This avoids an inconsistency where a '/' is implicitly inserted when
you append to the root of a source tree, but not when you append to
any other path.
The old way was not correct.
Here is an example:
```
$ nix-instantiate --eval --expr 'let x = a: throw "asdf"; in x 1' --show-trace
error: asdf
… while evaluating 'x'
at «string»:1:9:
1| let x = a: throw "asdf"; in x 1
| ^
… from call site
at «string»:1:29:
1| let x = a: throw "asdf"; in x 1
| ^
```
and yet also:
```
$ nix-instantiate --eval --expr 'let x = a: throw "asdf"; in x' --show-trace
<LAMBDA>
```
Here is the thing: in both cases we are evaluating `x`!
Nix is a higher-order languages, and functions are a sort of value. When
we write `x = a: ...`, `a: ...` is the expression that `x` is being
defined to be, and that is already a value. Therefore, we should *never*
get an trace that says "while evaluating `x`", because evaluating `a:
...` is *trival* and nothing happens during it!
What is actually happening here is we are applying `x` and evaluating
its *body* with arguments substituted for parameters. I think the
simplest way to say is just "while *calling* `x`", and so that is what I
changed it to.
It calls strlen() on the input (rather than simply copying at most
`size` bytes), which can fail if the input is not zero-terminated and
is inefficient in any case.
Fixes#7347.
E.g. instead of
error: Package ‘steam’ in /__virtual__/4/pkgs/games/steam/steam.nix:43 has an unfree license (‘unfreeRedistributable’), refusing to evaluate.
you now get
error: Package ‘steam’ in «github:nixos/nixpkgs/b82ccafb54163ab9024e893e578d840577785fea»/pkgs/games/steam/steam.nix:43 has an unfree license (‘unfreeRedistributable’), refusing to evaluate.
E.g.
… while evaluating the attribute 'buildCommand' of the derivation 'vm-test-run-github-flakes'
at «github:NixOS/nixpkgs/2fa57ed190fd6c7c746319444f34b5917666e5c1»/pkgs/stdenv/generic/make-derivation.nix:278:7:
