This can make type errors a little easier to understand.

(cherry picked from commit 2aa89daa)

This improves searchability.

(cherry picked from commit 6f093073)

Bumped to 15.09 because older channels, when downloaded from
nixos.org, require firefox to be accessed via `pkgs.firefox`

(cherry picked from commit 75ec68f9)

This reverts commit a0ef2126. This
doesn't work in 'nix run' and nix-shell because setns() fails in
multithreaded programs, and Boehm GC mark threads are uncancellable.

Fixes #2646.

(cherry picked from commit 01d07b1e)

backport nix-daemon fork safety to 2.2

Since macOS 10.14 this has become an error, causing problems if the
nix-daemon loads nix during substitution (this is a forked process).

Workaround for #2523.

(cherry picked from commit 8ac1130c)

[2.2] docs: fix typo in AllowDirectReads

It was just missing an `l` but should be fixed anyway.

(cherry picked from commit 34fade47)

Issue #2623.

(cherry picked from commit 4ae6e849)

This reverts commit e8b0efdc.

This reverts commit 80f464d9.

EvalState::resetFileCache: clear parse cache as well as eval cache

remove some dead code in nix/repl.cc

Escape square brackets in configure.ac

This patch attempts to solve the regression introduced in e8b0efdc
(#2607).

Update nlohmann-json 3.0.1 -> 3.5.0

Remove some bashisms from configure.ac

[Needs testing and review] Remove dead code

The manual reference was commented out, and the sole reference to this
program other than the configure script. Removed both.

Fixes #2546.

(at least the basic reproduction I've been testing)

"configure.ac" is used to generate "configure", which is supposed to be
run with /bin/sh (as suggested by the shebang line of "configure"). As a
result it is a good idea to remove any /bin/sh-incompatible syntax from
configure.ac. Otherwise, systems that do not use Bash as their /bin/sh
are unable to run "configure" due to syntax errors.

Fixes a static analyzer warning:

Expression 'isDrv' is always false.

Fixes: 3beb6f6e ('Show derivations more concisely')

https://github.com/nlohmann/json/releases/tag/v3.5.0

```
$ curl -L
"https://github.com/nlohmann/json/releases/download/v3.4.0/json.hpp" -o
src/nlohmann/json.hpp
```

nix-store: remove debugging print

tests/fetchurl: fix after changing default hash from 512 to 256

Clarify nix-instantiate --read-write-mode

SRI hashes (https://www.w3.org/TR/SRI/) combine the hash algorithm and
a base-64 hash. This allows more concise and standard hash
specifications. For example, instead of

  import <nix/fetchurl.nl> {
    url = https://nixos.org/releases/nix/nix-2.1.3/nix-2.1.3.tar.xz;
    sha256 = "5d22dad058d5c800d65a115f919da22938c50dd6ba98c5e3a183172d149840a4";
  };

you can write

  import <nix/fetchurl.nl> {
    url = https://nixos.org/releases/nix/nix-2.1.3/nix-2.1.3.tar.xz;
    hash = "sha256-XSLa0FjVyADWWhFfkZ2iKTjFDda6mMXjoYMXLRSYQKQ=";
  };

In fixed-output derivations, the outputHashAlgo is no longer mandatory
if outputHash specifies the hash (either as an SRI or in the old
"<type>:<hash>" format).

'nix hash-{file,path}' now print hashes in SRI format by default. I
also reverted them to use SHA-256 by default because that's what we're
using most of the time in Nixpkgs.

Suggested by @zimbatm.

docs: add missing prerequisites: brotli, boost, libseccomp

nix repl: don't create result symlinks

probably typo