String.find/String.rfind USize proposal

text/0000-usize-indexing

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+- Feature Name: string_find_usize
+- Start Date: 2024-12-09
+- RFC PR: (leave this empty)
+- Pony Issue: (leave this empty)
+
+# Summary
+
+Change the return type of `String.find`  and `String.rfind` from `ISize` to `USize`, aligning its return type with `Array.find`.
+
+# Motivation
+
+The current return type of `String.find` and `String.rfind` is `ISize`, which stems from its historical behavior of returning `-1` when no match was found. However, these methods now return `error` for such cases. Returning `USize` makes more sense because indices are always non-negative, and it aligns with `Array.find`'s method signature.
+
+# Detailed design
+
+The method signature of [String.find](https://github.com/ponylang/ponyc/blob/736220a34364a864fe0fd1f091a85852ded84d23/packages/builtin/string.pony#L640) and [String.rfind](https://github.com/ponylang/ponyc/blob/736220a34364a864fe0fd1f091a85852ded84d23/packages/builtin/string.pony#L669) would change to return `USize` instead of `ISize`.
+
+Specifically, the method signatures would be changed from this:
+
+```pony
+fun find(s: String box, offset: ISize = 0, nth: USize = 0): ISize ?
+fun rfind(s: String box, offset: ISize = -1, nth: USize = 0): ISize ?
+```
+
+to this:
+
+```pony
+fun find(s: String box, offset: ISize = 0, nth: USize = 0): USize ?
+fun rfind(s: String box, offset: ISize = -1, nth: USize = 0): USize ?
+```
+
+# How We Teach This
+
+N/A
+
+# How We Test This
+
+Standard CI and the compiler should identify most areas that need to be addressed. Additionally, any documentation, code examples, and guides should be reviewed and updated to reflect the new behavior.
+
+# Drawbacks
+
+It's a breaking change as the method signature has to change. Existing code relying on `String.find` and `String.rfind` will need to be migrated.
+
+# Alternatives
+
+Keep the existing method signature to avoid breaking changes introduced by this proposal.
+
+# Unresolved questions
+
+N/A