Make Watcher object safe

[erickt]

This changes the Watcher trait to be object safe, which allows users to
dynamically select which backend to use. This can be helpful on systems that
support multiple file watching systems that have different tradeoffs. For
example, Chromium's file watcher on OS X will use an fsevent backend
when watching recursive directories, and a kqueues backend when not.

In order to implement this, this makes a few changes to Watcher:

• removes new_immediate constructor.
• removes the <P: AsRef<Path>> from the watch and unwatch.

This then replaces (and renames) calls with notify::recommended_watcher()
to get similar behavior as the old constructor.

[0xpr03]

I think<P: AsRef<Path>> was nice to leave freedom in how you want to specify your path. Any reason to remove that ? I guess we have to remove that to allow dynamic switching via non-generic interfaces.

[erickt]

I think<P: AsRef> was nice to leave freedom in how you want to specify your path. Any reason to remove that ? I guess we have to remove that to allow dynamic switching via non-generic interfaces.

Correct, object safe traits can't have generic parameters. It's unfortunate to have to manually pass in Path::new(), but I'm guessing most callsites only specify one or two paths by hand, so it might not be that painful in practice.

[0xpr03]

Note that you currently seem to have syntax errors according to CI.

[erickt]

Note that you currently seem to have syntax errors according to CI.

Should be fixed now!

[0xpr03]

Thanks, this will be a good step towards a possible dynamic runtime change, which was in the never released -next branch

[0xpr03]

Note: we may want to document and test for this, so it isn't reverted later on with a trait under the impression of usability improvements

[erickt]

Thanks!

Note: we may want to document and test for this, so it isn't reverted later on with a trait under the impression of usability improvements

I did add a simple test for this in src/lib.rs, where I assert that the Watcher trait is object safe. So we can't accidentally remove this feature.

[0xpr03]

Ah yeah, had to test but that's already enough to verify it. Then I'll only have to add some docs later on for a more in-depth changelog