GlobalAlloc docs example crashes

[ijackson]

I cut and pasted the example for GlobalAlloc from the stdlib docs into the playground. Contrary to the implications in the documentation, the program does not run to completion. Instead, it prints

memory allocation of 4 bytes failed
timeout: the monitored command dumped core
/playground/tools/entrypoint.sh: line 11:     8 Aborted                 timeout --signal=KILL ${timeout} "$@"

I think this is fine behaviour. The example is wrong and should be fixed somehow.

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=54f5b4b24c19b64962d35ec4031ee4a5

I was doing this because I am trying to write a test case for the stdlib which involves a stunt allocator. So far my #[global_alloc] annotation in a file in library/std/tests/ does not seem to be effective. I suspect that whatever is getting in my way might also get in the way of adding a test case specifically for #[global_alloc]. Looks like src/test/ui/ is the right place and I should read the dev docs...

The example from std::alloc::System works correctly.

[ijackson]

@rustbot modify labels +T-libs

[ijackson]

@rustbot claim

[vitalyd]

My $.02 is that #81864 is a bit too involved for an example.

The existing example can be fixed by delegating to the System allocator up until main() runs, at which point the sample allocator can take effect. Something like this.