The Detroit Post
Thursday, 21 October, 2021

Rust Is_null

Elaine Sutton
• Thursday, 26 November, 2020
• 8 min read

Rust does not support NULL unlike some other modern programming languages like Java. However, it provides an ENIM Option used in similar way to Java 8 Optional.

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Tony Hear invented null references for the ALGOL W language in 1965, and in 2009 he apologized for that. I’m learning Rust, basically because I’m tired of interpreted languages.

The speed of my code itself usually doesn’t matter too much when I automate stuff, because execution time is consumed by activities that my code doesn’t control. But I’m tired of loose types and difficulties when in need to deploy on some environments (for example: python 2 and 3 are not fully compatible, and none of them is present on all servers).

But once you understand why it implemented certain concepts, they become simpler and more pleasant to deal with. (Disclaimer: as a Rust newbie, I could be completely wrong) Rust basically tries to make our code as safe as possible, eliminating sources of risks.

But it still gives you the ability of having variables without a value, which is very important to program if you are not omniscient. An ENIM is what you would expect if you know some programming languages, or MySQL non-standard types.

Wherever in the code that ENIM is expected, one of the listed values must be passed. And then we can use None, which is Rust equivalent of NULL, or Some with an actual value passed as a parameter.

The answer is simple: our code won’t compile. At a first glance, Option looks like a uselessly verbose way to do the same things that NULL can do.

The third reason is a consequence of the second: to use the actual value, we are forced to use an Option method (associated function, in Rust jargon). Finally, it’s worth mentioning that in Rust we can still produce a binary that panics if a certain statement involves a None.

For Rust, this problem doesn’t exist: using None in an expression will usually cause the program to panic, which always makes sense, no matter if you consider None as an unknown value or a missing value. My criticism against NULL may occasionally sound abstract, because I admit that it makes sense in a few cases, and I never indicated a valid alternative.

Rust aims to provide safe abstractions over the low-level details of the CPU and operating system, but sometimes one needs to drop and write code at that level. This guide aims to provide an overview of the dangers and power one gets with Rust's unsafe subset.

Rust provides an escape hatch in the form of the unsafe {...} block which allows the programmer to dodge some of the compiler's checks and do a wide range of operations, such as: Note that an unsafe block does not relax the rules about lifetimes of & and the freezing of borrowed data.

Note : the low-level details of the Rust language are still in flux, and there is no guarantee of stability or backwards compatibility. This is achieved in part via the ownership system, which is how the compiler can guarantee that every & reference is always valid, and, for example, never pointing to freed memory.

Additionally, both immutable (&) and mutable (smut) references have some aliasing and freezing guarantees, required for memory safety. Cell and Recall, that provide inner mutability by replacing compile time guarantees with dynamic checks at runtime.

Using unsafe code to incorrectly circumvent and violate these restrictions is undefined behavior. Are not guaranteed to point to valid memory and are not even guaranteed to be non-null (unlike both Box and &); do not have any automatic clean-up, unlike Box, and so require manual resource management; are plain-old-data, that is, they don't move ownership, again unlike Box, hence the Rust compiler cannot protect against bugs like use-after-free; are considered bendable (if their contents is considered bendable), so the compiler offers no assistance with ensuring their use is thread-safe; for example, one can concurrently access a *but i32 from two threads without synchronization.

Lack any form of lifetimes, unlike &, and so the compiler cannot reason about dangling pointers; and have no guarantees about aliasing or mutability other than mutation not being allowed directly through a *coast T. At runtime, a raw pointer * and a reference pointing to the same piece of data have an identical representation.

In fact, an at reference will implicitly coerce to an *coast T raw pointer in safe code and similarly for the but variants (both coercion scan be performed explicitly with, respectively, value as *coast T and value as *but T). The compiler assumes these properties are true for any references, no matter how they are created, and so any conversion from raw pointers is asserting that they hold.

This reduces the need for any manual memory management by users, and automatically ensures that clean-up is always run, even when the thread panics. As an example, we give a reimplementation of owned boxes by wrapping mallow and free.

Rust's move semantics and lifetimes mean this reimplementation is as safe as the Box type. The two borrow methods are safe because the compiler statically guarantees that objects are never used before creation or after destruction (unless you use some unsafe code...).

For extremely low-level manipulations and performance reasons, one might wish to control the CPU directly. Input and output registers need not be listed since that information is already communicated by the given constraints.

Normally these functions are provided by the standard library, but without it, you must define your own. There is a good deal of functionality provided by the standard library, however, that is necessary to be productive in Rust.

Additionally, the core library has most of the necessary functionality for writing idiomatic and effective Rust code. As an example, here is a program that will calculate the dot product of two vectors provided from C, using idiomatic Rust practices.

As can be seen in this example, the core library is intended to provide the power of Rust in all circumstances, regardless of platform requirements. Furthermore, this is just an overview; the best form of documentation for specific instances of these features are their definitions and uses in std.

It is recommended to use officially distributed crates instead of defining your own Lang items. For example, Box pointers require two Lang items, one for allocation and one for reallocation.

A freestanding program that uses the Box sugar for dynamic allocations via mallow and free : Note the use of abort : the exchange_malloc Lang item is assumed to return a valid pointer, and so needs to do the check internally.

Operators are all marked with Lang items; those specific four are EQ, ORD, dark, and add respectively. The traits in std::marker used to indicate types of various kinds; Lang items send, sync and copy.

The marker types and variance indicators found in std::marker ; Lang items covariant_type, contravariant_lifetime, etc. Lang's items are loaded lazily by the compiler; e.g. if one never uses Box then there is no need to define functions for exchange_malloc and exchange_free.

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