When diagnosing infinite recursion references to nullptr `Env` can be formed.
This happens only with `ExprBlackHole` is evaluated, which always leads to
`InfiniteRecursionError`.
UBSAN log for one such case:
```
../src/libexpr/eval-inline.hh:94:31: runtime error: reference binding to null pointer of type 'Env'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../src/libexpr/eval-inline.hh:94:31 in
```
In these trivial cases the final vector size (or lower bound on the size) is known,
so we can avoid some vector reallocations. This is not very important, but is just
good practice and general hygiene.
This is good practice to avoid pessimisations.
Left comments for the reasoning why ctors should be noexcept.
There are some tricky cases where we intentionally want throwing move ctors/assignments.
But those cases should really be reviewed, since some of those can be replaced
with more idiomatic copy/move-and-swap.
The ability to substitute inputs was removed in #10612 because it was
broken: with user-specified inputs containing a `narHash` attribute,
substitution resulted in an input that lacked the attributes returned
by the real fetcher (such as `lastModified`).
To fix this, we introduce a new input attribute `final`. If `final =
true`, fetching the input cannot add or change any attributes.
We only attempt to substitute inputs that have `final = true`. This is
implied by lock file entries; we only write a lock file if all its
entries are "final".
The user can specified `final = true` in `fetchTree`, in which case it
is their responsibility to ensure that all attributes returned by the
fetcher are included in the `fetchTree` call. For example,
nix eval --impure --expr 'builtins.fetchTree { type = "github"; owner = "NixOS"; repo = "patchelf"; final = true; narHash = "sha256-FSoxTcRZMGHNJh8dNtKOkcUtjhmhU6yQXcZZfUPLhQM="; }'
succeeds in a store path with the specified NAR hash exists or is
substitutable, but fails with
error: fetching final input '{"final":true,"narHash":"sha256-FSoxTcRZMGHNJh8dNtKOkcUtjhmhU6yQXcZZfUPLhQM=","owner":"NixOS","repo":"patchelf","type":"github"}' resulted in different input '{"final":true,"lastModified":1718457448,"narHash":"sha256-FSoxTcRZMGHNJh8dNtKOkcUtjhmhU6yQXcZZfUPLhQM=","owner":"NixOS","repo":"patchelf","rev":"a0f54334df36770b335c051e540ba40afcbf8378","type":"github"}'
... and remove a few unused arguments.
This adds pkg-config to a two or three packages that don't use it,
but we shouldn't let that bother us. It's like our personal stdenv.
This caused nlohmann/json.hpp to leak into a lot of compilation units,
which is slow (when not using precompiled headers).
Cuts build time from 46m24s to 42m5s (real time with -j24: 2m42s to
2m24s).
As a prelude to making "or" work like a normal variable, emit a warning
any time the "fn or" production is used in a context that will change
how it is parsed when that production is refactored.
In detail: in the future, OR_KW will be moved to expr_simple, and the
cursed ExprCall production that is currently part of the expr_select
nonterminal will be generated "normally" in expr_app instead. Any
productions that accept an expr_select will be affected, except for the
expr_app nonterminal itself (because, while expr_app has a production
accepting a bare expr_select, its other production will continue to
accept "fn or" expressions). So all we need to do is emit an appropriate
warning when an expr_simple representing a cursed ExprCall is accepted
in one of those productions without first going through expr_app.
As the warning message describes, users can suppress the warning by
wrapping their problematic "fn or" expressions in parentheses. For
example, "f g or" can be made future-proof by rewriting it as
"f (g or)"; similarly "[ x y or ]" can be rewritten as "[ x (y or) ]",
etc. The parentheses preserve the current grouping behavior, as in the
future "f g or" will be parsed as "(f g) or", just like
"f g anything-else" is grouped. (Mechanically, this suppresses the
warning because the problem ExprCalls go through the
"expr_app : expr_select" production, which resets the cursed status on
the ExprCall.)
