Possible Vec::truncate improvement

[leonardo-m]

The docs of Vec::truncate say:

"If len is greater than the vector's current length, this has no effect."

And indeed if you look at the source code:
https://doc.rust-lang.org/nightly/src/alloc/vec.rs.html#740-742

There's:

if len > self.len {
    return;
}

But isn't it better to use len >= self.len instead?

[ecstatic-morse]

See #74172.

[pickfire]

This doesn't just improve truncate itself but also resize. Resize runs truncate even on the same size which runs the rest of the code.

The previous change results in an additional branch but we could still improve this if we could prevent that.

[workingjubilee]

#78884 and #74172 both attempted to fix this, but due to inlining rules it is more complex than it appears to be.

Quoting scottmcm:

For anyone else who arrives and looks at this: this can happen because the old code, after inlining, resulted in if 0 > some_unsigned_number, which is of course never possible and thus gets optimized away. Changing the code so that the check is >= makes it possible again, and thus it is no longer optimized out.

[ChrisDenton]

#80857 added a comment to the code with an explanation of why we're doing this. Should this be closed now? For the reasons outlined above, changing this is not planned.

[toomuchsalt]

Shouldn't this be reviewed again, as clear doesn't call truncate after #96002 ?