test(e2e): add issue #131 scenario-1 validation (per-target codegen cxxflag on own entry)

Mirrors the issue's `[targets.test_contracts] cxxflags=[...]` shape: a single
bin target whose per-target codegen flag must affect only its own `main`, not
shared code. Uses -fno-exceptions (preprocessor-observable via __EXCEPTIONS) as
a portable stand-in for -fcontract-evaluation-semantic=observe, which needs a
contracts-enabled toolchain not guaranteed across the Linux/macOS/Windows CI
matrix. Checks: reaches the entry, does NOT leak to a shared unit, appears on
the target's main edge in build.ninja, and the binary builds + runs.

Author: Sunrisepeak

tests/e2e/60_issue131_per_target_cxxflag.sh

@@ -0,0 +1,66 @@
+#!/usr/bin/env bash
+# requires: gcc
+# Issue #131 scenario 1, verbatim shape: a single bin target carries a per-target
+# *codegen* cxxflag that must affect ONLY its own entry `main`, not shared code:
+#
+#   [targets.test_contracts]
+#   kind = "bin"
+#   main = "tests/test_contracts.cpp"
+#   cxxflags = ["-fcontract-evaluation-semantic=observe"]
+#
+# We use `-fno-exceptions` as a portable stand-in for the contracts semantic
+# flag (which needs a contracts-enabled toolchain not guaranteed across the
+# Linux/macOS/Windows CI matrix). The mechanism under test is identical: a real
+# per-target codegen flag scoped to that target's exclusive entry. Its effect is
+# preprocessor-observable via `__EXCEPTIONS` (defined iff exceptions are on), so
+# both "reached the entry" and "did NOT leak to shared" are checked at compile
+# time, plus the flag is asserted on the target's main edge in build.ninja.
+set -e
+
+TMP=$(mktemp -d)
+trap "rm -rf $TMP" EXIT
+cd "$TMP"
+mkdir -p proj/src proj/tests
+cd proj
+
+# Shared source — globbed, compiled once with DEFAULT flags (exceptions on).
+# If the per-target flag leaked here, __EXCEPTIONS would be undefined → error.
+cat > src/shared.cpp <<'EOF'
+#ifndef __EXCEPTIONS
+#error "per-target cxxflag leaked into a shared compile unit"
+#endif
+extern "C" int shared_val(void) { return 42; }
+EOF
+
+# The target's own entry — compiled WITH the per-target flag (-fno-exceptions),
+# so __EXCEPTIONS must be undefined here.
+cat > tests/test_contracts.cpp <<'EOF'
+#ifdef __EXCEPTIONS
+#error "per-target cxxflag did not reach this target's own entry"
+#endif
+extern "C" int shared_val(void);
+int main() { return shared_val() == 42 ? 0 : 1; }
+EOF
+
+cat > mcpp.toml <<'EOF'
+[package]
+name    = "proj"
+version = "0.1.0"
+
+[targets.test_contracts]
+kind     = "bin"
+main     = "tests/test_contracts.cpp"
+cxxflags = ["-fno-exceptions"]
+EOF
+
+# Compiles only if the flag reached the entry (entry check) AND did not leak to
+# the shared unit (shared check) — both are #error guards above.
+"$MCPP" build > build.log 2>&1 || { cat build.log; echo "build failed"; exit 1; }
+
+ninja_file="$(find target -name build.ninja | head -1)"
+[[ -n "$ninja_file" ]] || { echo "no build.ninja"; exit 1; }
+grep -q -- "-fno-exceptions" "$ninja_file" || { echo "per-target cxxflag missing from build.ninja"; exit 1; }
+
+"$MCPP" run test_contracts > /dev/null 2>&1 || { echo "run failed"; exit 1; }
+
+echo "OK"