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libexpr: Support structured error classes

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While preparing PRs like #9753, I've had to change error messages in
dozens of code paths. It would be nice if instead of

    EvalError("expected 'boolean' but found '%1%'", showType(v))

we could write

    TypeError(v, "boolean")

or similar. Then, changing the error message could be a mechanical
refactor with the compiler pointing out places the constructor needs to
be changed, rather than the error-prone process of grepping through the
codebase. Structured errors would also help prevent the "same" error
from having multiple slightly different messages, and could be a first
step towards error codes / an error index.

This PR reworks the exception infrastructure in `libexpr` to
support exception types with different constructor signatures than
`BaseError`. Actually refactoring the exceptions to use structured data
will come in a future PR (this one is big enough already, as it has to
touch every exception in `libexpr`).

The core design is in `eval-error.hh`. Generally, errors like this:

    state.error("'%s' is not a string", getAttrPathStr())
      .debugThrow<TypeError>()

are transformed like this:

    state.error<TypeError>("'%s' is not a string", getAttrPathStr())
      .debugThrow()

The type annotation has moved from `ErrorBuilder::debugThrow` to
`EvalState::error`.
This commit is contained in:
Rebecca Turner 2024-01-22 17:08:29 -08:00
parent c62c21e29a
commit c6a89c1a16
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40 changed files with 653 additions and 545 deletions
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8
src/libexpr/nixexpr.cc
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@ -296,10 +296,10 @@ void ExprVar::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> &
enclosing `with'. If there is no `with', then we can issue an
"undefined variable" error now. */
if (withLevel == -1)
throw UndefinedVarError({
.msg = hintfmt("undefined variable '%1%'", es.symbols[name]),
.errPos = es.positions[pos]
});
es.error<UndefinedVarError>(
"undefined variable '%1%'",
es.symbols[name]
).atPos(pos).debugThrow();
for (auto * e = env.get(); e && !fromWith; e = e->up)
fromWith = e->isWith;
this->level = withLevel;