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feat: rewrite strings.SplitN(..., 2) to strings.Cut #70

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feat: rewrite strings.SplitN(..., 2) to strings.Cut #70

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03bef50
feat: rewrite strings.SplitN(..., 2) to strings.Cut
dnwe Sep 11, 2025
d2c3479
feat: rewrite more strings.Split usage to Cut
dnwe Sep 11, 2025
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13 changes: 13 additions & 0 deletions ruleguard.rules.go
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Original file line number Diff line number Diff line change
Expand Up @@ -322,6 +322,19 @@ func joinpath(m dsl.Matcher) {
Report(`did you mean path.Join() or filepath.Join() ?`)
}

func rewriteSplitN(m dsl.Matcher) {
m.Match(
`$x := strings.SplitN($y, $delim, 2)[0]`,
`$x := strings.SplitN($y, $delim, -1)[0]`,
`$x := strings.Split($y, $delim)[0]`,
).
Suggest(`$x, _, _ := strings.Cut($y, $delim)`).
Report(`Use strings.Cut instead of strings.SplitN for cleaner string splitting`)
m.Match(`$x := strings.SplitN($y, $delim, 2)[1]`).
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@ccoVeille ccoVeille Sep 11, 2025

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My point was also to add SplitN -1 and Split for [1]

As you can see there are some out there.

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@dnwe dnwe Sep 11, 2025 •
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So I wrote this rule to catch "someone wrote valid and reasonable code with a clear intent (split into exactly two values), but it could be done in a cleaner more readable way that achieves the exact same result, here is an autofix to tidy that up for you" approach – a bit like the new gopls modernize package.

For $x := strings.Split($y, $delim)[1] and $x := strings.SplitN($y, $delim, -1)[1] the autofix is only valid if there are exactly two elements because the [1] equivalent in strings.Cut is "after", i.e., all elements after the delimiter, not just the second element alone

So whilst these two lines are equivalent:

port := strings.Split("host:port", ":")[1]
_, port, _ := strings.Cut("host:port", ":")

These two are not

want := strings.Split("a,b,c", ",")[1]
_, got, _ := strings.Cut("a,b,c", ",")
string differs (-got, +want):
  string(
- 	"b,c",
+ 	"b",
  )

(playground link)

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@ccoVeille ccoVeille Sep 12, 2025

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You are right.

Suggest(`_, $x, _ := strings.Cut($y, $delim)`).
Report(`Use strings.Cut instead of strings.SplitN for cleaner string splitting`)
}

func readfull(m dsl.Matcher) {
m.Match(`$n, $err := io.ReadFull($_, $slice)
if $err != nil || $n != len($slice) {
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21 changes: 21 additions & 0 deletions splitn.yml
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Original file line number Diff line number Diff line change
@@ -0,0 +1,21 @@
rules:
- id: rewrite-splitn-to-cut-first
patterns:
- pattern-either:
- pattern: $X := strings.SplitN($Y, $DELIM, 2)[0]
- pattern: $X := strings.SplitN($Y, $DELIM, -1)[0]
- pattern: $X := strings.Split($Y, $DELIM)[0]
message: Replace strings.SplitN with strings.Cut for cleaner string splitting
languages: [go]
severity: ERROR
fix: |
$X, _, _ := strings.Cut($Y, $DELIM)

- id: rewrite-splitn-to-cut-second
pattern: |
$X := strings.SplitN($Y, $DELIM, 2)[1]
message: Replace strings.SplitN with strings.Cut for cleaner string splitting
languages: [go]
severity: ERROR
fix: |
_, $X, _ := strings.Cut($Y, $DELIM)