[Backend] Establish standalone build infrastructure

Add the thin monorepo CMake entry point and an independently configurable backend project. Enforce the Clang/C++23 toolchain, expose Make/Ninja-neutral tests and developer checks, export an installable RCCBackend package, and introduce the first Support error abstraction.

20 files changed, 650 insertions, 22 deletions

diff --git a/.clang-format b/.clang-format
index 5d323cc..fc4f3fc 100644
--- a/.clang-format
+++ b/.clang-format
@@ -1,2 +1,20 @@
-BasedOnStyle: Google
+BasedOnStyle: LLVM
+Language: Cpp
+Standard: Latest
+ColumnLimit: 100
+IndentWidth: 2
+ContinuationIndentWidth: 4
+UseTab: Never
+BreakBeforeBraces: Attach
+PointerAlignment: Right
+ReferenceAlignment: Pointer
+DerivePointerAlignment: false
+NamespaceIndentation: None
+FixNamespaceComments: true
+SortIncludes: CaseSensitive
+IncludeBlocks: Regroup
+AllowShortFunctionsOnASingleLine: Empty
+AllowShortIfStatementsOnASingleLine: Never
+AllowShortLoopsOnASingleLine: false
+AllowShortCaseLabelsOnASingleLine: false
 LineEnding: LF
diff --git a/.clang-format-ignore b/.clang-format-ignore
index d495386..3d19633 100644
--- a/.clang-format-ignore
+++ b/.clang-format-ignore
@@ -1,4 +1,11 @@
 # Do not attempt to format .td files; we have too many formatting needs across
 # the various files to allow automatic formatting.
 **/*.td
-rcc-legacy/
\ No newline at end of file
+
+# MIR and structured test inputs are compatibility fixtures, not C++ sources.
+**/*.mir
+**/test/Inputs/**
+
+# Generated files are formatted by their generator, if applicable.
+**/generated/**
+rcc-legacy/
diff --git a/.clang-tidy b/.clang-tidy
index 856ac61..dcad2ae 100644
--- a/.clang-tidy
+++ b/.clang-tidy
@@ -1,16 +1,29 @@
 HeaderFilterRegex: ''
+WarningsAsErrors: ''
+FormatStyle: file
 Checks: >
   -*,
   clang-diagnostic-*,
-  llvm-*,
-  misc-*,
-  -misc-const-correctness,
-  -misc-include-cleaner,
-  -misc-no-recursion,
-  -misc-non-private-member-variables-in-classes,
-  -misc-unused-parameters,
-  -misc-use-anonymous-namespace,
-  readability-identifier-naming
+  bugprone-*,
+  -bugprone-easily-swappable-parameters,
+  performance-*,
+  portability-*,
+  llvm-header-guard,
+  llvm-namespace-comment,
+  llvm-qualified-auto,
+  misc-definitions-in-headers,
+  misc-misplaced-const,
+  misc-new-delete-overloads,
+  misc-non-copyable-objects,
+  modernize-use-default-member-init,
+  modernize-use-equals-default,
+  modernize-use-equals-delete,
+  modernize-use-nullptr,
+  modernize-use-override,
+  readability-container-size-empty,
+  readability-identifier-naming,
+  readability-redundant-*,
+  readability-simplify-boolean-expr
 
 CheckOptions:
   # Type
@@ -26,11 +39,9 @@ CheckOptions:
   - { key: readability-identifier-naming.FunctionCase,              value: camelBack  }
   - { key: readability-identifier-naming.MethodCase,                value: camelBack  }
   # Variable
-  - { key: readability-identifier-naming.VariableCase,              value: lower_case }
-  - { key: readability-identifier-naming.ParameterCase,             value: lower_case }
-  - { key: readability-identifier-naming.MemberCase,                value: lower_case }
