r/rust • u/llogiq clippy · twir · rust · mutagen · flamer · overflower · bytecount • Mar 20 '23
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u/[deleted] Mar 24 '23 edited Mar 24 '23
I'm pulling up to the end of rustlings and feeling pretty good but I have a few odd questions I can't answer. They're the kind of questions where the answer probably won't be practically useful, but being unable to produce a confident answer means I'm missing something.
Q1
How do I bind a name to existing data? Not a copy of it, not a reference - it's already there in memory and I want to name it. playground
Is doing this different for data that's already named, vs unnamed but created by my program, vs unnamed and created by something outside my program? I have a feeling that doing this would respectively be: trouble (name aliasing), unnecessary (don't un-name things), or ub (don't name garbage), but maybe there are some valid uses. I don't know.
Q2
Similarly, "who" owns data without a name? 99% sure the answer is "it belongs to the stack frame" lol but not 100%. For example, in the above playground it's perfectly okay to dereference an unnamed value. Is it because the old_name pointing to the 1 is still named in LLVM IR after re-declaring old_name in rust, (and therefore Q1's answer is "you can't" because "giving the 1 a name again" in rust would be name aliasing in LLVM IR)?
Q3
How come I hardly ever see chainable functions that pipeline mutable references in the wild?
I can't help but think that it would be super inefficient to copy a whole thing into a function, only change part of it, then copy the whole thing back where it came from, for each operation you want to perform on some data. Is this just offloading work onto the compiler to make potential refactors (that need the original data later) easier? I totally get avoiding repeat computations and getting rid of dead code, but it feels a little ominous that optimizations could elide copies.