feat: Feature-flagged logging

[Pscheidl]

Disabling logging in production via release_max_level_off disables logging for the whole resulting lib/binary, not just the serde-env crate. A workaround would be an opt-in feature, disabled by default.

[Pscheidl]

Created a tiny project to prove this log flag actually does behave like this. https://github.com/Pscheidl/logtest

Libraries should avoid using the max level features because they’re global and can’t be changed once they’re set.

[Xuanwo]

Oh, that's a valid point. However, adding a feature flag for every log call can be quite tedious. Would it be possible to create an internal module called logging and provide different logging backends for it? For instance, we could use a dummy logging in the release build and use log in the debug build.

[Pscheidl]

Oh, that's a valid point. However, adding a feature flag for every log call can be quite tedious. Would it be possible to create an internal module called logging and provide different logging backends for it? For instance, we could use a dummy logging in the release build and use log in the debug build.

It can be done (updated this MR), but it creates another problem. Unused variables on release. I don't know how to create a workaround for this, without duplicating macros from the log crate.

pavel@machine:~/Work/serde-env$ cargo build --release
   Compiling serde-env v0.1.1 (/home/pavel/Work/serde-env)
warning: unused variable: `name`
   --> src/de.rs:264:9
    |
264 |         name: &'static str,
    |         ^^^^ help: if this is intentional, prefix it with an underscore: `_name`
    |
    = note: `#[warn(unused_variables)]` on by default

warning: unused variable: `name`
   --> src/de.rs:292:9
    |
292 |         name: &'static str,
    |         ^^^^ help: if this is intentional, prefix it with an underscore: `_name`

warning: `serde-env` (lib) generated 2 warnings
    Finished release [optimized] target(s) in 0.35s

[Xuanwo]

Let's merge first, we can polish this part later.