-  - { key: readability-identifier-naming.PrivateMemberSuffix,       value: _          }
-  - { key: readability-identifier-naming.ProtectedMemberSuffix,     value: _          }
+  - { key: readability-identifier-naming.VariableCase,              value: CamelCase  }
+  - { key: readability-identifier-naming.ParameterCase,             value: CamelCase  }
+  - { key: readability-identifier-naming.MemberCase,                value: CamelCase  }
   # Constant
   - { key: readability-identifier-naming.GlobalConstantCase,        value: CamelCase  }
   - { key: readability-identifier-naming.ConstexprVariableCase,     value: CamelCase  }
diff --git a/CMakeLists.txt b/CMakeLists.txt
new file mode 100644
index 0000000..597963a
--- /dev/null
+++ b/CMakeLists.txt
@@ -0,0 +1,34 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+cmake_minimum_required(VERSION 3.25)
+
+project(
+  RCC
+  VERSION 0.1.0
+  DESCRIPTION "RCC compiler collection"
+  LANGUAGES CXX
+)
+
+list(PREPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules")
+
+include(RCCPreventInSourceBuild)
+include(RCCProjects)
+include(CTest)
+
+rcc_add_enabled_projects("${CMAKE_CURRENT_SOURCE_DIR}")
+
+if(TARGET rcc-backend-format)
+  add_custom_target(format DEPENDS rcc-backend-format)
+endif()
+
+if(TARGET rcc-backend-format-check)
+  add_custom_target(format-check DEPENDS rcc-backend-format-check)
+endif()
+
+if(TARGET rcc-backend-tidy)
+  add_custom_target(tidy DEPENDS rcc-backend-tidy)
+endif()
+
+if(TARGET rcc-backend-analyze)
+  add_custom_target(analyze DEPENDS rcc-backend-analyze)
+endif()
diff --git a/README.md b/README.md
index 9ab0765..19f6a44 100644
--- a/README.md
+++ b/README.md
@@ -1,12 +1,46 @@
 # RCC Project
 
-Welcome to the RCC Project!
+RCC is a compiler collection under active development. The current implementation
+focuses on a standalone machine-code backend.
 
-The RCC Project is free software. See the file LICENSE
-for copying permission.
+## Repository layout
 
-This project aims to build a highly optimized compiler collection.
-Obviously, its name comes from GNU Compiler Collection (GCC).
+```text
+backend/    Standalone backend project
+cmake/      Monorepo orchestration support
+docs/       Architecture and implementation records
+```
 
-The development of RCC is ongoing. Your contribution is welcome.
+Future frontend, middle-end, and driver projects will be sibling components.
 
+## Build the enabled projects
+
+RCC requires Clang and C++23. Both Ninja and Unix Makefiles are supported.
+
+```bash
+cmake -S . -B build -G Ninja \
+  -DCMAKE_CXX_COMPILER=clang++ \
+  -DRCC_ENABLE_PROJECTS=backend
+cmake --build build
+ctest --test-dir build --output-on-failure
+```
+
+## Build the backend independently
+
+```bash
+cmake -S backend -B build-backend -G Ninja \
+  -DCMAKE_CXX_COMPILER=clang++
+cmake --build build-backend
+ctest --test-dir build-backend --output-on-failure
+```
+
+Replace `Ninja` with `Unix Makefiles` to build with Make.
+
+Developer checks are exposed as `rcc-backend-format-check`,
+`rcc-backend-tidy`, and `rcc-backend-analyze` in a standalone backend build.
+The monorepo exposes the aggregate `format-check`, `tidy`, and `analyze`
+targets.
