Clarify docs on implementing Into.

[clarfonthey]

This was suggested by @dtolnay in #40380.

This explicitly clarifies in what circumstances you should implement Into instead of From.

[rust-highfive]

r? @alexcrichton

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

[dtolnay]

This works when I tried it. https://play.rust-lang.org/?gist=29711851d1acb274cd4ba9fd3e05d781

structWrapper(String);implFrom<Wrapper>forString{fnfrom(w:Wrapper) -> String{
w.0}}fnmain(){}

[Mark-Simulacrum]

Travis failed:

[01:13:49] failures:
[01:13:49]
[01:13:49] ---- convert.rs - convert::Into (line 178) stdout ----
[01:13:49] thread 'rustc' panicked at 'test compiled while it wasn't supposed to', /checkout/src/librustdoc/test.rs:263
[01:13:49] note: Run with `RUST_BACKTRACE=1` for a backtrace.
[01:13:49]
[01:13:49] ---- convert.rs - convert::Into (line 194) stdout ----
[01:13:49] error[E0425]: cannot find value `w` in this scope
[01:13:49] --> <anon>:7:9
[01:13:49] |
[01:13:49] 7 | w.0
[01:13:49] | ^ not found in this scope
[01:13:49]
[01:13:49] error[E0053]: method `into` has an incompatible type for trait
[01:13:49] --> <anon>:6:5
[01:13:49] |
[01:13:49] 6 | / fn into(self) -> Wrapper {
[01:13:49] 7 | | w.0
[01:13:49] 8 | | }
[01:13:49] | |_____^ expected struct `std::string::String`, found struct `main::Wrapper`
[01:13:49] |
[01:13:49] = note: expected type `fn(main::Wrapper) -> std::string::String`
[01:13:49] found type `fn(main::Wrapper) -> main::Wrapper`
[01:13:49]
[01:13:49] error: aborting due to previous error(s)
[01:13:49]
[01:13:49] thread 'rustc' panicked at 'Box<Any>', /checkout/src/librustc/session/mod.rs:214
[01:13:49] failures:
[01:13:49] convert.rs - convert::Into (line 178)
[01:13:49] convert.rs - convert::Into (line 194)
[01:13:49]
[01:13:49] test result: FAILED. 1200 passed; 2 failed; 12 ignored; 0 measured; 0 filtered out

[steveklabnik]

no need for the {}s

[clarfonthey]

I fixed this

[dtolnay]

The compile_fail code block actually compiles fine.

[shepmaster]

@clarcharr a gentle reminder that this PR is awaiting your deft hand to fix the failures and respond to review feedback.

[clarfonthey]

@shepmaster my deft hand has responded.

[Mark-Simulacrum]

Looks like this still needs a few more updates to get Travis passing.

[01:04:37] failures:
[01:04:37] [01:04:37] ---- convert.rs - convert::Into (line 191) stdout ----
[01:04:37] error[E0053]: method `into` has an incompatible type for trait
[01:04:37] --> <anon>:6:5
[01:04:37] |
[01:04:37] 6 | / fn into(self) -> Wrapper<T> {
[01:04:37] 7 | | self.0
[01:04:37] 8 | | }
[01:04:37] | |_____^ expected struct `std::vec::Vec`, found struct `main::Wrapper`
[01:04:37] |
[01:04:37] = note: expected type `fn(main::Wrapper<T>) -> std::vec::Vec<T>`
[01:04:37] found type `fn(main::Wrapper<T>) -> main::Wrapper<T>`
[01:04:37] [01:04:37] error: aborting due to previous error(s)
[01:04:37] [01:04:37] thread 'rustc' panicked at 'Box<Any>', /checkout/src/librustc/session/mod.rs:214
[01:04:37] note: Run with `RUST_BACKTRACE=1` for a backtrace.
[01:04:37] [01:04:37] [01:04:37] failures:
[01:04:37] convert.rs - convert::Into (line 191)
[01:04:37] [01:04:37] test result: FAILED. 1203 passed; 1 failed; 13 ignored; 0 measured; 0 filtered out

[clarfonthey]

My deft hand failed me once again :(

(fixed the error)

[Mark-Simulacrum]

No need for the . here.

[clarfonthey]

Fixed

[clarfonthey]

Rebased and made the changes.

[Mark-Simulacrum]

r? @steveklabnik

[steveklabnik]

@bors: r+ rollup

thanks!

[bors]

📌 Commit 54bbe23 has been approved by steveklabnik