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, 3 insertions, 3 deletions

diff --git a/src/nix/flake.cc b/src/nix/flake.cc
index 0da9b1ce9..a11e39534 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -808,10 +808,10 @@ struct CmdFlakeInitCommon : virtual Args, EvalCommand
         auto templateDir = templateDirAttr->getString();
 
         if (!store->isInStore(templateDir))
-            throw TypeError(
+            evalState->error<TypeError>(
                 "'%s' was not found in the Nix store\n"
                 "If you've set '%s' to a string, try using a path instead.",
-                templateDir, templateDirAttr->getAttrPathStr());
+                templateDir, templateDirAttr->getAttrPathStr()).debugThrow();
 
         std::vector<Path> changedFiles;
         std::vector<Path> conflictedFiles;
@@ -1281,7 +1281,7 @@ struct CmdFlakeShow : FlakeCommand, MixJSON
                 {
                     auto aType = visitor.maybeGetAttr("type");
                     if (!aType || aType->getString() != "app")
-                        throw EvalError("not an app definition");
+                        state->error<EvalError>("not an app definition").debugThrow();
                     if (json) {
                         j.emplace("type", "app");
                     } else {