How to build on Ubuntu 25

[jorendorff]

I think the manual is out of date. The "installation" page says "C2Rust requires LLVM 6 or 7 and its corresponding libraries and clang compiler" and offers instructions for how to install C2Rust on Ubuntu 16.04, 18.04, and 18.10.

I have Ubuntu 25.04. The distribution packages LLVM versions 14, 15, 17, 18, 19, and 20. I think it should still be possible to build c2rust at least, but I haven't yet figured out how to do it. It definitely isn't compatible with the LLVM 20 include files.

I tried every environment variable I could think of to convince the build to use LLVM 17, but c2rust-ast-exporter still fails to build with errors indicating it's incorrectly including LLVM 20 files:

~/src/c2rust/c2rust-ast-exporter$ CMAKE_PREFIX_PATH="/usr/lib/llvm-17/lib/cmake:/usr/lib/llvm-17/lib/cmake/llvm:/usr/lib/llvm-17/lib/cmake/clang" LLVM_DIR=/usr/lib/llvm-17/lib/cmake/llvm Clang_DIR=/usr/lib/llvm-17/lib/cmake/clang LLVM_CONFIG_PATH=/usr/bin/llvm-config-17 LLVM_LIB_DIR=/usr/lib/llvm-17/lib cargo build
   Compiling c2rust-ast-exporter v0.18.0 (/home/jorendorff/src/c2rust/c2rust-ast-exporter)
error: failed to run custom build command for `c2rust-ast-exporter v0.18.0 (/home/jorendorff/src/c2rust/c2rust-ast-exporter)`

Caused by:
  process didn't exit successfully: `/home/jorendorff/src/c2rust/target/debug/build/c2rust-ast-exporter-482b46abecceb40b/build-script-build` (exit status: 101)
  --- stdout
  CMAKE_TOOLCHAIN_FILE_x86_64-unknown-linux-gnu = None
  CMAKE_TOOLCHAIN_FILE_x86_64_unknown_linux_gnu = None
  HOST_CMAKE_TOOLCHAIN_FILE = None
  CMAKE_TOOLCHAIN_FILE = None
  CMAKE_GENERATOR_x86_64-unknown-linux-gnu = None
  CMAKE_GENERATOR_x86_64_unknown_linux_gnu = None
  HOST_CMAKE_GENERATOR = None
  CMAKE_GENERATOR = None
  CMAKE_PREFIX_PATH_x86_64-unknown-linux-gnu = None
  CMAKE_PREFIX_PATH_x86_64_unknown_linux_gnu = None
  HOST_CMAKE_PREFIX_PATH = None
  CMAKE_PREFIX_PATH = Some("/usr/lib/llvm-17/lib/cmake:/usr/lib/llvm-17/lib/cmake/llvm:/usr/lib/llvm-17/lib/cmake/clang")
  CMAKE_x86_64-unknown-linux-gnu = None
  CMAKE_x86_64_unknown_linux_gnu = None
  HOST_CMAKE = None
  CMAKE = None
  running: "cmake" "/home/jorendorff/src/c2rust/c2rust-ast-exporter/src" "-DLLVM_DIR=/usr/lib/llvm-17/lib/cmake/llvm" "-DClang_DIR=/usr/lib/llvm-17/lib/cmake/clang" "-DCMAKE_INSTALL_PREFIX=/home/jorendorff/src/c2rust/target/debug/build/c2rust-ast-exporter-fe59333d5d805438/out" "-DCMAKE_C_FLAGS= -ffunction-sections -fdata-sections -fPIC -m64" "-DCMAKE_C_COMPILER=/usr/bin/cc" "-DCMAKE_CXX_FLAGS= -ffunction-sections -fdata-sections -fPIC -m64" "-DCMAKE_CXX_COMPILER=/usr/bin/c++" "-DCMAKE_ASM_FLAGS= -ffunction-sections -fdata-sections -fPIC -m64" "-DCMAKE_ASM_COMPILER=/usr/bin/cc" "-DCMAKE_BUILD_TYPE=Debug"
  -- Could NOT find LibEdit (missing: LibEdit_INCLUDE_DIRS LibEdit_LIBRARIES) 
  -- Could NOT find LibEdit (missing: LibEdit_INCLUDE_DIRS LibEdit_LIBRARIES) 
  -- Configuring done (0.3s)
  -- Generating done (0.0s)
  -- Build files have been written to: /home/jorendorff/src/c2rust/target/debug/build/c2rust-ast-exporter-fe59333d5d805438/out/build
  running: "cmake" "--build" "." "--target" "clangAstExporter" "--config" "Debug"
  [  8%] Performing update step for 'tinycbor_build'
  -- Already at requested ref: d393c16f3eb30d0c47e6f9d92db62272f0ec4dc7
  [ 16%] Performing patch step for 'tinycbor_build'
  patching file Makefile
  Reversed (or previously applied) patch detected!  Skipping patch.
  1 out of 1 hunk ignored -- saving rejects to file Makefile.rej
  patching file Makefile.configure
  Reversed (or previously applied) patch detected!  Skipping patch.
  1 out of 1 hunk ignored -- saving rejects to file Makefile.configure.rej
  [ 25%] Performing configure step for 'tinycbor_build'
  [ 33%] Performing build step for 'tinycbor_build'
  [ 41%] Performing install step for 'tinycbor_build'
  [ 50%] Completed 'tinycbor_build'
  [ 66%] Built target tinycbor_build
  [ 75%] Building CXX object CMakeFiles/clangAstExporter.dir/AstExporter.cpp.o

