'''[https://www.rust-lang.org/%|%Rust]''' is a [programming language]
designed to be performant, safe, and productive.


** Description **

Rust combines concepts from [C], [C++], [Haskell], and other languages.  In
order to guarantee '''type safety''' and compile a program to performant
machine code, the compiler tracks the [type] of each [variable] and function.
Where the compiler can not '''infer''' a type of a variable, parameter, or
result, it must be '''annotated with a type'''.  In order to guarantee memory
safety, the compiler also tracks the '''lifetime''' of each variable.  Where
the compiler can not infer the lifetime of a variable, parameter or result, it
must be '''annotated with a lifetime'''.

Like Tcl, Rust does not employ a [garbage collection%|%garbage collector], but
instead assigns each value an '''owning block of code''', and deallocates
resources for the value when the program exits that block of code.  Assigning
the value to a new variable or passing it to another function moves the
'''ownership''' to that variable or function, deallocating the current variable
to '''prevent aliasing'''.  As a result, a variable may not be valid for the
entire duration of a block of code.

Like [C], '''&''' produces a '''reference''' to a value.  Taking a reference is
referred to as '''borrowing''' because the compiler tracks each reference,
ensures that it does not outlive its referent, and additionally ensures that at
each moment there is only one mutable reference.  This avoids a whole class of
memory errors that [C] programs are susceptible to.  '''*''' follows a
reference to access the value. 

A variable is '''immutable''' by default, meaning that it does not afford write
access to the data bound to it. If a variable is annotated with '''mut''', the
variable may then be used to mutate the bound value.  The value bound to a
mutable variable may be borrowed, but the compiler ensures that it is never
borrowed more than once at any given point during execution.  This eliminates
'''data races''' in Rust programs.

A program is composed primarily of '''structs''' and the functions that operate
on them.  A struct in turn is composed of '''primitive values''', structs,
enums, and references to any of those things or to functions.  An '''enum''' is
a type that enumerates any number of possible variants, each of which may be
struct.  A function that accepts an enum uses '''match'''  to tailor its
operation to each possible variant of the enum.

Programs and libraries are organized into '''modules''', which contain any of
the various programming artifacts Rust provides, namely structures, variables,
functions, traits, and trait implementations.

A '''trait''' is a specification for group of functions that act as an
'''interface''' to a struct.  Any number of traits may be '''implemented''' for
a given struct.  '''impl''' is used to provide a set of function definitions
that implement the trait for a particular type of struct.  To maintain
'''coherence''', a module may only implement a trait if either the trait or the
structure is local to the module.

Rust has various features for programming the structure of the program
itself, i.e. for '''metaprogramming''':  A '''[macro]''' language,
'''[closure%|%closures]''', syntax for '''[templating]''' generic structs and
functions, and '''pattern matching''' similar to that of [Haskell], on language
'''items''' for such purposes as '''destructuring variable assignment''' and
'''code branching according to data type'''.

Rust is actually segmented into two different languages:   '''safe Rust''' and
'''unsafe Rust'''.  In the safe parts, '''nothing is undefined''', '''nothing
is null''', only '''safe type conversions''' are possible, and only '''safe
memory operations''' are allowed.  There is, however, an escape hatch:  A
function '''annotated as unsafe''' takes upon itself the responsibility to
guarantee the same things that the Rust compiler guarantees.  , so the compiler
makes no further effort to check that function for conformance.  This makes it
possible to write such a function in another language such as [C], if needed.

One of the design goals of Rust is for the compiler to be able, as much as
possible to '''infer the intent''' of the programmer.  The compiler also takes
a more active role than most compilers in making suggestions for fixing
problems that it finds.  This helps to smooth the learning curve.



** Memory Management:  Tcl Vs Rust **

Both Tcl and Rust provide automatic memory management without using a garbage
collector, and both are memory-safe.  Where Tcl uses reference counting, Rust
uses its '''borrow rules''' to constrain the use of references, and tracks all
references at compile time to ensure that a value may be cleaned up immediately
when the program exits from the block that owns it.  Tcl's internal reference
counting relies on the developer to properly increment and decrement
references, which is error-prone since proper reference counting can not be
checked for correctness.  Because Rust would automate the task of reference
counting, relieving the developers of Tcl from this burden, it may prove to be
an ideal implementation language for Tcl. 


** Object Orientation **

Rust does not provide traditional '''object-oriented programming''''
facilities.  Inheritance has proven to be a liability, so Rust doesnt have it.
Instead, the emphasis is on structures and interfaces that those structures
support.  Each structure has a type, and where inheritance might be used in
other languages, it is common instead to create a new type of structure and
implement customized traits for it.  Composition is achieved by having a field
in one structure be a reference to another structure.  This design allows for
'''zero-cost abstractions''', since all the details of function dispatch are
worked out at compile time.  Where needed, Rust also provides facilities that
provide for runtime dispatch.


** Rust Vs. Tcl **

Both languages are founded on the concept of immutable value, but Tcl values
are much more abstract, both semantically and technically.  In Tcl,
[copy-on-write] can be implemented underneath the surface of the language
whereas in Rust, a programmer details directly with such issues.  This level of
abstraction is the most fundamental difference between the languages.

As value-oriented languages, both languages have no concept of '''[null]''',
which is anything but a value.

Both languages are '''memory-safe''', assuming that the underlying
implementation/compiler is free of bugs.  Rust is '''type-safe''', both in the
sense of memory layout and data structure, and also in the sense of the
semantics of the data and the operations on it, as only the intended structure
is accepted as the argument of a function or the value of a variable.  while
Tcl is not type-safe, it is also not type-unsafe:  Each procedure may choose to
implement the necessary runtime type verification.  However, even then a
procedure has no way to determine which program component the declared the data
type, and therefore can never achieve the level of semantic type safety that
Rust does.

For a program written in Rust, the additional burden of annotating data types
and tracking data mutability and ownership is high compared to practice of
taking things at face value in Tcl, and all the type annotation, templating,
and boilerplate code makes a program written in Rust far more verbose, and
development far slower.  Tcl is far more concise than Rust, but far less
performant.  In addition to performance, another big win for Rust is that all
the additional type information a programmer provides in the source code allows
the compiler to verify some aspects of its correctness, and probably reduces
the size any accompanying test suite by at least the same amount of code and
effort.  If a program compiles successfully, it's already some indication that
the program works as expected.  An equivalent program in Tcl would have to
ensure the same level of correctness via a series of unit tests.

----
"When you use Rust, it is sometimes outright preposterous how much knowledge of language, and how much of programming ingenuity and curiosity you need in order to accomplish the most trivial things. When you feel particularly desperate, you go to rust/issues and search for a solution for your problem. Suddenly, you find an issue with an explanation that it is theoretically impossible to design your API in this way, owing to some subtle language bug. The issue is Open and dated Apr 5, 2017." [https://hirrolot.github.io/posts/rust-is-hard-or-the-misery-of-mainstream-programming.html]



<<categories>> programming language