+
+## License
+
+RCC is free software distributed under GPL-3.0-or-later. See `LICENSE`.
diff --git a/backend/.clang-format b/backend/.clang-format
new file mode 100644
index 0000000..fc4f3fc
--- /dev/null
+++ b/backend/.clang-format
@@ -0,0 +1,20 @@
+BasedOnStyle: LLVM
+Language: Cpp
+Standard: Latest
+ColumnLimit: 100
+IndentWidth: 2
+ContinuationIndentWidth: 4
+UseTab: Never
+BreakBeforeBraces: Attach
+PointerAlignment: Right
+ReferenceAlignment: Pointer
+DerivePointerAlignment: false
+NamespaceIndentation: None
+FixNamespaceComments: true
+SortIncludes: CaseSensitive
+IncludeBlocks: Regroup
+AllowShortFunctionsOnASingleLine: Empty
+AllowShortIfStatementsOnASingleLine: Never
+AllowShortLoopsOnASingleLine: false
+AllowShortCaseLabelsOnASingleLine: false
+LineEnding: LF
diff --git a/backend/.clang-tidy b/backend/.clang-tidy
new file mode 100644
index 0000000..a0e18ce
--- /dev/null
+++ b/backend/.clang-tidy
@@ -0,0 +1,49 @@
+HeaderFilterRegex: '(^|/)(include|lib|tools|utils|unittests)/'
+WarningsAsErrors: ''
+FormatStyle: file
+Checks: >
+  -*,
+  clang-diagnostic-*,
+  bugprone-*,
+  -bugprone-easily-swappable-parameters,
+  performance-*,
+  portability-*,
+  llvm-header-guard,
+  llvm-namespace-comment,
+  llvm-qualified-auto,
+  misc-definitions-in-headers,
+  misc-misplaced-const,
+  misc-new-delete-overloads,
+  misc-non-copyable-objects,
+  modernize-use-default-member-init,
+  modernize-use-equals-default,
+  modernize-use-equals-delete,
+  modernize-use-nullptr,
+  modernize-use-override,
+  readability-container-size-empty,
+  readability-identifier-naming,
+  readability-redundant-*,
+  readability-simplify-boolean-expr
+
+CheckOptions:
+  - { key: readability-identifier-naming.NamespaceCase,             value: lower_case }
+  - { key: readability-identifier-naming.ClassCase,                 value: CamelCase  }
+  - { key: readability-identifier-naming.StructCase,                value: CamelCase  }
+  - { key: readability-identifier-naming.UnionCase,                 value: CamelCase  }
+  - { key: readability-identifier-naming.EnumCase,                  value: CamelCase  }
+  - { key: readability-identifier-naming.TypeAliasCase,             value: CamelCase  }
+  - { key: readability-identifier-naming.TypedefCase,               value: CamelCase  }
+  - { key: readability-identifier-naming.TypeTemplateParameterCase, value: CamelCase  }
+  - { key: readability-identifier-naming.FunctionCase,              value: camelBack  }
+  - { key: readability-identifier-naming.MethodCase,                value: camelBack  }
+  - { key: readability-identifier-naming.VariableCase,              value: CamelCase  }
+  - { key: readability-identifier-naming.ParameterCase,             value: CamelCase  }
+  - { key: readability-identifier-naming.MemberCase,                value: CamelCase  }
+  - { key: readability-identifier-naming.GlobalConstantCase,        value: CamelCase  }
+  - { key: readability-identifier-naming.ConstexprVariableCase,     value: CamelCase  }
+  - { key: readability-identifier-naming.ClassConstantCase,         value: CamelCase  }
+  - { key: readability-identifier-naming.EnumConstantCase,          value: CamelCase  }
+  - { key: readability-identifier-naming.MacroDefinitionCase,       value: UPPER_CASE }
+  - { key: readability-identifier-naming.IgnoreMainLikeFunctions,   value: 1          }
+  - { key: readability-redundant-member-init.IgnoreBaseInCopyConstructors, value: 1    }
+  - { key: modernize-use-default-member-init.UseAssignment,         value: 1          }
diff --git a/backend/CMakeLists.txt b/backend/CMakeLists.txt
new file mode 100644
index 0000000..2d5cc82
--- /dev/null
+++ b/backend/CMakeLists.txt
@@ -0,0 +1,85 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+cmake_minimum_required(VERSION 3.25)
+
+set(RCC_BACKEND_VERSION 0.1.0)
+set(RCC_BACKEND_IS_STANDALONE OFF)
+set(RCC_BACKEND_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}")
+set(RCC_BACKEND_BINARY_DIR "${CMAKE_CURRENT_BINARY_DIR}")
+
+if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
+  project(
+    RCCBackend
+    VERSION ${RCC_BACKEND_VERSION}
+    DESCRIPTION "Independent RCC machine-code backend"
+    LANGUAGES CXX
+  )
+  set(RCC_BACKEND_IS_STANDALONE ON)
+  include(CTest)
+endif()
+
+if(NOT CMAKE_CXX_COMPILER_ID MATCHES "^(Apple)?Clang$")
+  message(
+    FATAL_ERROR
+      "RCC Backend must be compiled with Clang; found '${CMAKE_CXX_COMPILER_ID}'"
+  )
+endif()
+
+if(CMAKE_SOURCE_DIR STREQUAL CMAKE_BINARY_DIR)
+  message(
+    FATAL_ERROR
+      "RCC Backend does not support in-source builds. Configure a separate build directory."
