String brackets

[charlesstaats]

Access length-one substrings via the notation s[pos], where s is a string and pos is an integer.

[Copilot]

Pull request overview

Adds first-class s[pos] indexing support for string, enabling access to length-one substrings via operator[] (including negative indices) and documenting + testing the new behavior.

Changes:

• Add string.operator[] as a virtual field on the primitive string type, with int key type support for subscripting.
• Implement the runtime builtin backing stringBrackets (with negative-index handling and out-of-range returning empty string).
• Add a dedicated test and update the manual to describe s[pos] behavior.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
types.h	Adds overrides and a primitiveTy::keyType() override needed for string subscripting.
types.cc	Wires operator[] for string as a virtual field and implements primitiveTy::keyType() for string.
runstring.in	Implements the runtime thunk/helper used to execute string.operator[].
exp.cc	Updates subscript expression type inference to treat string indexing as string.
tests/datastructures/stringBracketsTest.asy	Adds coverage for positive/negative/out-of-range indexing and signature checks.
doc/asymptote.texi	Documents s[pos] indexing semantics and adds index entries for operator[].

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[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated no new comments.