| Issue |
144981
|
| Summary |
[clang] Support layering checks for C++ modules
|
| Labels |
clang
|
| Assignees |
|
| Reporter |
matts1
|
One highly useful feature of build systems is the ability to ensure strict dependencies. For more details, see this [blog post](https://blog.bazel.build/2017/06/28/sjd-unused_deps.html).
## What are strict deps
TLDR: Strict deps enforce that code can only depend on its direct dependencies, and not transitive dependencies.
Consider the following code:
```
// foo.cppm
import bar;
import baz;
```
```
// bar.cppm
import baz
```
```
# BUILD
cc_module(
name = "foo",
interface = "foo.cppm",
deps = [":bar"]
)
cc_module(
name = "bar",
interface = "bar.cppm",
deps = [":baz"]
)
```
This code *should* not compile. `foo` imports `baz`, but does not declare a dependency on `baz`. However, build systems have no easy mechanism to enforce this, as they have to output compile steps along the lines of:
```
clang++ --precompile baz.cppm -o baz.pcm
clang++ --precompile bar.cppm -fmodule-file=baz=baz.pcm -o bar.pcm
clang++ --precompile foo.cppm -fmodule-file=baz=baz.pcm -fmodule-file=bar=bar.pcm -o foo.pcm
```
Note that we cannot simply remove the `-fmodule-file=baz=...`, otherwise we would get an error if the module `bar` ever tried to return a value of a type declared in `baz`.
## Proposal
I propose that clang adds a new option `-findirect-module-file`. This has the same semantics as `-fmodule-file`, with the only difference being that if you ever attempt to import the module from the main TU, you get an error.
It might also be a good idea to add `-Wstrict-module-deps` or similar, so that this option can be first applied as warnings to an existing codebase, instead of errors.
This would allow the build system to generate:
```
clang++ --precompile baz.cppm -o baz.pcm
clang++ --precompile bar.cppm -fmodule-file=baz=baz.pcm -o bar.pcm
clang++ --precompile foo.cppm -findirect-module-file=baz=baz.pcm -fmodule-file=bar=bar.pcm -o foo.pcm
```
Thus allowing foo to fail to compile in the first place.
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