[ChrisRackauckas-Claude]

What changed and why

This supersedes #123, whose contributor-fork head cannot be updated by the currently authenticated account. The commit retains Kristoffer Carlsson as author.

drop_expr previously specialized on every concrete RuntimeGeneratedFunction type, including its expression ID, compiling a new method instance for each generated function. This adds a specialization barrier, extracts the required type parameters through small helpers, and preserves the current thread-safe _BodyCache accessors.

Regression evidence

Unfixed master, with the new test applied:

env GROUP=Core timeout 3600 julia +1.10 --startup-file=no --project=. -e "using Pkg; Pkg.test()"
Test Failed at test/core_tests.jl:147
  Expression: specializations == 0
   Evaluated: 20 == 0

The same command with this fix:

Testing RuntimeGeneratedFunctions tests passed

Verification

GROUP=Core julia +1.10 --project=. -e "using Pkg; Pkg.test()"
Testing RuntimeGeneratedFunctions tests passed

GROUP=Core julia +release --project=. -e "using Pkg; Pkg.test()"
Testing RuntimeGeneratedFunctions tests passed

GROUP=Core julia +1.13 --project=. -e "using Pkg; Pkg.test()"
Testing RuntimeGeneratedFunctions tests passed

GROUP=QA julia +1.10 --project=. -e "using Pkg; Pkg.test()"
QA | 18 passed / 18 total

GROUP=QA julia +release --project=. -e "using Pkg; Pkg.test()"
QA | 20 passed / 20 total

julia +release --project=docs docs/make.jl
Documenter completed doctests, document checks, link checks, and HTML rendering successfully.

julia +release --project=<Runic package> -m Runic --check --diff .
No output; exit status 0.

typos src/RuntimeGeneratedFunctions.jl test/core_tests.jl test/shared/drop_expr_specializations.jl
No output; exit status 0.

Not verified locally

• macOS and Windows runners
• 32-bit Julia

Reviewer judgment call

The regression test uses Julias documented --trace-compile CLI output in a fresh subprocess and asserts that calls over 20 distinct RGF types emit no concrete drop_expr precompile statements. This directly detects the compilation blow-up without relying on private compiler-introspection APIs.

Ignore this PR until it has been reviewed by @ChrisRackauckas.