feat(log): Allow a log formatter per target (#3065)

Co-authored-by: Fabian-Lars <github@fabianlars.de>

.changes/allow-log-formatter-per-target.md

@@ -0,0 +1,6 @@
+---
+"log": "minor"
+"log-js": "minor"
+---
+
+Allow specifying a log formatter per target using the `format` method on `Target`.

plugins/log/src/lib.rs

@@ -182,10 +182,13 @@ pub enum TargetKind {
     Dispatch(fern::Dispatch),
 }
 
+type Formatter = dyn Fn(FormatCallback, &Arguments, &Record) + Send + Sync + 'static;
+
 /// A log target.
 pub struct Target {
     kind: TargetKind,
     filters: Vec<Box<Filter>>,
+    formatter: Option<Box<Formatter>>,
 }
 
 impl Target {
@@ -194,6 +197,7 @@ impl Target {
         Self {
             kind,
             filters: Vec::new(),
+            formatter: None,
         }
     }
 
@@ -205,6 +209,15 @@ impl Target {
         self.filters.push(Box::new(filter));
         self
     }
+
+    #[inline]
+    pub fn format<F>(mut self, formatter: F) -> Self
+    where
+        F: Fn(FormatCallback, &Arguments, &Record) + Send + Sync + 'static,
+    {
+        self.formatter.replace(Box::new(formatter));
+        self
+    }
 }
 
 pub struct Builder {
@@ -276,6 +289,13 @@ impl Builder {
         self
     }
 
+    pub fn clear_format(mut self) -> Self {
+        self.dispatch = self.dispatch.format(|out, message, _record| {
+            out.finish(format_args!("{message}"));
+        });
+        self
+    }
+
     pub fn format<F>(mut self, formatter: F) -> Self
     where
         F: Fn(FormatCallback, &Arguments, &Record) + Sync + Send + 'static,
@@ -384,6 +404,9 @@ impl Builder {
             for filter in target.filters {
                 target_dispatch = target_dispatch.filter(filter);
             }
+            if let Some(formatter) = target.formatter {
+                target_dispatch = target_dispatch.format(formatter);
+            }
 
             let logger = match target.kind {
                 #[cfg(target_os = "android")]