Remove unused data segments

[fitzgen]

I used wasm-snip to remove all the panicking and formatting code from my .wasm, but my data section is still mostly fragments of diagnostic messages for panicking and formatting, and wasm-gc and wasm-opt can't remove them.

This is almost 20% of my .wasm's code size.

We need some way to mechanically remove this stuff -- doing it by hand is not feasible.

[koute]

You can easily remove the data sections automatically; the problem is mostly figuring out which ones you can safely remove, as any references to them in the bytecode look like normal integers, AFAIK.

[pepyakin]

I think it is hard to solve in general. Segments can be not separated (so you can't always know exact length of data blob) and these integers can be crafted.

[alexcrichton]

I believe https://github.com/rust-lang-nursery/rust-wasm/issues/2 combined with https://reviews.llvm.org/D42511 should solve this.

[pepyakin]

@alexcrichton I think there still left this problem https://github.com/rust-lang-nursery/rust-wasm/issues/19#issuecomment-358455513

[alexcrichton]

Done now that LLD is merged!