Function sections should be enabled by default in the static compiler

[Tobba]

This would greatly reduce the size of staticaly linked binaries if ld is invoked with -gc-sections

[alexcrichton]

In the past I found that this significantly slowed down the linker, and this is what -Zlto was added for. Things may have changed though!

The --gc-sections flag can have other unforseen side effects, and I'm not sure if we'd want to officially support being able to run it. I remember it stripping out metadata, but there are indeed ways to get around that, so perhaps I didn't dig enough.

[alexcrichton]

The good news is that this is just a flag that we pass through to LLVM somewhere and it'll just magically do it all for us, so with the new -C flag this will be easy to add.

[thestinger]

I don't think this should be enabled by default. It makes the remaining code larger and slower, and the same positive effects can be gained by doing link-time optimization. Making binaries smaller will require altering the design of the standard library, as using link-time optimization strips out even more than --gc-sections with function sections but doesn't result in binaries anywhere near the size you can get with C or C++.

[alexcrichton]

I compiled rust with make RUSTFLAGS='-C llvm-args=-ffunction-sections' to do some analysis (on linux). I found that all libraries saw about a 20% size increase, and link times for hello world doubled from ~100ms to ~200ms.

With this in mind I don't think that we're going to want to turn this on by default. I'm going to close this in favor of #12455