Don't redefine structs that have been forward declared

[7Hazard]

Don't redefine structs that have been forward declared. This will keep track of all struct symbols to know whether a struct has already been defined, similarly to how functions are tracked (seen_function, saw_function).
Resolves #1864

[emilio]

Thanks for the patch. This looks sensible but I'm not sure it's the right approach, we shouldn't be generating forward declarations to begin with, see is_forward_declaration in comp.rs.

I tried to reproduce the issue and I couldn't with a simple example like:

void foo(struct bar*);

typedef struct bar {
    float roll;
    float pitch;
    float yaw;
} bar;

void baz(struct bar*);

So there's clearly something else going on. We should also add a test-case to the test suite to avoid regressing it.

[7Hazard]

I was also hesitant in this approach. I realized the codegen reaches this part when it shouldn't https://github.com/rust-lang/rust-bindgen/blob/master/src/codegen/mod.rs#L1823.

I was able to reproduce the error locally with the example you provided.
This seems to be done as a post-process measure but doesn't take into consideration that the struct was defined in the meantime of the codegen. Im not familiar with the codebase but I will try to find what triggers this.

[emilio]

I was able to reproduce the error locally with the example you provided.

Oh, that's interesting. Which libclang version you have installed?

[7Hazard]

clang version 11.0.0
Target: x86_64-pc-windows-msvc
Thread model: posix
InstalledDir: C:\Program Files\LLVM\bin

Using the windows installer provided in LLVM's page.

[7Hazard]

So I can't find why exactly what triggers the forward declaration Rust definitions and how to prevent them from triggering if the forward declaration was defined later on by the C header, but I thought I'd share the solution im currently using in my fork.

[bors-servo]

☔ The latest upstream changes (presumably #2284) made this pull request unmergeable. Please resolve the merge conflicts.