[ASDAlexander77]

Stack promotion so far treated every call into another module as an escape. With this change, a library records which parameters of its functions are only read, and a program linked statically against it reads those facts. An object passed to such a parameter can then go on the stack. For example, Math.max(a, b, c) now builds its argument array on the stack, under both --emit=exe and --emit=jit.

Spec: tslang/docs/superpowers/specs/2026-10-18-borrow-facts-across-modules-design.md. Plan: tslang/docs/superpowers/plans/2026-10-18-borrow-facts-across-modules.md.

Producing the facts

• The symbol: every module compiled ahead of time exports __tsborrow_<file>_<hash>. It is a char array with one line per function: <symbol> <borrowed parameter indexes>.
• Analysis: the facts come from the stack promotion escape analysis, run trusting no vtable, since an importer may subclass any class.
• Always written: the symbol is emitted whatever --export and --no-stack-promotion say.
• Renamed functions: a function renamed by @linkname or @dllname publishes no facts.

Reading the facts

Facts are read only where the file read is the file that runs:

• --emit=exe: the default library's static library (now linked by its path rather than searched for), and every --lib that resolves to a static library or object file (also linked by that path).
• --emit=jit: the default library's DLL, read from the file before anything is loaded. A JIT cache entry lists that DLL as a dependency.
• Never: DLL links, -shared imports, --emit=obj. Missing or old facts mean every call escapes, as before.

Matching

• Direct calls only: only a direct call to a declared function uses a fact. Virtual calls on library classes still escape.
• Ambiguous names: a fact is dropped when another link input defines the same symbol: another --lib, an --obj, a -shared import, or a module the JIT cache compiled separately. If any --lib is searched for by the linker, no fact is trusted at all.
• Declarations bound elsewhere: a declaration renamed by @linkname or @dllname takes no fact.

Also

• Print flag: tslang --print-borrow-facts=<file> prints what an object file, static library or DLL publishes.
• Docs: docs/memory-models.md has a new "Calls into other modules" section.

Testing

• Producer: test-stack-facts-lib-* covers gc, rc, own and none.
• Reader and consumer: test-borrow-facts-* builds a static library and a program with --lib, checks every decision, and compares the output with an unpromoted build. Negative cases:
  • --emit=obj
  • a library built without facts
  • @linkname
  • a second library or --obj defining the same symbol
  • a searched --lib
  • a @static -shared import
  • --lib naming a DLL
  • an import library
• Default library: test-borrow-facts-defaultlib covers the real default library. It is skipped unless that library was built with this compiler. It passed against a locally rebuilt rc default library, in both exe and JIT modes, and also checks that a stray TypeScriptDefaultLib.lib in the working folder is not linked.
• Teeth checks:
  • With a producer that lists every parameter, the program printed 0 instead of 2, and the tests fail.
  • Without the -shared filter, the -shared test fails.
  • Without the DLL reader, the DLL test fails.
• Full suite: 4672/4672 with promotion on, and 4672/4672 with -DTSLANG_TEST_NO_STACK_PROMOTION=ON.

Not covered

• Linux: the archive and .so paths are written but untested; this was built and tested on Windows only.
• Default library: the default library has to be rebuilt with this compiler before it publishes facts. Until then nothing changes for programs using it.
• Implicit libraries: the libraries the compiler links implicitly (CRT, gc, runtime) are trusted not to define the default library's symbols.

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