Add excess capacity description for Vec::leak

[chansuke]

Fixes #79240

[rust-highfive]

r? @shepmaster

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[m-ou-se]

This is probably not the behaviour we want to commit to, see the discussion here: rust-lang/rfcs#2969 (comment)

[chansuke]

trying to fix free(): double free detected in tcache 2

[bors]

☔ The latest upstream changes (presumably #80530) made this pull request unmergeable. Please resolve the merge conflicts.

[shepmaster]

r? @m-ou-se

[rust-log-analyzer]

The job x86_64-gnu-llvm-9 failed! Check out the build log: (web) (plain)

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

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---
Suite("src/test/assembly") not skipped for "bootstrap::test::Assembly" -- not in ["src/tools/tidy"]
Check compiletest suite=assembly mode=assembly (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)

running 27 tests
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 finished in 0.067 seconds
Suite("src/test/incremental") not skipped for "bootstrap::test::Incremental" -- not in ["src/tools/tidy"]
Check compiletest suite=incremental mode=incremental (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
---
Suite("src/test/debuginfo") not skipped for "bootstrap::test::Debuginfo" -- not in ["src/tools/tidy"]
Check compiletest suite=debuginfo mode=debuginfo (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)

running 116 tests
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test result: ok. 78 passed; 0 failed; 38 ignored; 0 measured; 0 filtered out; finished in 2.28s

 finished in 2.349 seconds
Suite("src/test/ui-fulldeps") not skipped for "bootstrap::test::UiFullDeps" -- not in ["src/tools/tidy"]
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failures:

---- src/vec/mod.rs - vec::Vec<T, A>::leak (line 1757) stdout ----
Test executable failed (terminated by signal).
stderr:
stderr:
free(): double free detected in tcache 2


failures:
    src/vec/mod.rs - vec::Vec<T, A>::leak (line 1757)
    src/vec/mod.rs - vec::Vec<T, A>::leak (line 1757)

test result: FAILED. 539 passed; 1 failed; 1 ignored; 0 measured; 0 filtered out; finished in 11.83s

error: test failed, to rerun pass '--doc'


command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "test" "--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" "alloc" "--" "--quiet"


failed to run: /checkout/obj/build/bootstrap/debug/bootstrap --stage 2 test --exclude src/tools/tidy
Build completed unsuccessfully in 0:20:45

[JohnCSimon]

Ping from triage - @chansuke
can you please resolve the build failure? thank you.

@rustbot label: -S-waiting-on-review +S-waiting-on-author

[chansuke]

@JohnCSimon I apologize for the late response. I will take a look at it this weekend.

[chansuke]

@rustbot label -S-waiting-on-author +S-waiting-on-review

[m-ou-se]

This example doesn't demonstrate any leaking. .into_boxed_slice() returns a Box, which the slice variable will own.

The issue this PR was originally trying to fix is that the documentation doesn't promise this function drops excess capacity. However, I don't think that's something we want to fix. Instead, I think it'd be better if leak explicitly does not drop the excess capacity. But for that, there's an RFC which has been standing still for a while: rust-lang/rfcs#2969

[chansuke]

@m-ou-se Thank you for your support and help. I've been stucking and would like to close my PR.