+  )
+endif()
+
+set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
+
+list(PREPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules")
+
+include(GNUInstallDirs)
+include(CMakePackageConfigHelpers)
+include(RCCBackendBuild)
+include(RCCBackendDeveloperTools)
+
+option(RCC_BACKEND_BUILD_TESTS "Build RCC Backend tests" "${BUILD_TESTING}")
+option(RCC_BACKEND_WARNINGS_AS_ERRORS "Treat compiler warnings as errors" OFF)
+option(RCC_BACKEND_ENABLE_CLANG_TIDY "Run clang-tidy during each compilation" OFF)
+
+rcc_backend_initialize_toolchain()
+
+add_subdirectory(lib)
+
+if(RCC_BACKEND_BUILD_TESTS)
+  enable_testing()
+  add_subdirectory(unittests)
+endif()
+
+rcc_backend_add_developer_targets()
+
+install(DIRECTORY include/ DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}")
+
+configure_package_config_file(
+  "${CMAKE_CURRENT_SOURCE_DIR}/cmake/RCCBackendConfig.cmake.in"
+  "${CMAKE_CURRENT_BINARY_DIR}/RCCBackendConfig.cmake"
+  INSTALL_DESTINATION "${CMAKE_INSTALL_LIBDIR}/cmake/RCCBackend"
+)
+
+write_basic_package_version_file(
+  "${CMAKE_CURRENT_BINARY_DIR}/RCCBackendConfigVersion.cmake"
+  VERSION "${RCC_BACKEND_VERSION}"
+  COMPATIBILITY SameMajorVersion
+)
+
+install(
+  EXPORT RCCBackendTargets
+  FILE RCCBackendTargets.cmake
+  NAMESPACE RCCBackend::
+  DESTINATION "${CMAKE_INSTALL_LIBDIR}/cmake/RCCBackend"
+)
+
+install(
+  FILES
+    "${CMAKE_CURRENT_BINARY_DIR}/RCCBackendConfig.cmake"
+    "${CMAKE_CURRENT_BINARY_DIR}/RCCBackendConfigVersion.cmake"
+  DESTINATION "${CMAKE_INSTALL_LIBDIR}/cmake/RCCBackend"
+)
diff --git a/backend/cmake/RCCBackendConfig.cmake.in b/backend/cmake/RCCBackendConfig.cmake.in
new file mode 100644
index 0000000..910572b
--- /dev/null
+++ b/backend/cmake/RCCBackendConfig.cmake.in
@@ -0,0 +1,7 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+@PACKAGE_INIT@
+
+include("${CMAKE_CURRENT_LIST_DIR}/RCCBackendTargets.cmake")
+
+check_required_components(RCCBackend)
diff --git a/backend/cmake/modules/RCCBackendBuild.cmake b/backend/cmake/modules/RCCBackendBuild.cmake
new file mode 100644
index 0000000..79e7bb1
--- /dev/null
+++ b/backend/cmake/modules/RCCBackendBuild.cmake
@@ -0,0 +1,93 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+function(rcc_backend_initialize_toolchain)
+  string(REGEX MATCH "^[0-9]+" RCCBackendClangMajor "${CMAKE_CXX_COMPILER_VERSION}")
+
+  find_program(
+    RCC_BACKEND_CLANG_FORMAT
+    NAMES "clang-format-${RCCBackendClangMajor}" clang-format
+    REQUIRED
+  )
+  find_program(
+    RCC_BACKEND_CLANG_TIDY
+    NAMES "clang-tidy-${RCCBackendClangMajor}" clang-tidy
+    REQUIRED
+  )
+
+  foreach(Tool IN ITEMS RCC_BACKEND_CLANG_FORMAT RCC_BACKEND_CLANG_TIDY)
+    execute_process(
+      COMMAND "${${Tool}}" --version
+      OUTPUT_VARIABLE ToolVersionOutput
+      ERROR_VARIABLE ToolVersionError
