It works on Net 8 as expected, but trying the otherwise same derivation with the Net 9 SDK threw that error, for whatever reason. I'll probably have to try it with Net 8 (via fetch-drv?) and see how much the ldd output differs for these _build binaries.

Here's the Net 9 ldd on _build for example:

$  ldd _build
        linux-vdso.so.1 (0x00007ffffa5f8000)
        libpthread.so.0 => /nix/store/8p33is69mjdw3bi1wmi8v2zpsxir8nwd-glibc-2.40-66/lib/libpthread.so.0 (0x00007aef94629000)
        libdl.so.2 => /nix/store/8p33is69mjdw3bi1wmi8v2zpsxir8nwd-glibc-2.40-66/lib/libdl.so.2 (0x00007aef94624000)
        libstdc++.so.6 => not found
        libm.so.6 => /nix/store/8p33is69mjdw3bi1wmi8v2zpsxir8nwd-glibc-2.40-66/lib/libm.so.6 (0x00007aef9453c000)
        libgcc_s.so.1 => /nix/store/f6famnry0piih1d3hh6y73fxx05jxsa8-xgcc-14.3.0-libgcc/lib/libgcc_s.so.1 (0x00007aef9450e000)
        libc.so.6 => /nix/store/8p33is69mjdw3bi1wmi8v2zpsxir8nwd-glibc-2.40-66/lib/libc.so.6 (0x00007aef94200000)
        /lib64/ld-linux-x86-64.so.2 => /nix/store/8p33is69mjdw3bi1wmi8v2zpsxir8nwd-glibc-2.40-66/lib64/ld-linux-x86-64.so.2 (0x00007aef94645000)

Did MS or an Nixpkgs remove linking against libstdc between Net 8 and Net 9? That's the only issue that I can immediately find, for example...

It works on Net 8 as expected, but trying the otherwise same derivation with the Net 9 SDK threw that error, for whatever reason. I'll probably have to try it with Net 8 (via fetch-drv?) and see how much the ldd output differs for these _build binaries.

Here's the Net 9 ldd on _build for example:

$  ldd _build
        linux-vdso.so.1 (0x00007ffffa5f8000)
        libpthread.so.0 => /nix/store/8p33is69mjdw3bi1wmi8v2zpsxir8nwd-glibc-2.40-66/lib/libpthread.so.0 (0x00007aef94629000)
        libdl.so.2 => /nix/store/8p33is69mjdw3bi1wmi8v2zpsxir8nwd-glibc-2.40-66/lib/libdl.so.2 (0x00007aef94624000)
        libstdc++.so.6 => not found
        libm.so.6 => /nix/store/8p33is69mjdw3bi1wmi8v2zpsxir8nwd-glibc-2.40-66/lib/libm.so.6 (0x00007aef9453c000)
        libgcc_s.so.1 => /nix/store/f6famnry0piih1d3hh6y73fxx05jxsa8-xgcc-14.3.0-libgcc/lib/libgcc_s.so.1 (0x00007aef9450e000)
        libc.so.6 => /nix/store/8p33is69mjdw3bi1wmi8v2zpsxir8nwd-glibc-2.40-66/lib/libc.so.6 (0x00007aef94200000)
        /lib64/ld-linux-x86-64.so.2 => /nix/store/8p33is69mjdw3bi1wmi8v2zpsxir8nwd-glibc-2.40-66/lib64/ld-linux-x86-64.so.2 (0x00007aef94645000)

Did MS or an Nixpkgs remove linking for libstdc++.so.6 between Net 8 and Net 9? That's the only issue that I can immediately find, for example...

This executable must come from a nuget package?

I believe the binary comes from for rid in $dotnetRuntimeIds; do dotnet restore --runtime "$rid" "build/_build.csproj"; done

But how is it finding any of those libs?
Honestly, that's a good question that I don't have an answer to.

This executable must come from a nuget package?

I believe the binary comes from for rid in $dotnetRuntimeIds; do dotnet restore --runtime "$rid" "build/_build.csproj"; done

But how is it finding any of those libs?

Honestly, that's a good question that I don't have an answer to.

