libexpr: Support structured error classes

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`.

(cherry picked from commit c6a89c1a1659b31694c0fbcd21d78a6dd521c732)
Change-Id: Iced91ba4e00ca9e801518071fb43798936cbd05a

1 files changed, 0 insertions, 2 deletions

diff --git a/src/libcmd/repl.cc b/src/libcmd/repl.cc
index 80b252c31..8ff8d04c4 100644
--- a/src/libcmd/repl.cc
+++ b/src/libcmd/repl.cc
@@ -427,8 +427,6 @@ StringSet NixRepl::completePrefix(const std::string & prefix)
             // Quietly ignore parse errors.
         } catch (EvalError & e) {
             // Quietly ignore evaluation errors.
-        } catch (UndefinedVarError & e) {
-            // Quietly ignore undefined variable errors.
         } catch (BadURL & e) {
             // Quietly ignore BadURL flake-related errors.
         }