RFC: Deprecate FromStr in favor of TryFrom<&str>

text/0000-deprecate-fromstr.md

@@ -0,0 +1,147 @@
+- Feature Name: deprecate_fromstr
+- Start Date: 2020-05-11
+- RFC PR: [#2924](https://github.com/rust-lang/rfcs/pull/2924)
+- Rust Issue: TBD
+
+# Summary
+[summary]: #summary
+
+Deprecate [`FromStr`](https://doc.rust-lang.org/std/str/trait.FromStr.html) in favor of [`TryFrom<&str>`](https://doc.rust-lang.org/std/convert/trait.TryFrom.html) and [`From<&str>`](https://doc.rust-lang.org/std/convert/trait.From.html).
+
+# Motivation
+[motivation]: #motivation
+
+[`FromStr`](https://doc.rust-lang.org/std/str/trait.FromStr.html) was created when [`TryFrom`](https://doc.rust-lang.org/std/convert/trait.TryFrom.html) didn't exist. Now that it is stable, [`FromStr`](https://doc.rust-lang.org/std/str/trait.FromStr.html) becomes superfluous. 
+
+[`FromStr`](https://doc.rust-lang.org/std/str/trait.FromStr.html) definition is virtually identical to [`TryFrom<&str>`](https://doc.rust-lang.org/std/convert/trait.TryFrom.html):
+```rust
+pub trait FromStr: Sized {
+    type Err;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err>;
+}
+
+// Where T is &str
+pub trait TryFrom<T>: Sized {
+    type Error;
+
+    fn try_from(value: T) -> Result<Self, Self::Error>;
+}
+```
+
+Infallible conversions become more idiomatic:
+```rust
+struct Dummy(String);
+
+impl std::str::FromStr for Dummy {
+    type Err = core::convert::Infallible;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        Ok(Dummy(s.to_owned()))
+    }
+}
+
+//vs
+
+impl From<&str> for Dummy {
+
+    fn from(s: &str) -> Self {
+        Dummy(s.to_owned())
+    }
+}
+```
+
+[`FromStr`](https://doc.rust-lang.org/std/str/trait.FromStr.html) limits lifetimes in a way that prevents borrowing the passed string:
+```rust
+struct Dummy<'a>(&'a str);
+
+// This doesn't compile
+impl<'a> std::str::FromStr for Dummy<'a> {
+    type Err = core::convert::Infallible;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        Ok(Dummy(s))
+    }
+}
+
+//This works
+impl<'a> From<&'a str> for Dummy<'a> {
+
+    fn from(s: &'a str) -> Self {
+        Dummy(s)
+    }
+}
+
+```
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+Basic implementation:
+```rust
+use std::convert::TryFrom;
+use std::num::ParseIntError;
+
+#[derive(Debug, PartialEq)]
+struct Point {
+    x: i32,
+    y: i32,
+}
+
+impl TryFrom<&str> for Point {
+    type Error = ParseIntError;
+
+    fn try_from(s: &str) -> Result<Self, Self::Error> {
+        let coords: Vec<&str> = s
+            .trim_matches(|p| p == '(' || p == ')')
+            .split(',')
+            .collect();
+
+        let x_fromstr = i32::try_from(coords[0])?;
+        let y_fromstr = i32::try_from(coords[1])?;
+
+        Ok(Point {
+            x: x_fromstr,
+            y: y_fromstr,
+        })
+    }
+}
+```
+
+Example:
+```rust
+let p = Point::try_from("(1,2)");
+assert_eq!(p.unwrap(), Point { x: 1, y: 2 })
+```
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+- Mark [`FromStr`](https://doc.rust-lang.org/std/str/trait.FromStr.html) as deprecated
+- Mark [`str::parse()`](https://doc.rust-lang.org/std/primitive.str.html#method.parse) as deprecated
+- Implement [`TryFrom<&str>`](https://doc.rust-lang.org/std/convert/trait.TryFrom.html) for all types implementing [`FromStr`](https://doc.rust-lang.org/std/str/trait.FromStr.html)
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+To be discussed
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+To be discussed
+
+# Prior art
+[prior-art]: #prior-art
+
+Some discussion about a tangential problem: https://github.com/rust-lang/rfcs/issues/2143
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+To be discussed
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+To be discussed