Prefer span slices over substrings in the F# instructions - #20642
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A string slice that is only inspected is a `ReadOnlySpan<char>`; a `Substring` or `ToString()` copies it on every call. Materialise only the value that leaves the function or is stored, and use `ReadOnlyMemory<char>` for a slice that outlives the stack frame. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Copilot review overview
🟡 Changes recommended
Address the identified guidance inconsistencies and System.Memory applicability issue.
Review effort: Lite
Findings: 1
What changed in this PR
Updates F# guidance to prefer span-based string slicing and avoid unnecessary allocations.
Changes:
- Recommends
ReadOnlySpan<char>for temporary inspection. - Recommends
ReadOnlyMemory<char>for stored or long-lived slices. - Adds allocation-free comparison examples.
| File | Summary |
|---|---|
.github/instructions/FSharp.instructions.md |
Adds span and memory slicing guidance; requires corrections for package availability, span lifetimes, and comparison consistency. |
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The rule above asks for one on every `StartsWith`, and the span examples have none, which reads as a contradiction rather than what it is: the span overloads that take no comparison compare the characters, so they are ordinal already, while the `string` overloads default to the current culture. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
MemoryExtensions has StringComparison overloads for ReadOnlySpan<char> even on netstandard2.0, so spans need no exception from the rule; the overloads without one are the generic element-wise ones and only happen to be ordinal. The span Contains also covers the missing String.Contains overload. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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A string slice that is only inspected (compared, trimmed, scanned, matched against a prefix) should be a
ReadOnlySpan<char>, not aSubstringorToString()that copies it on every call. Only the value that leaves the function or is stored gets materialised, and a slice that outlives the stack frame is aReadOnlyMemory<char>.src/CompilerreferencesSystem.Memoryonnetstandard2.0, so the API is available everywhere the rule applies.🤖 Generated with Claude Code