$  readelf -d _build
Dynamic section at offset 0x106c8 contains 33 entries:
  Tag                Type           Name/Value
  0x000000000000001d (RUNPATH)      Library runpath: [$ORIGIN/netcoredeps]
  0x0000000000000001 (NEEDED)       Shared library: [libpthread.so.0]
  0x0000000000000001 (NEEDED)       Shared library: [libdl.so.2]
  0x0000000000000001 (NEEDED)       Shared library: [libstdc++.so.6]
  0x0000000000000001 (NEEDED)       Shared library: [libm.so.6]
  0x0000000000000001 (NEEDED)       Shared library: [libgcc_s.so.1]
  0x0000000000000001 (NEEDED)       Shared library: [libc.so.6]
  0x000000000000001e (FLAGS)        BIND_NOW
  0x000000006ffffffb (FLAGS_1)      NOW PIE
  0x0000000000000015 (DEBUG)        0x0
  0x0000000000000007 (RELA)         0x1ba0
  0x0000000000000008 (RELASZ)       744 (bytes)
  0x0000000000000009 (RELAENT)      24 (bytes)
  0x000000006ffffff9 (RELACOUNT)    20
  0x0000000000000017 (JMPREL)       0x1e88
  0x0000000000000002 (PLTRELSZ)     2448 (bytes)
  0x0000000000000003 (PLTGOT)       0x12928
  0x0000000000000014 (PLTREL)       RELA
  0x0000000000000006 (SYMTAB)       0x340
  0x000000000000000b (SYMENT)       24 (bytes)
  0x0000000000000005 (STRTAB)       0xfc4
  0x000000000000000a (STRSZ)        3032 (bytes)
  0x000000006ffffef5 (GNU_HASH)     0xfa8
  0x0000000000000019 (INIT_ARRAY)   0x12630
  0x000000000000001b (INIT_ARRAYSZ) 72 (bytes)
  0x000000000000001a (FINI_ARRAY)   0x12628
  0x000000000000001c (FINI_ARRAYSZ) 8 (bytes)
  0x000000000000000c (INIT)         0x10f84
  0x000000000000000d (FINI)         0x10fa0
  0x000000006ffffff0 (VERSYM)       0xda8
  0x000000006ffffffe (VERNEED)      0xe88
  0x000000006fffffff (VERNEEDNUM)   5
  0x0000000000000000 (NULL)         0x0

Here's what I'm trying out now:

preConfigure = ''
  dotnet --version > DotnetCliVersion.txt
'';
preBuild = ''
  patchShebangs --build build.sh
  for rid in $dotnetRuntimeIds; do dotnet restore --runtime "$rid" "build/_build.csproj"; done
  ./build.sh Codegen --configuration ${buildType}
'';

Build itself failed.

preConfigure = ''
  dotnet --version > DotnetCliVersion.txt
  for rid in $dotnetRuntimeIds; do dotnet restore --runtime "$rid" "build/_build.csproj"; done
'';
preBuild = ''
  patchShebangs --build build.sh
  ./build.sh Codegen --configuration ${buildType}
'';

...
Executing dotnetBuildHook
patching script interpreter paths in build.sh
build.sh: interpreter directive changed from "#!/usr/bin/env bash" to "/nix/store/q7sqwn7i6w2b67adw0bmix29pxg85x3w-bash-5.3p3/bin/bash"
GNU bash, version 5.3.3(1)-release (x86_64-pc-linux-gnu)
Microsoft (R) .NET Core SDK version 9.0.305
You must install or update .NET to run this application.

App: /build/source/build/bin/Debug/_build
Architecture: x64
Framework: 'Microsoft.NETCore.App', version '8.0.0' (x64)
.NET location: /nix/store/pkvy24cwwrsg3q3rqkhak8k6j98ns5gy-dotnet-sdk-9.0.305/share/dotnet

The following frameworks were found:
  9.0.9 at [/nix/store/pkvy24cwwrsg3q3rqkhak8k6j98ns5gy-dotnet-sdk-9.0.305/share/dotnet/shared/Microsoft.NETCore.App]

Learn more:
https://aka.ms/dotnet/app-launch-failed

To install missing framework, download:
https://aka.ms/dotnet-core-applaunch?framework=Microsoft.NETCore.App&framework_version=8.0.0&arch=x64&rid=linux-x64&os=linux