Document how MaybeUninit<Struct> can be initialized.

[rodrimati1992]

No description provided.

[rust-highfive]

r? @dtolnay

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[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

configure: rust.channel         := nightly
configure: rust.debug-assertions := True
configure: llvm.assertions      := True
configure: dist.missing-tools   := True
configure: build.configure-args := ['--enable-sccache', '--disable-manage-submodu ...
configure: writing `config.toml` in current directory
configure: 
configure: run `python /checkout/x.py --help`
configure: 

[dtolnay]

Nice!

[dtolnay]

@bors r+

[bors]

📌 Commit 0974026 has been approved by dtolnay

[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

configure: rust.channel         := nightly
configure: rust.debug-assertions := True
configure: llvm.assertions      := True
configure: dist.missing-tools   := True
configure: build.configure-args := ['--enable-sccache', '--disable-manage-submodu ...
configure: writing `config.toml` in current directory
configure: 
configure: run `python /checkout/x.py --help`
configure: 
---
 Documenting core v0.0.0 (/checkout/library/core)
error: unresolved link to `std::ptr::addr_of_mut`
   --> library/core/src/mem/maybe_uninit.rs:175:44
    |
175 | /// You can use `MaybeUninit<T>`, and the [`std::ptr::addr_of_mut`] macro, to initialize structs field by field:
    |                                            ^^^^^^^^^^^^^^^^^^^^^^^ no item named `std` in scope
    |
    = note: `-D broken-intra-doc-links` implied by `-D warnings`
error: unresolved link to `std::ptr::addr_of_mut`
   --> library/core/src/mem/maybe_uninit.rs:175:44
    |
    |
175 | /// You can use `MaybeUninit<T>`, and the [`std::ptr::addr_of_mut`] macro, to initialize structs field by field:
    |                                            ^^^^^^^^^^^^^^^^^^^^^^^ no item named `std` in scope
error: aborting due to 2 previous errors

error: could not document `core`


Caused by:
  process didn't exit successfully: `/checkout/obj/build/bootstrap/debug/rustdoc --edition=2018 --crate-type lib --crate-name core library/core/src/lib.rs --target x86_64-unknown-linux-gnu -o /checkout/obj/build/x86_64-unknown-linux-gnu/stage0-std/x86_64-unknown-linux-gnu/doc --error-format=json --json=diagnostic-rendered-ansi --markdown-css rust.css --markdown-no-toc -Z unstable-options --resource-suffix 1.51.0 --index-page /checkout/src/doc/index.md -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-std/x86_64-unknown-linux-gnu/release/deps -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-std/release/deps --cfg=bootstrap -Dwarnings -Winvalid_codeblock_attributes --crate-version '1.51.0-nightly
  (c25232a55
  2021-01-31)'` (exit code: 1)


command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "rustdoc" "--target" "x86_64-unknown-linux-gnu" "-Zbinary-dep-depinfo" "-j" "16" "--release" "--locked" "--color" "always" "--features" "panic-unwind backtrace compiler-builtins-c" "--manifest-path" "/checkout/library/test/Cargo.toml" "-p" "core" "--" "--markdown-css" "rust.css" "--markdown-no-toc" "-Z" "unstable-options" "--resource-suffix" "1.51.0" "--index-page" "/checkout/src/doc/index.md"


failed to run: /checkout/obj/build/bootstrap/debug/bootstrap doc --stage 0 library/std
Build completed unsuccessfully in 0:00:09

[dtolnay]

^

@bors r-

[lukaslueg]

Language nitpick: would be leaked --> leaks

[the8472]

Maybe the leak can be avoided by reordering the code a bit?

let name = ...
let list = ...

unsafe {
   addr_of_mut!((*ptr).name).write(name);
   addr_of_mut!((*ptr).list).write(list);
}

[rodrimati1992]

@the8472 In this case yes, but I wanted this to translate to how one would initialize a large struct field by field.
If the struct had been a heap allocated struct with a large array field instead of a Vec, it wouldn't be possible to construct the array on the stack before moving it to the field in debug builds, since it would overflow the stack.

[the8472]

Well, afaik currently there is no guarantee that passing a large array directly to function arguments gets optimized any better than first assigning it to a named variable. You're at the mercy of the optimizer in either case since there's no placement API. rust-lang/rfcs#2884 is still under discussion.

I think to initialize a large array without blowing the stack in debug mode you would have to loop over each item.

[dtolnay]

@bors r+

[bors]

📌 Commit 21c2343 has been approved by dtolnay