impl From<char> for String

[matthiaskrgr]

This allows us to write

fn char_to_string() -> String {
    'a'.into()
}

which was not possible before.

[rust-highfive]

r? @sfackler

(rust_highfive has picked a reviewer for you, use r? to override)

[matthiaskrgr]

I would like to add

fn char_to_string() -> String {
'a'.into()
}

as a test, what is the best place to do so?

[matthiaskrgr]

I guess this can go into src/liballoc/tests/string.rs

[tesuji]

Does doctest work?

[Mark-Simulacrum]

r? @dtolnay for FCP, code changes look good to me.

[dtolnay]

@rust-lang/libs:
@rfcbot fcp merge

impl From<char> for String

[rfcbot]

Team member @dtolnay has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @SimonSapin
• @dtolnay
• @sfackler
• @withoutboats

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[sfackler]

It appears that this is the first From<T> for String impl for types other than String, str, and references of those. What is the plan moving forward for these - should every type implementing Display also implement a From conversion?

[Mark-Simulacrum]

IMO, char makes sense as it has "string literal" like feeling, but I wouldn't go farther than that personally without good reason.

@matthiaskrgr can you say more about your use case for this?

[hellow554]

I can't find the post, but either on github or on urlo this proposal was denied, because it would allocate memory out of the sudden. And IMHO this is true. It is not obvious that a small into (or a ?, which performs into automatically) will allocate memory behind the scenes.
I'm somewhat against this‽

[matthiaskrgr]

can you say more about your use case for this?

When I investigated #73462 I found it strange that I could
"a".into() but not 'a'.into() for a fn conv_to_string() -> String

[withoutboats]

I can't find the post, but either on github or on urlo this proposal was denied, because it would allocate memory out of the sudden. And IMHO this is true. It is not obvious that a small into (or a ?, which performs into automatically) will allocate memory behind the scenes.

So will most of the other conversions into String, such as &str to String. There's no expectation that Into<String> does not allocate.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[dtolnay]

@bors r+

[bors]

📌 Commit 2cde493 has been approved by dtolnay

[pickfire]

Will there also be?

impl From<char> for &str

[dtolnay]

No; that impl would require the binary to contain a 4 MB table containing the utf8 encoding of every possible char in order for it to return a reference into.