Make various items `#[non_exhaustive]`

[argv-minus-one]

Makes various items #[non_exhaustive]. Fixes #32.

Compatibility

⚠ This is a breaking change. In particular, code that constructs a FileLoggerConfig, TerminalLoggerConfig, or KVFilterParameters will need to be changed to use the new method on these types.

⚠ This raises the minimum supported Rust version of this crate to 1.40, which is fairly recent (December 2019).

Changes Made

Items Made Non-Exhaustive

• LoggerBuilder, LoggerConfig

  This makes it non-breaking to add more logging destinations in the future.

• ErrorKind

  This makes it non-breaking to add more specific kinds of errors in the future.

• file::FileLoggerConfig, terminal::TerminalLoggerConfig

  This makes it non-breaking to add more configuration settings to these loggers (but see “Non-Exhaustive structs” below).

• types::KVFilterParameters

  Made non-exhaustive because slog_kvfilter::KVFilter is effectively non-exhaustive (its fields are all private).

• types::Format, types::SourceLocation

  These could conceivably have more variants added at some point.

• types::OverflowStrategy

  Made non-exhaustive because slog_async::OverflowStrategy is non-exhaustive.

Items Not Made Non-Exhaustive

• null::NullLoggerBuilder, null::NullLoggerConfig

  A logger that does nothing will probably never need configuration.

• types::Severity

  Not non-exhaustive because slog::Level is not non-exhaustive.

• types::TimeZone

  I'm not sure whether you intend to support using specific time zones (besides UTC and the local time zone) in the future, so I've left this one alone.

Non-Exhaustive structs

Note that marking a struct as non-exhaustive changes how to construct it. Instead of using the traditional Struct { .. } syntax, a constructor method must be used.

// This does not work.
let p = KVFilterParameters {
    severity: Severity::Trace,
    .. Default::default()
};

// This works.
let mut p = KVFilterParameters::new();
p.severity = Severity::Trace;

I've marked three structs non-exhaustive: FileLoggerConfig, TerminalLoggerConfig, and KVFilterParameters. I've added a new method to each of them.

Rustdoc generates a short explanation of non-exhaustive structs. I've added a longer explanation to the documentation for KVFilterParameters, since that one seems meant to be constructed by hand, whereas the other two seem meant to be constructed by serde.

[codecov-commenter]

Codecov Report

Merging #33 into master will decrease coverage by 0.57%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
- Coverage   42.24%   41.67%   -0.58%     
==========================================
  Files           8        8              
  Lines         658      667       +9     
==========================================
  Hits          278      278              
- Misses        380      389       +9     

Impacted Files	Coverage Δ
src/build.rs	0.00% <ø> (ø)
src/config.rs	0.00% <ø> (ø)
src/error.rs	0.00% <ø> (ø)
src/file.rs	65.36% <0.00%> (-0.52%)	:arrow_down:
src/terminal.rs	0.00% <0.00%> (ø)
src/types.rs	21.34% <0.00%> (-0.75%)	:arrow_down:

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