+      RESULT_VARIABLE ToolVersionResult
+    )
+    if(NOT ToolVersionResult EQUAL 0)
+      message(
+        FATAL_ERROR
+          "Unable to query ${Tool}: ${ToolVersionOutput}${ToolVersionError}"
+      )
+    endif()
+
+    string(REGEX MATCH "version[ \t]+([0-9]+)" ToolVersionMatch "${ToolVersionOutput}")
+    if(NOT CMAKE_MATCH_1 STREQUAL RCCBackendClangMajor)
+      message(
+        FATAL_ERROR
+          "${Tool} must have Clang major version ${RCCBackendClangMajor}: "
+          "${ToolVersionOutput}"
+      )
+    endif()
+  endforeach()
+
+  set(RCC_BACKEND_CLANG_FORMAT "${RCC_BACKEND_CLANG_FORMAT}" PARENT_SCOPE)
+  set(RCC_BACKEND_CLANG_TIDY "${RCC_BACKEND_CLANG_TIDY}" PARENT_SCOPE)
+endfunction()
+
+function(rcc_backend_configure_target Target)
+  target_compile_features("${Target}" PUBLIC cxx_std_23)
+  set_target_properties(
+    "${Target}"
+    PROPERTIES
+      CXX_EXTENSIONS OFF
+      CXX_STANDARD 23
+      CXX_STANDARD_REQUIRED ON
+  )
+
+  target_compile_options(
+    "${Target}"
+    PRIVATE
+      -Wall
+      -Wextra
+      -Wpedantic
+      -Wconversion
+      -Wsign-conversion
+      -Wshadow
+      -Wnon-virtual-dtor
+      -Wold-style-cast
+      -Wcast-align
+      -Woverloaded-virtual
+      -Wnull-dereference
+      -Wdouble-promotion
+      -Wformat=2
+  )
+
+  if(RCC_BACKEND_WARNINGS_AS_ERRORS)
+    target_compile_options("${Target}" PRIVATE -Werror)
+  endif()
+
+  if(RCC_BACKEND_ENABLE_CLANG_TIDY)
+    set_target_properties(
+      "${Target}"
+      PROPERTIES
+        CXX_CLANG_TIDY
+          "${RCC_BACKEND_CLANG_TIDY};--warnings-as-errors=*"
+    )
+  endif()
+endfunction()
+
+function(rcc_backend_register_format_files)
+  set_property(GLOBAL APPEND PROPERTY RCC_BACKEND_FORMAT_FILES ${ARGN})
+endfunction()
+
+function(rcc_backend_register_tidy_files)
+  set_property(GLOBAL APPEND PROPERTY RCC_BACKEND_TIDY_FILES ${ARGN})
+endfunction()
diff --git a/backend/cmake/modules/RCCBackendDeveloperTools.cmake b/backend/cmake/modules/RCCBackendDeveloperTools.cmake
new file mode 100644
index 0000000..d3693cd
--- /dev/null
+++ b/backend/cmake/modules/RCCBackendDeveloperTools.cmake
@@ -0,0 +1,50 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+function(rcc_backend_add_developer_targets)
+  get_property(FormatFiles GLOBAL PROPERTY RCC_BACKEND_FORMAT_FILES)
+  get_property(TidyFiles GLOBAL PROPERTY RCC_BACKEND_TIDY_FILES)
+
+  if(FormatFiles)
+    list(REMOVE_DUPLICATES FormatFiles)
+
+    add_custom_target(
+      rcc-backend-format
+      COMMAND "${RCC_BACKEND_CLANG_FORMAT}" -i ${FormatFiles}
+      COMMAND_EXPAND_LISTS
+      COMMENT "Formatting RCC Backend C++ sources"
+      VERBATIM
+    )
+
+    add_custom_target(
+      rcc-backend-format-check
+      COMMAND "${RCC_BACKEND_CLANG_FORMAT}" --dry-run --Werror ${FormatFiles}
