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Pretty-printing support for Rust generators #62572

tmandry opened this issue Jul 10, 2019 · 1 comment

Pretty-printing support for Rust generators #62572

tmandry opened this issue Jul 10, 2019 · 1 comment


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@tmandry tmandry commented Jul 10, 2019

Support printing Rust generators in a way that looks more like enums. Generators (as of #59897) are represented in Rust as variant layouts with some extra fields on the outer layout. These extra fields (which are upvars, captured from the environment of the generator closure) seem to cause printing to become less pretty.

Today printing a Rust generator from rust-gdb looks like this:

$2 = generator_objects::main::generator {__0: 0x7fffffffd204, <<variant>>: {__state: 3, 0: generator_objects::main::generator::Unresumed, 1: generator_objects::main::generator::Returned, 2: generator_objects::main::generator::Panicked, 3: generator_objects::main::generator::Suspend0 {c: 6, d: 7}, 4: generator_objects::main::generator::Suspend1 {c: 6, d: 7}}}

Note that we print the fields of every single variant, even though the discriminant is marked in the DWARF output.

Here's the test which produced the above output. The above excerpt is from the second print in the test. __0 in the output is the upvar for a from the environment (it should be called a instead of __0, but this is a problem in the compiler).

I also opened this bug on GDB; I'm not sure how much we upstream pretty printing support today.

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@nikomatsakis nikomatsakis commented Aug 13, 2019

Check-in from async-await wg: marking as "deferred", since this is not a blocker for stabilization.

Dylan-DPC added a commit to Dylan-DPC/rust that referenced this issue May 2, 2021
Fix debuginfo for generators

First, all fields except the discriminant (including `outer_fields`) should be put into structures inside the variant part, which gives an equivalent layout but offers us much better integration with debuggers.

Second, artificial flags in generator variants should be removed.
 - Literally, variants are not artificial. We have `yield` statements, upvars and inner variables in the source code.
 - Functionally, we don't want debuggers to suppress the variants. It contains the state of the generator, which is useful when debugging. So they shouldn't be marked artificial.
 - Debuggers may use artificial flags to find the active variant. In this case, marking variants artificial will make debuggers not work properly.

Fixes rust-lang#62572.
Fixes rust-lang#79009.

And refer
@bors bors closed this in 2553053 May 2, 2021
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4 participants