* Inline format arg in documentation and a few call places
* Add a unit test for inlined format args
I was surprised to discover that log crate has no edition value... Any reason to keep it at 2015?
Attempt number two/three? Too many in any case.
Previously I proposed a design that followed a `struct` like syntax:
```rust
info!("my message: {}", arg, {
key1: "value1",
key2: 123,
});
```
However it turns out that this does not work well with named arguments
as reported in issues #369 and #372. The implementation was eventually
reverted in pr #374.
This new design takes inspiration from the `tracing` crate which already
supports key-value pairs in logging events. The basic idea is to put the
key-value pairs before the message and arguments. Applying the same
structure like syntax as above we would get something like the
following.
```rust
info!({
key1: "value1",
key2: 123,
}, "my message: {}", arg);
```
But personally I'm not a big fan of this formatting, let's try putting
everything on a single line instead.
```rust
info!({ key1: "value1", key2: 123 }, "my message: {}", arg);
```
A little better, but at this point the structure like syntax is really
more annoying then helpful. So, instead I've done away it, opting
instead use the following syntax.
```rust
info!(key1 = "value1", key2 = 123, "my message: {}", arg);
```
Two major differences:
* Removed the brackets.
* Colons (`:`) are replaced with equal/assignment signs (`=`).
This gives us syntax similar to variable assignment.
But then we run in some limitations of the macro syntax, specifically
that `expr` fragments aren't allowed after `expr` fragments. To fix this
I went with the easiest option of changing the last comma (`,`) after
the key-value pairs to a semicolon (`;`). Making the final syntax look
like the following.
```rust
info!(key1 = "value1", key2 = 123; "my message: {}", arg);
info!(target: "my_target", key1 = "value1", key2 = 123; "my message: {}", arg);
log!(target: "my_target", log::Level::Info, key1 = "value1", key2 = 123; "my message: {}", arg);
```
Which, in my opinion and all things considered, it's too bad looking.
