Clarify safety of layout_for_ptr

[CAD97]

There is one functional change in the language update: we no longer document the behavior when given an external type, letting it fall into the "unknown" unsized tail kinds which are forbidden from use. This is in preparation for hopefully stabilizing this functionality in some form; the behavior of external types is still up in the air (recent temperature being to attempt adding a new default bound to current editions in order to forbid querying the layout external types entirely).

[rustbot]

r? @thomcc

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[CAD97]

• Tracking issue link: #69835

[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)

GITHUB_ENV=/home/runner/work/_temp/_runner_file_commands/set_env_1f71be7f-828c-4585-8350-aec79c2c1a64
GITHUB_EVENT_NAME=pull_request
GITHUB_EVENT_PATH=/home/runner/work/_temp/_github_workflow/event.json
GITHUB_GRAPHQL_URL=https://api.github.com/graphql
GITHUB_HEAD_REF=layout-for-ptr
GITHUB_JOB=pr
GITHUB_PATH=/home/runner/work/_temp/_runner_file_commands/add_path_1f71be7f-828c-4585-8350-aec79c2c1a64
GITHUB_REF=refs/pull/117991/merge
GITHUB_REF_NAME=117991/merge
GITHUB_REF_PROTECTED=false
---
   Compiling compiler_builtins v0.1.103
error: unresolved link to `ABI`
   --> library/core/src/mem/mod.rs:492:18
    |
492 | /// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to in
    |
    |
    = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`
    = note: `-D rustdoc::broken-intra-doc-links` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(rustdoc::broken_intra_doc_links)]`
error: unresolved link to `size_of`
   --> library/core/src/alloc/layout.rs:184:44
    |
    |
184 |     ///   equivalent to [`size_of::<T>()`][size_of]. The pointer is unused.
    |                                            ^^^^^^^ no item named `size_of` in scope
    |
    = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`
error: could not document `core`

Caused by:
Caused by:
  process didn't exit successfully: `/checkout/obj/build/bootstrap/debug/rustdoc --edition=2021 --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/x86_64-unknown-linux-gnu/doc -Zunstable-options --check-cfg 'cfg(feature, values("debug_refcell", "panic_immediate_abort"))' --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat -C metadata=a1aec6da431e1dc8 -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-std/x86_64-unknown-linux-gnu/doc/x86_64-unknown-linux-gnu/release/deps -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-std/x86_64-unknown-linux-gnu/doc/release/deps --cfg=bootstrap -Csymbol-mangling-version=legacy -Zunstable-options '--check-cfg=cfg(feature,values(any()))' -Zunstable-options '--check-cfg=cfg(bootstrap,values())' '--check-cfg=cfg(stdarch_intel_sde,values())' '--check-cfg=cfg(no_fp_fmt_parse,values())' '--check-cfg=cfg(no_global_oom_handling,values())' '--check-cfg=cfg(no_rc,values())' '--check-cfg=cfg(no_sync,values())' '--check-cfg=cfg(freebsd12,values())' '--check-cfg=cfg(freebsd13,values())' '--check-cfg=cfg(backtrace_in_libstd,values())' '--check-cfg=cfg(target_env,values("libnx"))' '--check-cfg=cfg(target_arch,values("asmjs","spirv","nvptx","xtensa"))' --document-private-items --document-hidden-items -Dwarnings '-Wrustdoc::invalid_codeblock_attributes' --crate-version '1.76.0-nightly (f4836219f 2023-11-16)' '-Zcrate-attr=doc(html_root_url="https://doc.rust-lang.org/nightly/")' '-Zcrate-attr=warn(rust_2018_idioms)' -Z unstable-options --resource-suffix 1.76.0 --markdown-css rust.css --markdown-no-toc --index-page /checkout/src/doc/index.md` (exit status: 1)
Build completed unsuccessfully in 0:00:09
  local time: Thu Nov 16 22:25:24 UTC 2023
  network time: Thu, 16 Nov 2023 22:25:24 GMT
##[error]Process completed with exit code 1.

