Rust：理解特征“From”的实现

Question

How to understand trait FromWhatever in rust

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.

Answer 1

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).