fix typedesc field access

[ringabout]

This pull request introduces a new restriction on typedesc field access for object and tuple types, limiting it to typeof contexts by default. It also adds a legacy compiler flag to restore the previous behavior, updates affected code and tests to comply with the new rule, and provides test coverage for the legacy mode.

Language Semantics and Compiler Options:

• Typedesc field access on object/tuple types (e.g., Foo[int].val) is now only allowed in a typeof context. Use the new --legacy:typedescFieldAccess flag to restore the old behavior if needed. [1] [2] [3]

Codebase and Test Adjustments:

• Updated code in nimprof.nim, tmapconcept.nim, ttypedesc3.nim, and tcustom_pragma.nim to use typeof when accessing typedesc fields, ensuring compatibility with the new restriction. [1] [2] [3] [4] [5]
• Added a new test (ttypedesc_field_legacy.nim) to verify that the legacy flag (--legacy:typedescFieldAccess) restores the previous behavior, allowing typedesc field access outside of typeof.

[Copilot]

Pull request overview

This PR restricts typedesc field access on object/tuple types (e.g. Foo[int].val) so that it only works inside a typeof context. A new legacy switch --legacy:typedescFieldAccess restores the previous behavior. Existing tests and stdlib usages that relied on the old behavior are updated to wrap such accesses in typeof(...).

Changes:

• In tryReadingTypeField, only resolve fields on tyObject/tyTuple typedescs when inside a typeof context or under the legacy flag.
• Add the typedescFieldAccess legacy feature and document the breaking change in changelog.md.
• Update tests and lib/pure/nimprof.nim to use typeof(...), and add a regression test for the legacy flag.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
compiler/semexprs.nim	Gate object/tuple typedesc field access on inTypeofContext or the legacy feature.
compiler/options.nim	Add typedescFieldAccess to the legacy features enum.
changelog.md	Document the breaking change and how to opt out.
lib/pure/nimprof.nim	Wrap StackTrace.lines accesses in typeof(...) to keep working under the new rule.
tests/metatype/ttypedesc3.nim	Update to use typeof(Foo[int].val).
tests/metatype/ttypedesc_field_legacy.nim	New regression test for --legacy:typedescFieldAccess.
tests/concepts/tmapconcept.nim	Update SparseSeq.data access to use typeof(...).
tests/pragmas/tcustom_pragma.nim	Update Baz.x access to use typeof(...).

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