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.

[kmeisthax]

Fun fact: I may or may not have spent a few hours banging my head against performance issues that ultimately were resolved by replacing a truncate with a set_len. (See this commit)

Yeah, I know the optimizer is supposed to handle it, and in a perfect world that would be it, but...

• The procedure for enabling the optimizer is non-obvious to programmers learning Rust and constitutes a DX hazard. It's technically mentioned in the book but as an option that can be easily skimmed over, not a critical part of Rust's performance strategy.
• As mentioned elsewhere, LLVM doesn't support debugging optimized code. If you cannot debug optimized code, then the optimizer is not useful for developers!
• You're moving the mental overhead of the check from core Rust to user code (like I had to do, replacing a truncate call with a set_len call).
  • Worse, someone might actually fork Vec and publish it on crates.io. Minor dependency proliferation isn't a good thing - do we want to become npm?
• The whole damned point of Rust is low-overhead, memory-safe native code. Objecting to removing overhead for the sake of "the optimizer will take care of it" is extremely un-Rust-like.