A host build ignores [target.<host triple>]: its cxx_runtime has no effect and no diagnostic says so #704

Observation

A host build does not read the [target.<triple>] section whose triple is the host's. The section's cxx_runtime has no effect, and the build does not report that it ignored the section.

Measured on 2026.9.26.1 and 2026.9.26.2, Linux x86_64, gcc@16.1.0:

[package]
name = "cxxrt"
version = "0.1.0"

[language]
standard = "c++23"
import_std = false

[target.x86_64-linux-gnu]
cxx_runtime = "toolchain-coupled"

[targets.cxxrt]
kind = "bin"
main = "src/main.cpp"

src/build/prepare.cppm consults targetOverrides only when overrides.target_triple is set (--target, [build] target, or a configured default target). The warning for [target.'cfg(linux)'.build] cxx_runtime directs the user to [target.<triple>].cxx_runtime, which a host build then ignores.

Consequence

A program that links Qt's official Linux libraries embeds libstdc++, and each Qt library requires libstdc++.so.6. QtCore carries RUNPATH $ORIGIN, so the loader does not search the executable's RPATH for QtCore's dependencies, and the program fails at start with libstdc++.so.6: cannot open shared object file. The engine's interposition check reports the embedded copy: 72 symbols are also provided by the toolchain's libstdc++.so.6. mcpp-plugins 0.13.0 (rules-qt) states the contract in [build] for this reason.

Proposed behaviour

1. A host build resolves the host triple and applies the matching [target.<triple>] section, using the same spelling-independent comparison as --target. The remaining keys of that section already describe the host target in every other respect.
2. Alternatively, the build reports a [target.<triple>] section whose triple is the host's and which the build did not read.

A further generalisation would extend the 2026.9.16.1 rule, under which an ELF program whose contract nobody stated adopts the contract of a C++ shared library that the build makes. The same rule would apply to a prebuilt shared library in the link that has NEEDED libstdc++.so.6. Qt users would then state nothing.