build: enable libstdc++ assertions

Benchmarks say that it does not regress performance by more than 1%
(which is where it gets really hard to measure accurately anyhow).

Meson appears to be planning to do this for us without asking us in a
release we will get in the future, and it seems good enough to ship
today:
https://mesonbuild.com/Release-notes-for-1-4-0.html#ndebug-setting-now-controls-c-stdlib-assertions

Benchmarks:

| Command | Mean [ms] | Min [ms] | Max [ms] | Relative |
|:---|---:|---:|---:|---:|
| `result-asserts/bin/nix --extra-experimental-features 'nix-command flakes' eval -f ../nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix` | 418.4 ± 25.0 | 396.9 | 451.2 | 1.01 ± 0.08 |
| `result/bin/nix --extra-experimental-features 'nix-command flakes' eval -f ../nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix` | 416.1 ± 23.9 | 397.1 | 445.4 | 1.00 |

| Command | Mean [s] | Min [s] | Max [s] | Relative |
|:---|---:|---:|---:|---:|
| `GC_INITIAL_HEAP_SIZE=10g result-asserts/bin/nix eval --extra-experimental-features 'nix-command flakes' --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'` | 4.147 ± 0.021 | 4.123 | 4.195 | 1.00 |
| `GC_INITIAL_HEAP_SIZE=10g result/bin/nix eval --extra-experimental-features 'nix-command flakes' --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'` | 4.149 ± 0.027 | 4.126 | 4.215 | 1.00 ± 0.01 |

| Command | Mean [s] | Min [s] | Max [s] | Relative |
|:---|---:|---:|---:|---:|
| `result-asserts/bin/nix --extra-experimental-features 'nix-command flakes' eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'` | 5.838 ± 0.023 | 5.799 | 5.867 | 1.01 ± 0.01 |
| `result/bin/nix --extra-experimental-features 'nix-command flakes' eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'` | 5.788 ± 0.044 | 5.715 | 5.876 | 1.00 |

| Command | Mean [s] | Min [s] | Max [s] | Relative |
|:---|---:|---:|---:|---:|
| `result-asserts/bin/nix --extra-experimental-features 'nix-command flakes' search --no-eval-cache github:nixos/nixpkgs/e1fa12d4f6c6fe19ccb59cac54b5b3f25e160870 hello` | 15.993 ± 0.081 | 15.829 | 16.096 | 1.01 ± 0.01 |
| `result/bin/nix --extra-experimental-features 'nix-command flakes' search --no-eval-cache github:nixos/nixpkgs/e1fa12d4f6c6fe19ccb59cac54b5b3f25e160870 hello` | 15.897 ± 0.075 | 15.807 | 16.047 | 1.00 |

Fixes: lix-project/lix#4

Change-Id: Id3a6f38274ba94d5d10b09edd19dfd96bc3e7d5f

doc/manual/rl-next/new-assertions.md

@@ -0,0 +1,23 @@
+---
+synopsis: Lix turns more internal bugs into crashes
+cls: [797, 626]
+---
+
+Lix now enables build options such as trapping on signed overflow and enabling
+libstdc++ assertions by default. These may find new bugs in Lix, which will
+present themselves as Lix processes aborting, potentially without an error
+message.
+
+If Lix processes abort on your machine, this is a bug. Please file a bug,
+ideally with the core dump (or information from it).
+
+On Linux, run `coredumpctl list`, find the crashed process's PID at
+the bottom of the list, then run `coredumpctl info THE-PID`. You can then paste
+the output into a bug report.
+
+On macOS, open the Console app from Applications/Utilities, select Crash
+Reports, select the crash report in question. Right click on it, select Open In
+Finder, then include that file in your bug report. [See the Apple
+documentation][apple-crashreport] for more details.
+
+[apple-crashreport]: https://developer.apple.com/documentation/xcode/acquiring-crash-reports-and-diagnostic-logs#Locate-crash-reports-and-memory-logs-on-the-device

local.mk

@@ -4,7 +4,7 @@
 # This has an overhead of 0-4% on gcc and unmeasurably little on clang, in
 # Nix evaluation benchmarks.
 DEFAULT_SANITIZE_FLAGS = -fsanitize=signed-integer-overflow -fsanitize-undefined-trap-on-error
-GLOBAL_CXXFLAGS += -Wno-deprecated-declarations -Werror=switch $(DEFAULT_SANITIZE_FLAGS)
+GLOBAL_CXXFLAGS += -Wno-deprecated-declarations -Werror=switch $(DEFAULT_SANITIZE_FLAGS) -D_GLIBCXX_ASSERTIONS=1
 GLOBAL_LDFLAGS += $(DEFAULT_SANITIZE_FLAGS)
 # Allow switch-enum to be overridden for files that do not support it, usually because of dependency headers.
 ERROR_SWITCH_ENUM = -Werror=switch-enum

meson.build

@@ -344,6 +344,12 @@ add_project_arguments(
   '-Wimplicit-fallthrough',
   '-Werror=switch',
   '-Werror=switch-enum',
+  # Enable assertions in libstdc++ by default. Harmless on libc++. Benchmarked
+  # at ~1% overhead in `nix search`.
+  #
+  # FIXME: remove when we get meson 1.4.0 which will default this to on for us:
+  # https://mesonbuild.com/Release-notes-for-1-4-0.html#ndebug-setting-now-controls-c-stdlib-assertions
+  '-D_GLIBCXX_ASSERTIONS=1',
   language : 'cpp',
 )
 
@@ -351,8 +357,7 @@ if cxx.get_id() in ['gcc', 'clang']
   # 2024-03-24: jade benchmarked the default sanitize reporting in clang and got
   # a regression of about 10% on hackage-packages.nix with clang. So we are trapping instead.
   #
-  # This has an overhead of 0-4% on gcc and unmeasurably little on clang, in
-  # Nix evaluation benchmarks.
+  # This has an unmeasurably low overhead in Nix evaluation benchmarks.
   #
   # N.B. Meson generates a completely nonsense warning here:
   # https://github.com/mesonbuild/meson/issues/9822