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19 May 2020 · #rust · #lifetimes
n
Table of Contents
nnIntron
I’ve held all of these misconceptions at some point and I see many beginners struggle with these misconceptions today. Some of my terminology might be non-standard, so here’s a table of shorthand phrases I use and what I intend for them to mean.
nnnnPhrasenShorthand fornnnnnTn1) a set containing all possible types or
2) some type within that setnnnowned typensome non-reference type, e.g. i32, String, Vec, etcnnn1) borrowed type or
2) ref typensome reference type regardless of mutability, e.g. &i32, &mut i32, etcnnn1) mut ref or
2) exclusive refnexclusive mutable reference, i.e. &mut Tnnn1) immut ref or
2) shared refnshared immutable reference, i.e. &TnnnnThe Misconceptionsn
In a nutshell: A variable’s lifetime is how long the data it points to can be statically verified by the compiler to be valid at its current memory address. I’ll now spend the next ~6500 words going into more detail about where people commonly get confused.
n1) T only contains owned typesn
This misconception is more about generics than lifetimes but generics and lifetimes are tightly intertwined in Rust so it’s not possible to talk about one without also talking about the other. Anyway:
n
When I first started learning Rust I understood that i32, &i32, and &mut i32 are different types. I also understood that some generic type variable T represents a set which contains all possible types. However, despite understanding both of these things separately, I wasn’t able to understand them together. In my newbie Rust mind this is how I thought generics worked:
nnnnnnnnnnnnType VariablenTn&Tn&mut TnnnExamplesni32n&i32n&mut i32nnnn
T contains all owned types. &T contains all immutably borrowed types.