[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)

GITHUB_ENV=/home/runner/work/_temp/_runner_file_commands/set_env_4de4bddb-0467-4e67-99c3-a1d66cbbea3b
GITHUB_EVENT_NAME=pull_request
GITHUB_EVENT_PATH=/home/runner/work/_temp/_github_workflow/event.json
GITHUB_GRAPHQL_URL=https://api.github.com/graphql
GITHUB_HEAD_REF=layout-for-ptr
GITHUB_JOB=pr
GITHUB_PATH=/home/runner/work/_temp/_runner_file_commands/add_path_4de4bddb-0467-4e67-99c3-a1d66cbbea3b
GITHUB_REF=refs/pull/117991/merge
GITHUB_REF_NAME=117991/merge
GITHUB_REF_PROTECTED=false
---
   Compiling compiler_builtins v0.1.103
error: unresolved link to `size_of`
   --> library/core/src/alloc/layout.rs:184:44
    |
184 |     ///   equivalent to [`size_of::<T>()`][size_of]. The pointer is unused.
    |                                            ^^^^^^^ no item named `size_of` in scope
    |
    = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`
    = note: `-D rustdoc::broken-intra-doc-links` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(rustdoc::broken_intra_doc_links)]`
error: could not document `core`

Caused by:
Caused by:
  process didn't exit successfully: `/checkout/obj/build/bootstrap/debug/rustdoc --edition=2021 --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/x86_64-unknown-linux-gnu/doc -Zunstable-options --check-cfg 'cfg(feature, values("debug_refcell", "panic_immediate_abort"))' --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat -C metadata=a1aec6da431e1dc8 -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-std/x86_64-unknown-linux-gnu/doc/x86_64-unknown-linux-gnu/release/deps -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-std/x86_64-unknown-linux-gnu/doc/release/deps --cfg=bootstrap -Csymbol-mangling-version=legacy -Zunstable-options '--check-cfg=cfg(feature,values(any()))' -Zunstable-options '--check-cfg=cfg(bootstrap,values())' '--check-cfg=cfg(stdarch_intel_sde,values())' '--check-cfg=cfg(no_fp_fmt_parse,values())' '--check-cfg=cfg(no_global_oom_handling,values())' '--check-cfg=cfg(no_rc,values())' '--check-cfg=cfg(no_sync,values())' '--check-cfg=cfg(freebsd12,values())' '--check-cfg=cfg(freebsd13,values())' '--check-cfg=cfg(backtrace_in_libstd,values())' '--check-cfg=cfg(target_env,values("libnx"))' '--check-cfg=cfg(target_arch,values("asmjs","spirv","nvptx","xtensa"))' --document-private-items --document-hidden-items -Dwarnings '-Wrustdoc::invalid_codeblock_attributes' --crate-version '1.76.0-nightly (5807d04be 2023-11-16)' '-Zcrate-attr=doc(html_root_url="https://doc.rust-lang.org/nightly/")' '-Zcrate-attr=warn(rust_2018_idioms)' -Z unstable-options --resource-suffix 1.76.0 --markdown-css rust.css --markdown-no-toc --index-page /checkout/src/doc/index.md` (exit status: 1)
Build completed unsuccessfully in 0:00:09
  local time: Thu Nov 16 22:44:13 UTC 2023
  network time: Thu, 16 Nov 2023 22:44:13 GMT
##[error]Process completed with exit code 1.

[thomcc]

I'm going to be away for a few months, so I'm rerolling my PRs so that folks don't have to wait for me. Sorry/thanks.

r? libs

[cuviper]

Please resolve the mingw-check failure.

@rustbot author

[RalfJung]

I'd love to see some progress here. :)

@CAD97 what is this blocked on? Figuring out who can approve it?

[CAD97]

Finding someone to approve it, but also me forgetting that there's a silly doc link error that needs to be fixed (which is why it's still labeled waiting on author). I'll address that and your comments today or tomorrow (ping me if I haven't in the next 48 hours, thanks).