Add RFC for std::inputln()

text/0000-inputln.md

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+- Feature Name: `inputln`
+- Start Date: 2021-11-16
+- RFC PR: [rust-lang/rfcs#0000](https://github.com/rust-lang/rfcs/pull/0000)
+- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
+
+# Summary
+[summary]: #summary
+
+Add an `inputln` function to `std` to read a line from standard input and return
+a `std::io::Result<String>`.
+
+# Motivation
+[motivation]: #motivation
+
+Building a small interactive program that reads input from standard input and
+writes output to standard output is well-established as a simple and fun way of
+learning and teaching a new programming language.  Case in point the chapter 2
+of the official Rust book is [Programming a Guessing Game], which suggests the
+following code:
+
+[Programming a Guessing Game]: https://doc.rust-lang.org/book/ch02-00-guessing-game-tutorial.html
+
+```rs
+let mut guess = String::new();
+
+io::stdin()
+    .read_line(&mut guess)
+    .expect("Failed to read line");
+```
+
+While the above code is perfectly clear to everybody who already knows Rust, it
+can be quite overwhelming for a beginner. What is `mut`? What is `&mut`?  The
+2nd chapter gives only basic explanations and assures that mutability and
+borrowing will be explained in detail in later chapters. Don't worry about that
+for now, everything will make sense later.  But the beginner might worry about
+something else: Why is something so simple so complicated with Rust? For example
+in Python you can just do `guess = input()`.  Is Rust always this cumbersome?
+Maybe they should rather stick with their current favorite programming language
+instead.
+
+This RFC therefore proposes the introduction of a `std::inputln` function so
+that the above example could be simplified to just:
+
+```rs
+let guess = inputln().expect("Failed to read line");
+```
+
+This would allow for a more graceful introduction to Rust. Letting beginners
+experience the exciting thrill of running their own first interactive Rust
+program, without being confronted with mutability and borrowing straight away.
+While mutability and borrowing are very powerful concepts, Rust does not force
+you to use them when you don't need them. The examples we use to teach Rust to
+complete beginners should reflect that.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+`std::inputln()` is a convenience wrapper around `std::io::Stdin::read_line`,
+introduced so that Rust beginners can create interactive programs without having
+to worry about mutability or borrowing.  The function allocates a new String
+buffer for you, and reads a line from standard input.  The result is returned as
+a `std::io::Result<String>`.
+
+If you are repeatedly reading lines from standard input and don't need to
+allocate a new String for each of them you should be using
+`std::io::Stdin::read_line` directly instead, so that you can reuse an existing
+buffer.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+```rs
+pub fn inputln() -> std::io::Result<String> {
+    let mut input = String::new();
+    std::io::stdin().read_line(&mut input)?;
+    Ok(input)
+}
+```
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+* Can lead to unnecessary buffer allocations in Rust programs when developers
+  don't realize that they could reuse a buffer instead. This could potentially
+  be remedied by a new Clippy lint.
+
+* There is no precedent for a function residing directly in the `std` module
+  (currently it only contains macros). So Rust programmers might out of habit
+  try to call `inputln!()`. This should however not pose a big hurdle because
+  `rustc` already provides a helpful error message:
+
+  ```
+    error: cannot find macro `inputln` in this scope
+    --> src/main.rs:13:5
+     |
+  13 |     inputln!();
+     |     ^^^^^^^
+     |
+     = note: `inputln` is in scope, but it is a function, not a macro
+   ```
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+> Why is this design the best in the space of possible designs?
+
+It is the simplest solution to the explained problem.
+
+> What other designs have been considered and what is the rationale for not
+> choosing them?
+
+The function could also be implemented as a macro but there is really no need for that.
+
+> What is the impact of not doing this?
+
+A higher chance of Rust beginners getting overwhelmed by mutability and borrowing.
+
+# Prior art
+[prior-art]: #prior-art
+
+Python has [input()], Ruby has [gets], C# has `Console.ReadLine()`
+... all of these return a string read from standard input.
+
+[input()]: https://docs.python.org/3/library/functions.html#input
+[gets]: https://ruby-doc.org/docs/ruby-doc-bundle/Tutorial/part_02/user_input.html
+
+Other standard libraries additionally:
+
+* accept a prompt to display to the user before reading from standard input
+  (e.g. Python and Node.js)
+
+* provide some functions to parse multiple values of specific data types
+  into ovariables (e.g. C's `scanf`, C++, Java's `Scanner`)
+
+Python's `input()` function accepts a `prompt` argument because Python's output
+is line buffered by default, meaning a `print()` without a newline would only be
+output after a manual flush. Node.js accepts a prompt because its
+[readline](https://nodejs.org/api/readline.html) interface is very high level.
+Both reasonings don't apply to Rust. With Rust a simple `print!()` call before
+invoking `inputln()` suffices to display an input prompt and more high-level
+interfaces are better provided by crates.
+
+While scanning utilities could also be added to the Rust standard library, how
+these should be designed is less clear, as well as whether or not they should be
+in the standard library in the first place. There exist many well established
+input parsing libraries for Rust that are only a `cargo install` away. The same
+argument does not apply to `inputln()` ... beginners should be able to get
+started with an interactive Rust program without having to worry about
+mutability, borrowing or having to install a third-party library.
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+> What parts of the design do you expect to resolve through the RFC process
+> before this gets merged?
+
+The name of the function is up to debate. `read_line()` would also be a
+reasonable choice, that does however potentially beg the question: Read from
+where? `inputln()` hints that the line comes from standard input.
+
+The location of the function is also up to debate. It could also reside in
+`std::io`, which would however come with the drawback that beginners need to
+either import it or prefix `std::io`, both of which seem like unnecessary
+hurdles.
+
+> What related issues do you consider out of scope for this RFC that could be
+> addressed in the future independently of the solution that comes out of this RFC?
+
+I consider the question whether or not scanning utilities should be added to the
+standard library to be out of the scope of this RFC.
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+Once this RFC is implemented the Chapter 2 of the Rust book could be simplified
+to introduce mutability and borrowing in a more gentle manner. Clippy could gain
+a lint to tell users to avoid unnecessary allocations due to repeated
+`inputln()` calls and suggest `std::io::Stdin::read_line` instead.
+
+With this addition Rust might lend itself more towards being the first
+programming language for students.