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Set out all the things to cover during static
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@ -274,13 +274,20 @@ is almost certainly a code smell.
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Static variables are related to `const` variables, but take a slightly different approach.
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When the compiler can guarantee that a *reference* is fixed for the life of a program,
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you end up with a `static` variable. Where `const` variables can never change value
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(because the value may have been copied to random pieces of code), `static` variables
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uniquely identify a location in memory. This allows them to change value under specific
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circumstances.
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you end up with a `static` variable (as opposed to *values* that are fixed for the
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duration a program is running). Because of this reference/value distinction,
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static variables behave much more like what people expect from "global" variables.
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We'll look at regular static variables first, and then address the `lazy_static!()`
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and `thread_local!()` macros later.
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1. What's going on with `lazy_static!()`?
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2. What's going on with `thread_local!()`?
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More generally, `static` variables are globally unique locations in memory,
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the contents of which are loaded as part of your program being read into main memory.
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They allow initialization with both raw values and (most) `const fn` calls, and must
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implement the [`Sync`](https://doc.rust-lang.org/std/marker/trait.Sync.html)
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marker trait. The initial value is loaded along with the program/library binary,
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though it can change during the time your program is running. While `static mut`
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variables are allowed, mutating a static is considered an `unsafe` operation.
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Unlike `const` though, interior mutability is accepted, and can be done with `safe` code.
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## **push** and **pop**: Stack Allocations
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