensure that these changes align with the intended functionality and adhere to the MoonBit language specifications.

[coveralls]

Pull Request Test Coverage Report for Build 3262

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 19 unchanged lines in 5 files lost coverage.
• Overall coverage decreased (-0.4%) to 82.783%

---

Files with Coverage Reduction	New Missed Lines	%
float/float.mbt	1	0.0%
result/result.mbt	1	74.14%
quickcheck/splitmix/random.mbt	1	75.0%
quickcheck/arbitrary.mbt	3	53.13%
rational/rational.mbt	13	71.64%

Totals
Change from base Build 3260:	-0.4%
Covered Lines:	4058
Relevant Lines:	4902

---

💛 - Coveralls

[hackwaly]

The mut is indented to make compiler unable to do unbox optimization on type T.

[hackwaly]

While there's no something likes //@warning "-unused-mut". I think we need a manual assignment statement to eliminate the warning?

[Guest0x0]

Currently T will not be unboxed globally even if its only field is immutable. If some value of T is used locally by accessing field only then that particular value may be unboxed, but that's not the case here, and mutable structs also get unboxed in that case.

This test itself may be controversial but let's discuss and fix that in a separate PR

[Guest0x0]

There are some bug fixes in yesterday's compiler release on the format of default trait impl in .mbti, and .mbti files on core should be updated. Since it breaks CI, I'll update the changes in this PR.