memory leak in DiagnosticBuilder.into_diagnostic()

@nnethercote

This code leaks the Box at self.0:

rust/src/librustc_errors/diagnostic_builder.rs

Lines 141 to 144 in d3c7934

    
           unsafe { 
        
               diagnostic = std::ptr::read(&self.0.diagnostic); 
        
               std::mem::forget(self); 
        
           };

The leak was introduced in 2fcd870. cc @nnethercote

We could avoid the leak and the unsafe by doing

let diagnostic = self.0.diagnostic.clone();
self.cancel();

or if we're concerned about performance here, we could maybe do something with drop_in_place().

Thanks for the report, I will measure the perf effects of the different alternatives.

I'd like to know how you found this -- code inspection? Some kind of tool?

I found it while running fuzz-rustc and noticing that I kept running out of memory. I turned on -Z sanitizer=address and this popped out.

Another alternative could be to call std::mem::replace() to swap in a dummy diagnostic, instead of cloning the existing one.

Below is an example compiler input that triggers the leak:

str// bass (FIXMoo {
}


         )

Simpler example that triggers the leak: just a single closing brace

Fix a leak in `DiagnosticBuilder::into_diagnostic`.

Auto merge of #69628 - nnethercote:fix-DiagnosticBuilder-into_diagnos…

Rollup merge of rust-lang#69628 - nnethercote:fix-DiagnosticBuilder-i…

Fix a leak in `DiagnosticBuilder::into_diagnostic`.

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memory leak in DiagnosticBuilder.into_diagnostic() #69600

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	unsafe {
	diagnostic = std::ptr::read(&self.0.diagnostic);
	std::mem::forget(self);
	};

memory leak in DiagnosticBuilder.into_diagnostic() #69600

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dwrensha commented on Mar 1, 2020

nnethercote commented on Mar 2, 2020

dwrensha commented on Mar 2, 2020

dwrensha commented on Mar 2, 2020

dwrensha commented on Mar 2, 2020

dwrensha commented on Mar 2, 2020

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