+      COMMAND_EXPAND_LISTS
+      COMMENT "Checking RCC Backend C++ formatting"
+      VERBATIM
+    )
+  endif()
+
+  if(TidyFiles)
+    list(REMOVE_DUPLICATES TidyFiles)
+
+    add_custom_target(
+      rcc-backend-tidy
+      COMMAND
+        "${RCC_BACKEND_CLANG_TIDY}" --warnings-as-errors=*
+        "-p=${CMAKE_BINARY_DIR}" ${TidyFiles}
+      COMMAND_EXPAND_LISTS
+      COMMENT "Running clang-tidy on RCC Backend"
+      VERBATIM
+    )
+
+    add_custom_target(
+      rcc-backend-analyze
+      COMMAND
+        "${RCC_BACKEND_CLANG_TIDY}" "--checks=-*,clang-analyzer-*"
+        --warnings-as-errors=* "-p=${CMAKE_BINARY_DIR}" ${TidyFiles}
+      COMMAND_EXPAND_LISTS
+      COMMENT "Running Clang Static Analyzer checks on RCC Backend"
+      VERBATIM
+    )
+  endif()
+endfunction()
diff --git a/backend/include/rcc/Support/Error.h b/backend/include/rcc/Support/Error.h
new file mode 100644
index 0000000..3d50b46
--- /dev/null
+++ b/backend/include/rcc/Support/Error.h
@@ -0,0 +1,49 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+#ifndef RCC_SUPPORT_ERROR_H
+#define RCC_SUPPORT_ERROR_H
+
+#include <cstdint>
+#include <expected>
+#include <string>
+#include <string_view>
+#include <utility>
+
+namespace rcc {
+
+enum class ErrorCode : std::uint8_t {
+  InvalidArgument,
+  ParseError,
+  VerificationError,
+  Unsupported,
+  IoError,
+  InternalError,
+};
+
+[[nodiscard]] std::string_view getErrorCodeName(ErrorCode Code) noexcept;
+
+class Error final {
+public:
+  Error(ErrorCode Code, std::string Message);
+
+  [[nodiscard]] ErrorCode code() const noexcept {
+    return Code;
+  }
+  [[nodiscard]] std::string_view message() const noexcept {
+    return Message;
+  }
+
+private:
+  ErrorCode Code;
+  std::string Message;
+};
+
+template <typename ValueT> using Expected = std::expected<ValueT, Error>;
+
+[[nodiscard]] inline std::unexpected<Error> makeUnexpected(ErrorCode Code, std::string Message) {
+  return std::unexpected<Error>(Error(Code, std::move(Message)));
+}
+
+} // namespace rcc
+
+#endif // RCC_SUPPORT_ERROR_H
diff --git a/backend/lib/CMakeLists.txt b/backend/lib/CMakeLists.txt
new file mode 100644
index 0000000..13f7c7b
--- /dev/null
+++ b/backend/lib/CMakeLists.txt
@@ -0,0 +1,3 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+add_subdirectory(Support)
diff --git a/backend/lib/Support/CMakeLists.txt b/backend/lib/Support/CMakeLists.txt
new file mode 100644
index 0000000..88c959d
--- /dev/null
+++ b/backend/lib/Support/CMakeLists.txt
@@ -0,0 +1,40 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+set(RCCBackendSupportSources Error.cpp)
+set(
+  RCCBackendSupportHeaders
+  "${RCC_BACKEND_SOURCE_DIR}/include/rcc/Support/Error.h"
+)
+
+add_library(RCCBackendSupport ${RCCBackendSupportSources})
+add_library(RCCBackend::Support ALIAS RCCBackendSupport)
+
