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Give stringbytesroundtrip.isExactString its own exactness logic #54722

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Merged
pelikhan merged 5 commits into from
Aug 22, 2026
+35 −1
Merged

Give stringbytesroundtrip.isExactString its own exactness logic #54722

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d73fc19
Initial plan
Copilot Aug 22, 2026
eabac01
Give stringbytesroundtrip.isExactString its own exactness logic
Copilot Aug 22, 2026
59dc734
Merge remote-tracking branch 'origin/main' into copilot/fix-stringbyt…
Copilot Aug 22, 2026
fc0e810
Unalias types in isExactString and cover alias-to-named string
Copilot Aug 22, 2026
4c45736
Merge branch 'main' into copilot/fix-stringbytesroundtrip-no-op-alias
github-actions[bot] Aug 22, 2026
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12 changes: 11 additions & 1 deletion pkg/linters/stringbytesroundtrip/stringbytesroundtrip.go
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Original file line number Diff line number Diff line change
Expand Up @@ -135,13 +135,23 @@ func reportWastefulCloneRoundTrip(pass *analysis.Pass, outer, inner *ast.CallExp
)
}

// isStringType reports whether t is a string basic type. Callers pass an

@github-actions github-actions Bot Aug 22, 2026

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[/codebase-design] The phrase "so this also matches named string types" in the doc comment is slightly misleading — the body only ever matches *types.Basic. The intent is that callers pass a pre-resolved underlying type (which may originate from a named type), so the comment is about caller convention, not the function's own behaviour.

💡 Suggested wording 
// isStringType reports whether t is a string basic type. Callers must pass an
// already-.Underlying()-resolved type; the function does not resolve named types
// itself.

This avoids implying the function handles *types.Named directly.

@copilot please address this.

// already-.Underlying()-resolved type, so this also matches named string types.
func isStringType(t types.Type) bool {
basic, ok := t.(*types.Basic)
return ok && basic.Kind() == types.String
}

// isExactString reports whether t denotes the predeclared string type, not a
// named type whose underlying type is string. Unlike isStringType, which
// expects an already-.Underlying()-resolved type, isExactString must be given
// the raw type so it can tell string from `type MyString string`. Aliases are
// resolved first, because an alias may denote either the predeclared string
// (`type A = string`) or a named string type (`type A = MyString`). That
// distinction matters because only the predeclared string can have both
// conversions removed; a named string type still needs an outer conversion.
func isExactString(t types.Type) bool {
return isStringType(t)
return isStringType(types.Unalias(t))
}

func isByteSliceType(t types.Type) bool {
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24 changes: 24 additions & 0 deletions pkg/linters/stringbytesroundtrip/testdata/src/stringbytesroundtrip/stringbytesroundtrip.go
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Expand Up @@ -37,6 +37,30 @@ func badNamedTypes() string {
return string([]byte(ms)) // want `string\(\[\]byte\(ms\)\) is a redundant round-trip; replace it with string\(ms\)`
}

// aliasString is an alias for the predeclared string type, so round-trips
// through it are fully redundant.
type aliasString = string

// namedAlias is an alias for a named string type, so an outer conversion is
// still required even though its underlying type is string.
type namedAlias = myString

func badMixedNamedTypes() {
s := "hello"
var ms myString = "hello"
var as aliasString = "hello"
var na namedAlias = "hello"

// Outer conversion is a named string type: the outer conversion must stay.
_ = myString([]byte(s)) // want `myString\(\[\]byte\(s\)\) is a redundant round-trip; replace it with myString\(s\)`
// Argument is a named string type: an outer conversion is still needed.
_ = string([]byte(ms)) // want `string\(\[\]byte\(ms\)\) is a redundant round-trip; replace it with string\(ms\)`
// Alias of the predeclared string type: both conversions can be removed.
_ = aliasString([]byte(as)) // want `aliasString\(\[\]byte\(as\)\) is a redundant round-trip; both conversions can be removed`
// Alias of a named string type: the outer conversion must stay.
_ = namedAlias([]byte(na)) // want `namedAlias\(\[\]byte\(na\)\) is a redundant round-trip; replace it with namedAlias\(na\)`
}

// helperString is a regular function, not a type conversion — must not be flagged.
func helperString(b []byte) string { return string(b) }

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