Understanding trait `From` implementation

Question

The Rust book explains following:

The From trait allows for a type to define how to create itself from another type, hence providing a very simple mechanism for converting between several types.

Sounds simple enough. Lets try as simple example as possible:

use std::str::FromStr; struct MyStructure {} // auto accepted suggestion from language server. impl FromStr for MyStructure { type Err = (); // I've added this fn from_str(_s: &str) -> Result<Self, Self::Err> { Ok(Self {}) // I've added this } } fn main() { const INPUT: &str = "test"; let _tmp: MyStructure = MyStructure::from(INPUT); }

playground

Compiling test_range_2 v0.1.0 (/home/pavel/Repositories/test_range_2) error[E0308]: mismatched types --> src/main.rs:15:47 | 15 | let _tmp: MyStructure = MyStructure::from(INPUT); | ----------------- ^^^^^ expected struct `MyStructure`, found `&str` | | | arguments to this function are incorrect | note: associated function defined here --> /home/pavel/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/convert/mod.rs:374:8 | 374 | fn from(_: T) -> Self; | ^^^^ For more information about this error, try `rustc --explain E0308`. error: could not compile `test_range_2` due to previous error

So I expected this behavior:

let target: TARGET_TYPE = TARGET_TYPE::from::<SOURCE_TYPE>(input_var: SOURCE_TYPE);

Compared to the example in the rust book:

let num = Number::from(30);

It seems to me like a reasonable assumption.

However, reading the error message: "expected struct MyStructure, found &str". Does that mean that syntax is like this?

let target: TARGET_TYPE = TARGET_TYPE::from::<TARGET_TYPE>(input_var: TARGET_TYPE);

If, that's true, then code in the rust book should also fail with the error "expected Number, found i32", but it doesn't.

I expected my solution to work because I've implemented the trait FromStr, and I'm trying to create object from &str (see the "from" and "str"?). Which is also the type that was autocompleted by language server after I've typed impl FromStr for MyStructure. What am I missing? I want to impl FromStr for all my types but that compiler is not making it easy.

Solution

use std::str::FromStr; struct MyStructure {} impl FromStr for MyStructure { type Err = (); fn from_str(_s: &str) -> Result<Self, Self::Err> { Ok(Self {}) } } impl From<&str> for MyStructure { fn from(_: &str) -> Self { Self {} } } fn main() { const INPUT: &str = "test"; let _tmp0 = MyStructure::from_str(INPUT); let _tmp1 = MyStructure::from(INPUT); }

I implemented trait FromStr but was referring to trait From<&str>. Since there is a trait in standard lib that looks like this:

trait From<T> -> T

compiler tried to use that in stead.

That page refers specifically to core::convert::From. FromStr is a different trait with different methods and expectations. They are not linked in such a way that would allow you to do MyStructure::from(INPUT). — E_net4
– E_net4, Commented Dec 7, 2022 at 15:04

Masklinn · Accepted Answer · 2022-12-07 15:05:40Z

From and FromStr are completely different and entirely unrelated traits.

The first one performs 1 -> 1 infaillible conversions, while the latter is used to parse strings (it's a pretty old trait, its modern equivalent is TryFrom but because FromStr hooks into str::parse() it remains convenient and popular).

Here you impl FromStr, which does nothing for your From::from call.

Instead you get told about a blanket implementation: converting a type to itself is a no-op, so the standard library has

impl<T> From<T> for T

which just returns itself.

Hence the compiler's error message: when it encounters

MyStructure::from(INPUT);

and tries to resolve this to an implementation, the only impl From<?> for MyStructure it finds is the blanket implementation,

impl From<MyStructure> for MyStructure

and thus it tells you that it expects an argument of type MyStructure (the only possible input for the implementations it found), but found an &str (what you actually gave it).

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