Add log format documentation to the book

book/src/SUMMARY.md

@@ -16,6 +16,7 @@
   - [panic! and assert!](./panic.md)
   - [Printers](./printers.md)
   - [Encoding](./encoding.md)
+  - [Customizing log output](./custom-log-output.md)
   - [JSON output](./json-output.md)
 - [Migrating from `v0.2.x` to `v0.3.0`](./migration-02-03.md)
 - [Design & impl details](./design.md)

book/src/custom-log-output.md

@@ -0,0 +1,186 @@
+# Customizing the log output
+
+The way a printer outputs logs can be customized by providing a format string to `defmt`. The format string takes a set of metadata and format specifiers which can be used to include or exclude certain information when printing the logs.
+
+## Basics
+
+> The following log will be used as reference in the examples below: `defmt::debug!("hello");`
+
+The simplest format string is `"{s}"`. This prints the log and nothing else:
+
+```text
+hello
+```
+
+Arbitrary text can be added to the format string, which will be printed as specified with each log.
+For example, `"Log: {s}"`:
+
+```text
+Log: hello
+```
+
+Multiple specifiers can be included within a format string, in any order. For example `"[{L}] {s}"` prints:
+
+```text
+[DEBUG] hello
+```
+
+## Metadata specifiers
+
+There are several metadata specifiers available that can be used in a format string.
+
+#### Log - `{s}`
+
+This specifier prints the actual log contents. For `defmt::info!("hello");`, this specifier prints `hello`.
+
+#### Crate name - `{c}`
+
+This specifier prints the name of the crate where the log is coming from.
+
+#### File name - `{f}`
+
+For a log coming from a file `/path/to/crate/src/foo/bar.rs`, this specifier prints `bar.rs`.
+
+This specifier can be used to print more detailed parts of the file path. The number of `f`s in the specifier determines how many levels up the path should be printed. For example, `{ff}` prints `foo/bar.rs`, and `{fff}` prints `src/foo/bar.rs`.
+
+#### File path - `{F}`
+
+For a log coming from a file `/path/to/crate/src/foo/bar.rs`, this specifier prints `/path/to/crate/src/foo/bar.rs`.
+
+#### Line number - `{l}`
+
+This specifier prints the line number where the log is coming from.
+
+#### Log level - `{L}`
+
+This specifier prints the log level. The log level is padded to 5 characters by default, for alignment purposes. For `defmt::info!("hello);`, this prints `INFO `.
+
+#### Module path - `{m}`
+
+This specifier prints the module path of the function where this log is coming from. This prints `my_crate::foo::bar` for the log shown below:
+
+```ignore
+// crate: my_crate
+mod foo {
+    fn bar() {
+        defmt::info!("hello");
+    }
+}
+```
+
+#### Timestamp - `{t}`
+
+This specifier prints the timestamp at which a log was logged, as formatted by `defmt::timestamp!`.
+
+## Customizing log segments
+
+The way a metadata specifier is printed can be customized by providing additional, optional format specifiers.
+
+Format parameters are provided by adding the formatting parameters after the metadata specifier, separated by colons (`:`), like this: `"{L:bold:5} {f:white:<10} {s}"`.
+
+### Color
+
+A log segment can be specified to be colored by providing a color in the format parameters.
+
+There are three different options for coloring a log segment:
+- using a supported color such as `red` or `green`.
+- `severity` colors the log segment using the predefined color for the log level of the log.
+- `werror` is similar to `severity`, but it only applies the color if the log level is `WARN` or `ERROR`.
+
+Only one coloring option can be provided in format parameters for a given log segment, i.e. `{L:red:green}` is not supported.
+
+The following colors are supported in the format parameters:
+- `black`
+- `red`
+- `green`
+- `yellow`
+- `blue`
+- `magenta`
+- `purple`
+- `cyan`
+- `white`
+- `bright black`
+- `bright red`
+- `bright green`
+- `bright yellow`
+- `bright blue`
+- `bright magenta`
+- `bright purple`
+- `bright cyan`
+- `bright white`
+
+### Styles
+
+A log segment can be specified to be printed with a given style by providing a style specifier in the format parameters.
+
+The style specifier must be one of the following strings:
+- `bold`
+- `italic`
+- `underline`
+- `strike`
+- `dimmed`
+
+Multiple styles can be applied to a single log segment, but they must not be repeated, i.e.
+`"{s:bold:underline:italic}"` is allowed, but `"{s:bold:bold}"` isn't.
+
+### Width, alignment and padding
+
+A log segment can be specified to be printed with a given minimum width and alignment by providing a format parameter.
+
+The alignment can be specified to be left (`<`), right (`>`), or center-aligned (`^`). If no alignment is specified, left alignment is used by default.
+
+The minimum width is specified after the alignment. For example, `"{L} {f:>10}: {s}"` is printed as follows:
+
+```text
+[DEBUG]    main.rs: hello
+```
+
+The log segment is padded with spaces by default in order to fill the specified segment width. A specifier can be padded with zeros by prefixing the width specifier with a zero, e.g. `{l:03}` prints a line number 24 as `024`.
+
+## Nested formatting
+
+Log segments can be grouped and formatted together by nesting formats. Format parameters for the grouped log segments must be provided after the group, separated by `%`.
+
+Nested formats allow for more intricate formatting. For example, `"{[{L:bold}]%underline} {s}"` prints
+
+```text
+[DEBUG] hello
+```
+
+where only `DEBUG` is formatted bold, and `[DEBUG]` is underlined.
+
+Formats can be nested several levels. This provides a great level of flexibility to customize the logger formatting.
+For example, the width and alignment of a group of log segments can be specified with nested formats.
+`"{{[{L}]%bold} {f}:{l}%35} {s}"` prints something like this:
+
+```text
+[DEBUG] main.rs:20                  hello
+[DEBUG] goodbye.rs:304              goodbye
+```
+
+## Restrictions
+
+- Format strings *must* include the `{s}` metadata specifier.
+- At the moment it is not possible to escape curly brackets (i.e. `{`, `}`) in the format string, therefore curly brackets cannot be printed as part of the logger format.
+- The same restriction exists for the `%` character.
+
+## Passing log format to printers
+
+The format string can be passed to `probe-rs`, `probe-run` and `defmt-print` using the `--log-format` option.
+
+This option can be passed to the printer in `.cargo/config.toml`, but due to limitations in `cargo`, the command has to be split as follows:
+
+```toml
+# .cargo/config.toml
+
+runner = [
+  "probe-rs",
+  "run",
+  "--chip",
+  "nRF52840_xxAA",
+  "--log-format",
+  "{L} {s}",
+]
+```
+
+The format of the logs printed by the host can also be customized using the `--host-log-format` option.