Subtract underflow when generating explicit paddings

[yshui]

Input C/C++ Header

#pragma pack(2)

struct Test {
        unsigned long x;
        char a;
        char b;
        char c;
};

Bindgen Invocation

$ bindgen input.h --explicit-padding

Actual Results

panicked at bindgen/codegen/struct_layout.rs:309:20:
attempt to subtract with overflow

Analysis

The reason this happens is because in saw_field_with_layout, padding bytes size is always calculated using the specified pack alignment, i.e. 2 bytes. however, if the field itself has a smaller alignment requirement, that should be used instead.

[yshui]

And padding is not generated for packed layout with align > 1, I will open another bug after this is fixed.

[Connor-GH]

This should probably still be open; affects me on 0.71.1.