+rcc_backend_configure_target(RCCBackendSupport)
+
+target_include_directories(
+  RCCBackendSupport
+  PUBLIC
+    "$<BUILD_INTERFACE:${RCC_BACKEND_SOURCE_DIR}/include>"
+    "$<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>"
+)
+
+set_target_properties(
+  RCCBackendSupport
+  PROPERTIES
+    EXPORT_NAME Support
+    OUTPUT_NAME RCCBackendSupport
+)
+
+install(
+  TARGETS RCCBackendSupport
+  EXPORT RCCBackendTargets
+  ARCHIVE DESTINATION "${CMAKE_INSTALL_LIBDIR}"
+  LIBRARY DESTINATION "${CMAKE_INSTALL_LIBDIR}"
+  RUNTIME DESTINATION "${CMAKE_INSTALL_BINDIR}"
+)
+
+rcc_backend_register_format_files(
+  "${CMAKE_CURRENT_SOURCE_DIR}/Error.cpp"
+  ${RCCBackendSupportHeaders}
+)
+rcc_backend_register_tidy_files("${CMAKE_CURRENT_SOURCE_DIR}/Error.cpp")
diff --git a/backend/lib/Support/Error.cpp b/backend/lib/Support/Error.cpp
new file mode 100644
index 0000000..dc7d590
--- /dev/null
+++ b/backend/lib/Support/Error.cpp
@@ -0,0 +1,28 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+#include "rcc/Support/Error.h"
+
+namespace rcc {
+
+std::string_view getErrorCodeName(ErrorCode Code) noexcept {
+  switch (Code) {
+  case ErrorCode::InvalidArgument:
+    return "invalid argument";
+  case ErrorCode::ParseError:
+    return "parse error";
+  case ErrorCode::VerificationError:
+    return "verification error";
+  case ErrorCode::Unsupported:
+    return "unsupported";
+  case ErrorCode::IoError:
+    return "I/O error";
+  case ErrorCode::InternalError:
+    return "internal error";
+  }
+
+  return "unknown error";
+}
+
+Error::Error(ErrorCode Code, std::string Message) : Code(Code), Message(std::move(Message)) {}
+
+} // namespace rcc
diff --git a/backend/unittests/CMakeLists.txt b/backend/unittests/CMakeLists.txt
new file mode 100644
index 0000000..13f7c7b
--- /dev/null
+++ b/backend/unittests/CMakeLists.txt
@@ -0,0 +1,3 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+add_subdirectory(Support)
diff --git a/backend/unittests/Support/CMakeLists.txt b/backend/unittests/Support/CMakeLists.txt
new file mode 100644
index 0000000..0c0aabc
--- /dev/null
+++ b/backend/unittests/Support/CMakeLists.txt
@@ -0,0 +1,16 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+set(RCCBackendSupportTestSources ErrorTest.cpp)
+
+add_executable(RCCBackendSupportTests ${RCCBackendSupportTestSources})
+rcc_backend_configure_target(RCCBackendSupportTests)
+target_link_libraries(RCCBackendSupportTests PRIVATE RCCBackend::Support)
+
+add_test(NAME RCCBackend.Support.Error COMMAND RCCBackendSupportTests)
+
+rcc_backend_register_format_files(
+  "${CMAKE_CURRENT_SOURCE_DIR}/ErrorTest.cpp"
+)
+rcc_backend_register_tidy_files(
+  "${CMAKE_CURRENT_SOURCE_DIR}/ErrorTest.cpp"
+)
diff --git a/backend/unittests/Support/ErrorTest.cpp b/backend/unittests/Support/ErrorTest.cpp
new file mode 100644
