🐛 fix(config): defer TOML set_env string substitution#3759
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The TOML loader eagerly resolved all substitutions in set_env values
via Unroll before constructing SetEnv. When set_env contained
references like {env_site_packages_dir}, this triggered a circular
dependency: set_env → env_site_packages_dir → create_python_env →
system_site_packages → environment_variables → set_env.
The INI loader already handles this by deferring string substitution
for SetEnv values. Apply the same pattern to the TOML loader: resolve
structural TOML constructs (refs, posargs, env lookups) but skip
string-level substitution, providing a real replacer for lazy
per-value resolution.
Closes tox-dev#3758
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Using substitutions like
{env_site_packages_dir}insideset_envin TOML format causes aRecursionErrorbecause the TOML loader eagerly resolves all string substitutions viaUnrollbefore constructingSetEnv. This creates a circular dependency:set_env→env_site_packages_dir→create_python_env()→system_site_packages→environment_variables→set_env.The INI loader already handles this correctly by detecting
SetEnvtype and deferring string substitution — raw strings are stored as-is and only resolved when individual env vars are accessed viaSetEnv.load(). 🔧 This applies the same pattern to the TOML loader: structural TOML constructs (replace: ref,replace: posargs,replace: env) are still resolved during config loading, but string-level{...}substitutions are deferred via a proper TOML-aware replacer passed toSetEnv.use_replacer().Discovered via pypa/virtualenv#3050 when migrating virtualenv's config from
tox.initotox.toml. The same configuration that worked in INI format caused infinite recursion in TOML because of the eager substitution.Closes #3758