Fix clippy:derivable_impls and other lints

[Enselic]

Using https://github.com/Enselic/cargo-public-items we can easily see the impact on the public API of this change, and it is as follows:

% git checkout master
% cargo public-items > /tmp/master
% git checkout fix-some-lints
% cargo public-items > /tmp/pr
% diff -U0 /tmp/master /tmp/pr
--- /tmp/before	2022-02-28 21:10:55.000000000 +0100
+++ /tmp/after	2022-02-28 21:10:44.000000000 +0100
@@ -15 +15 @@
-pub fn bugreport::collector::CommandLine::default() -> Self
+pub fn bugreport::collector::CommandLine::default() -> CommandLine
@@ -21 +21 @@
-pub fn bugreport::collector::CompileTimeInformation::default() -> Self
+pub fn bugreport::collector::CompileTimeInformation::default() -> CompileTimeInformation
@@ -30 +30 @@
-pub fn bugreport::collector::OperatingSystem::default() -> Self
+pub fn bugreport::collector::OperatingSystem::default() -> OperatingSystem
@@ -34 +34 @@
-pub fn bugreport::collector::SoftwareVersion::default() -> Self
+pub fn bugreport::collector::SoftwareVersion::default() -> SoftwareVersion

and as we can see the only difference is that the return type has changed from Self to explicit types, which is equivalent to the current public API.

[sharkdp]

There is one (hypothetical?) drawback of #[derive(Default)]: If a future developer adds new fields to one of these structs, they are not necessarily forced to think about whether or not the "canonical" default value is really what they want as the default value for this particular application. Having a custom impl Default forces developers to think about it because it requires a change.

But this is pretty weak argument compared to the benefit of not having to write this boilerplate code. And not the actual reason why I wrote them in the first place (I guess I just wasn't used to #[derive(Default)] yet) 😄

[Enselic]

Good point about devs not being forced to think about defaults if they add fields. I didn't think of that. But I agree that getting rid of boilerplate trumps that :)