This reverts commit 5a5b5d4. The optimisation elided the callee-side
ARC retain/release for const string/class/interface value params on the
premise that the caller keeps the argument alive for the whole call. That
premise fails for a TEMPORARY bound to a const param (e.g. `Use(A + ' ' + B)`):
the concatenation result's only reference is the argument slot, so without the
callee-side retain its refcount hits zero at the call boundary and it is freed
before the callee reads it — a use-after-free.
This bit the RTL hardest: `_StringCopy` / `StrHead` take `const string` params
and are called with built-at-runtime temporaries, so the emitted RTL was
miscompiled. Under self-hosting the defect is self-reproducing and only
manifests at the SECOND generation (the compiler that emits the broken RTL is
itself fine), which is why a one-step fixpoint did not expose it and the
compiler's own sources did not reliably trigger it. The symptom was a
deterministic crash compiling any program that uses a metaclass reference
(`C := TFoo`) or HasClassAttribute — which is why TestRunner (via
blaise.testing.runner.text) could not be built, blocking the whole suite.
The original change's valgrind e2e test passed only because it bound a string
LITERAL (immortal) to the const param, not a temporary.
Removed the now-invalid IR/e2e tests that asserted the elided behaviour
(string const params, and the interface-const variant from 088d12f which
relied on this commit's IsConstParam guard). Added
TestRun_ConstStringTemp_StaysAlive_Valgrind, which passes a concatenation
result as a const string param and reads it in the callee — the exact case the
optimisation broke. docs/future-improvements.adoc records how to re-attempt the
elision safely (condition on the argument, not the parameter; retain temporaries
either caller- or callee-side).
Verified: stage-2 build clean, TestRunner builds, full suite 0 failures,
FIXPOINT_OK.
03b59e8ff0