Optimize vec.truncate and destructors for types with no dtor at -O0

[bvssvni]

The internal representation of String is Vec<u8>, but vec.truncate loops to drop members.

Perhaps setting the length directly?

[rust-slacker]

Maybe vec.truncate could use std::intrinsics::needs_drop::<T>() to decide if the loop is needed?

[thestinger]

It's already optimized out when optimizations are enabled. These loops could use needs_drop to improve performance when optimizations are disabled, but I don't think it's very important.

[bvssvni]

vec.truncate leads to 70% decreased performance in some cases for game development, where compiling with optimization has negative impact on productivity.

[nightpool]

@bvssvni Even just O1 isn't reasonable for development?

[bvssvni]

It would be nice if adding a profile to Cargo.toml was not required for a such simple case.

[rust-slacker]

Would it make sense to have cargo default to O1?

[thestinger]

@rust-slacker: LLVM only fully supports debugging at -O0, unlike GCC which supports it at all optimization levels via a lot of work integrating support in passes.

[pcwalton]

Closing; I don't think we necessarily want to do this.