  --- stderr
  CMake Deprecation Warning at CMakeLists.txt:1 (cmake_minimum_required):
    Compatibility with CMake < 3.5 will be removed from a future version of
    CMake.

    Update the VERSION argument <min> value or use a ...<max> suffix to tell
    CMake that the project does not need compatibility with older versions.


  make[4]: warning: jobserver unavailable: using -j1.  Add '+' to parent make rule.
  make[4]: warning: jobserver unavailable: using -j1.  Add '+' to parent make rule.
  make[4]: warning: jobserver unavailable: using -j1.  Add '+' to parent make rule.
  /home/jorendorff/src/c2rust/c2rust-ast-exporter/src/AstExporter.cpp: In lambda function:
  /home/jorendorff/src/c2rust/c2rust-ast-exporter/src/AstExporter.cpp:2208:60: error: no matching function for call to ‘clang::FieldDecl::getBitWidthValue(clang::ASTContext&)’
   2208 |                                  array, D->getBitWidthValue(*this->Context));
        |                                         ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~
  In file included from /usr/lib/llvm-20/include/clang/AST/ASTContext.h:21,
                   from /usr/lib/llvm-20/include/clang/Frontend/ASTUnit.h:17,
                   from /usr/lib/llvm-20/include/clang/Frontend/FrontendAction.h:23,
                   from /usr/lib/llvm-20/include/clang/Frontend/FrontendActions.h:12,
                   from /home/jorendorff/src/c2rust/c2rust-ast-exporter/src/AstExporter.cpp:15:
  /usr/lib/llvm-20/include/clang/AST/Decl.h:3166:12: note: candidate: ‘unsigned int clang::FieldDecl::getBitWidthValue() const’
   3166 |   unsigned getBitWidthValue() const;
        |            ^~~~~~~~~~~~~~~~
  /usr/lib/llvm-20/include/clang/AST/Decl.h:3166:12: note:   candidate expects 0 arguments, 1 provided
  gmake[3]: *** [CMakeFiles/clangAstExporter.dir/build.make:76: CMakeFiles/clangAstExporter.dir/AstExporter.cpp.o] Error 1
  gmake[2]: *** [CMakeFiles/Makefile2:355: CMakeFiles/clangAstExporter.dir/all] Error 2
  gmake[1]: *** [CMakeFiles/Makefile2:362: CMakeFiles/clangAstExporter.dir/rule] Error 2
  gmake: *** [Makefile:254: clangAstExporter] Error 2
  thread 'main' panicked at '
  command did not execute successfully, got: exit status: 2

  build script failed, must exit now', /home/jorendorff/.cargo/registry/src/github.com-1ecc6299db9ec823/cmake-0.1.50/src/lib.rs:1098:5
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

I was able to build C2Rust in a buster VM, but it seems like since this project has folks actively working on it, it ought to build on current OSes; what am I missing? Thanks.

[bungcip]

based on the error message, it seem you try to compile 0.18 version. try using version from master