Add option to continue if an Error occurs in one section

[puzzlewolf]

Closes #124

What has been done:

• Add a configuration option skip_if_error to the build, git, rustc and sysinfo features, to allow skipping a feature and continuing if an Error occurs. configure_cargo is not fallible, so the option wouldn't make sense there.
• Emit a cargo:warning if an Error was skipped in this way.
• Add a test for skip_if_error for the git feature. This adds a new dependency tempfile, which is just needed for this one test of the git feature. Sadly, there is no way to define optional dev-dependencies (Allow optional dev-dependencies rust-lang/cargo#1596), so I deactivated the unused_crate_dependencies lint.
• Add documentation for the option and mention option_env!.
• Fix two unrelated nits.

Open questions:

• I'm not sure if skip_if_error is a good name, please suggest a better one :)
• It would be nice to add tests for the other features, too. Do you have an idea how to make them Error?
• Is disabling unused_crate_dependencies okay for you?
• If you prefer, I'll open a separate PR for the unrelated nits.
• Please tell me if you'd like the commits squashed :)

[puzzlewolf]

I don't quite understand why this add_entries doesn't need to be mut like all the others. For example, if I remove the mut from add_entries in configure_build, the compiler says

cargo check --features build
    Checking vergen v7.3.2 (/home/nora/Projects/vergen)                                        i
error[E0596]: cannot borrow `add_entries` as mutable, as it is not declared as mutable
   --> src/feature/build.rs:150:26
    |
124 |     let add_entries = || {
    |         ----------- help: consider changing this to be mutable: `mut add_entries`
...
128 |                     add_config_entries(config, *build_config, &OffsetDateTime::now_utc())?;
    |                                        ------ calling `add_entries` requires mutable binding due to mutable borrow of `config`
...
150 |             let result = add_entries();
    |                          ^^^^^^^^^^^ cannot borrow as mutable
(... analogous reports snipped ...)

but config is borrowed here in the same way as there 🤔? Any advice appreciated 😄

Anyway, this compiles and seems to do the right thing 🤷

[puzzlewolf]

Hey, could you please approve the CI run? I'm a first-time contributor to this project, so GitHub prevents me from running CI (and rightly so) :)

[CraZySacX]

Sorry was on vacation last week. It's running now.

[puzzlewolf]

Thank you :) No problem, I hope the vacation was nice 🍹

I messed up formatting, sorry 😓

[CraZySacX]

That's why the check is there :). I miss formatting all the time.

[codecov]

Codecov Report

Merging #128 (7dc4d81) into master (160336f) will decrease coverage by 0.70%.
The diff coverage is 74.82%.

@@            Coverage Diff             @@
##           master     #128      +/-   ##
==========================================
- Coverage   94.57%   93.86%   -0.71%     
==========================================
  Files          11       11              
  Lines        1105     1189      +84     
==========================================
+ Hits         1045     1116      +71     
- Misses         60       73      +13     

Impacted Files	Coverage Δ
src/lib.rs	63.15% <ø> (+5.26%)	⬆️
src/feature/build.rs	92.85% <54.54%> (-7.15%)	⬇️
src/feature/rustc.rs	93.68% <54.54%> (-5.07%)	⬇️
src/feature/si.rs	97.48% <54.54%> (-2.52%)	⬇️
src/feature/git.rs	91.72% <75.90%> (+3.49%)	⬆️
src/config.rs	89.60% <100.00%> (-2.65%)	⬇️
src/gen.rs	99.62% <100.00%> (+0.02%)	⬆️
src/error.rs	69.56% <0.00%> (-4.86%)	⬇️
... and 3 more

📣 Codecov can now indicate which changes are the most critical in Pull Requests. Learn more

[puzzlewolf]

Thanks for merging :) 🎉