Docs: "Cannot find source file" in common_issues

As described in #580, this error may be encountered when using
corrosion_add_cxxbridge, now that the generated headers are added as
`PUBLIC` sources.

doc/src/common_issues.md

@@ -6,6 +6,7 @@
 - [Linking Rust static libraries into Debug C/C++ binaries fails on Windows MSVC targets](#linking-rust-static-libraries-into-debug-cc-binaries-fails-on-windows-msvc-targets)
 - [Missing `soname` on Linux for `cdylibs`](#missing-soname-on-linux-for-cdylibs)
 - [Missing `install_name` on MacOS for `ccdylibs` / Hardcoded references to the build-directory](#missing-installname-on-macos-for-ccdylibs--hardcoded-references-to-the-build-directory)
+- [CMake Error (target_link_libraries): Cannot find source file](#cmake-error-target_link_libraries-cannot-find-source-file)
 
 ## Linking Debug C/C++ libraries into Rust fails on Windows MSVC targets
 
@@ -86,3 +87,36 @@ When building binaries using this shared library, you should set the build rpath
 your shared library, e.g. by setting `set(CMAKE_BUILD_RPATH ${YOUR_CUSTOM_OUTPUT_DIRECTORY})` before adding
 executables.
 For a practical example, you may look at [Slint PR 2455](https://github.com/slint-ui/slint/pull/2455).
+
+## CMake Error (target_link_libraries): Cannot find source file
+
+When using `corrosion_add_cxxbridge`, you may encounter an error similar to this in targets that depend on the cxxbridge target:
+
+```diff
+- CMake Error at ...../CMakeLists.txt:61 (target_link_libraries):
+-  Cannot find source file:
+-
+-    ...../corrosion_generated/..../somefile.h
+-
+-  Tried extensions .c .C .c++ .cc .cpp .cxx .cu .mpp .m .M .mm .ixx .cppm
+-  .ccm .cxxm .c++m .h .hh .h++ .hm .hpp .hxx .in .txx .f .F .for .f77 .f90
+-  .f95 .f03 .hip .ispc
+```
+
+Where `somefile.h` should be generated by CXX via `corrosion_add_cxxbridge`.
+In theory, CMake should already know that this is a generated file and just generate it when needed.
+
+However, in older versions of CMake the `GENERATED` property isn't correctly propagated.
+See also: [https://gitlab.kitware.com/cmake/cmake/-/issues/18399](https://gitlab.kitware.com/cmake/cmake/-/issues/18399)
+
+This has since been fixed with CMake 3.20: [https://cmake.org/cmake/help/v3.20/policy/CMP0118.html](https://cmake.org/cmake/help/latest/command/cmake_policy.html#version)
+However, the CMake policy CMP0118 must be enabled **in any dependent CMakeLists.txt** for the fix to work.
+
+The best fix is to call:
+```cmake
+cmake_minimium_required(VERSION 3.20 FATAL_ERROR)
+# (or any other version above 3.20)
+```
+As described [here](https://cmake.org/cmake/help/latest/command/cmake_policy.html#version), this implies a call to `cmake_policy` which enables CMP0118.
+
+Unfortunately this must be done in all (transitive) downstream dependencies that link to the bridge target, so cannot be done from within corrosion automatically.