Changing the sublice does not change the original slice #1554

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opened 2025-12-28 17:16:49 +00:00 by sami · 1 comment

sami commented

2025-12-28 17:16:49 +00:00

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Originally created by @Turalchik on GitHub (Sep 16, 2025).

func TestSlice(t *testing.T) {
	src := `package foo
	func Main() byte {
		a := make([]byte, 2)
		b := a[:1]
		b[0] = 42
		return a[0]
	}
	`
	eval(t, src, big.NewInt(42))
}

Current Behavior

Test returns 0.

Expected Behavior

Test should return 42.

Possible Solution

Perhaps you don't need to create a copy of the original slice when creating a subslice.

Steps to Reproduce

Run the test above.

Your Environment

v0.112.0

Originally created by @Turalchik on GitHub (Sep 16, 2025).  ```golang func TestSlice(t *testing.T) { src := `package foo func Main() byte { a := make([]byte, 2) b := a[:1] b[0] = 42 return a[0] } ` eval(t, src, big.NewInt(42)) } ``` ## Current Behavior Test returns 0. ## Expected Behavior Test should return 42. ## Possible Solution Perhaps you don't need to create a copy of the original slice when creating a subslice. ## Steps to Reproduce Run the test above. ## Your Environment  v0.112.0

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2025-12-28 17:16:49 +00:00

sami commented

2025-12-28 17:16:50 +00:00

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@roman-khimov commented on GitHub (Oct 1, 2025):

We can emulate subslices efficiently with buffers for known at compile time offsets ([:1], it's just a matter of tracking additional data for variables to emit code properly. But dynamic slicing ([:x]) is a problem requiring additional data, can be emulated with a structure, but this incurs significant performance penalty.

@roman-khimov commented on GitHub (Oct 1, 2025): We can emulate subslices efficiently with buffers for known at compile time offsets (`[:1]`, it's just a matter of tracking additional data for variables to emit code properly. But dynamic slicing (`[:x]`) is a problem requiring additional data, can be emulated with a structure, but this incurs significant performance penalty.