Assits: enable log statement

[matklad]

I use printf debugging a lot, and the problem with printfs is that you have to recompile the code, which is slow.

An alternative is to use loging, which can be configured at run time and does not need recompiling.

However, I find logs not very useful for debugging, as enabling logging is cumbersome, and you usually get way to much output.

Ideally, I'd love to be able to enable not only the whole log level, but specific log statements. I believe an IDE should help here:

• for each log statement, there's an assist "enable log statement"
• IDE maintains a persitent list of enabled log statemtnts
• when running tasks, IDE set's RUST_LOG env variable to only hit the specified statements.

[kinnison]

This sounds exceedingly useful. I'd add the following to the spec:

• The assist ought to be able to turn on entire modules' logging
• There must be a way to set the environment variable name since many programs use differently named environment variables.
• Ideally there could be some integration with tracing since that seems to be gaining popularity.

[matklad]

cc rust-cli/env_logger#144

[matklad]

cc https://github.com/hawkw/line-filter

[hawkw]

cc https://github.com/hawkw/line-filter

FWIW, I went ahead and published it: https://crates.io/crates/tracing-line-filter

I also added support for enabling events/spans by file path, which is what I imagine would be easier for a rust-analyzer assist to do.

The one potential concern is that in order for an assist to work, users would have to actually be using this filter, and configuring it from whatever env var rust-analyzer sets. If we can introduce a convention for this and enough people actually follow it, that's not a huge issue. But — and I don't know how practical this is — it could be good if rust-analyzer could somehow detect whether an appropriate tracing configuration is in use. If we could do that, we could only display the option to enable an event if an appropriate tracing config is in use, or make enabling an event when there isn't an appropriate filter display a message noting that this will do nothing unless a LineFilter is added.

[hawkw]

• Ideally there could be some integration with tracing since that seems to be gaining popularity.

Configuring the RUST_LOG env variable or a user-provided env var would be a fine integration point for tracing, since tracing provides an env_logger-like filter for controlling what traces are enabled. If users use this filter, a RUST_LOG-based system would work out of the box for them — EnvFilter's configuration syntax is a superset of env_logger's.