index 0000000..23140e3
--- /dev/null
+++ b/backend/unittests/Support/ErrorTest.cpp
@@ -0,0 +1,33 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+#include "rcc/Support/Error.h"
+
+#include <iostream>
+#include <string>
+
+namespace {
+
+int fail(const char *Message) {
+  std::cerr << Message << '\n';
+  return 1;
+}
+
+} // namespace
+
+int main() {
+  rcc::Expected<int> Value = 42;
+  if (!Value || *Value != 42)
+    return fail("Expected must preserve a successful value");
+
+  rcc::Expected<int> Failure = rcc::makeUnexpected(rcc::ErrorCode::ParseError, "bad MIR");
+  if (Failure)
+    return fail("Expected must preserve an error");
+  if (Failure.error().code() != rcc::ErrorCode::ParseError)
+    return fail("Error must preserve its code");
+  if (Failure.error().message() != "bad MIR")
+    return fail("Error must preserve its message");
+  if (rcc::getErrorCodeName(Failure.error().code()) != "parse error")
+    return fail("Error code must have a stable name");
+
+  return 0;
+}
diff --git a/cmake/modules/RCCPreventInSourceBuild.cmake b/cmake/modules/RCCPreventInSourceBuild.cmake
new file mode 100644
index 0000000..97b3392
--- /dev/null
+++ b/cmake/modules/RCCPreventInSourceBuild.cmake
@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+if(CMAKE_SOURCE_DIR STREQUAL CMAKE_BINARY_DIR)
+  message(
+    FATAL_ERROR
+      "RCC does not support in-source builds. Configure a separate build directory."
+  )
+endif()
diff --git a/cmake/modules/RCCProjects.cmake b/cmake/modules/RCCProjects.cmake
new file mode 100644
index 0000000..59c4fc5
--- /dev/null
+++ b/cmake/modules/RCCProjects.cmake
@@ -0,0 +1,40 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+set(RCC_KNOWN_PROJECTS backend)
+set(RCC_DEFAULT_PROJECTS backend)
+
+set(
+  RCC_ENABLE_PROJECTS
+  "${RCC_DEFAULT_PROJECTS}"
+  CACHE STRING
+  "Semicolon-separated RCC projects to build, or 'all'"
+)
+
+function(rcc_add_enabled_projects RCCSourceRoot)
+  if(RCC_ENABLE_PROJECTS STREQUAL "all")
+    set(EnabledProjects ${RCC_KNOWN_PROJECTS})
+  else()
+    set(EnabledProjects ${RCC_ENABLE_PROJECTS})
+  endif()
+
+  if(NOT EnabledProjects)
+    message(FATAL_ERROR "RCC_ENABLE_PROJECTS must name at least one project")
+  endif()
+
+  list(REMOVE_DUPLICATES EnabledProjects)
+
+  foreach(Project IN LISTS EnabledProjects)
+    if(NOT Project IN_LIST RCC_KNOWN_PROJECTS)
+      message(
+        FATAL_ERROR
+          "Unknown RCC project '${Project}'. Known projects: ${RCC_KNOWN_PROJECTS}"
+      )
+    endif()
+  endforeach()
+
+  # Component order is controlled here rather than by the user-provided list.
+  # Future inter-project dependency closure will also be resolved here.
+  if("backend" IN_LIST EnabledProjects)
+    add_subdirectory("${RCCSourceRoot}/backend" "${CMAKE_BINARY_DIR}/backend")
+  endif()
+endfunction()