Add method.

src/handler.rs

@@ -25,6 +25,9 @@ impl Default for RgbColors {
     }
 }
 
+/// Function to format a message before rendering.
+pub type MessageFormatter = dyn Fn(String) -> String;
+
 /**
 Create a custom [`MietteHandler`] from options.
 
@@ -55,6 +58,7 @@ pub struct MietteHandlerOpts {
     pub(crate) context_lines: Option<usize>,
     pub(crate) tab_width: Option<usize>,
     pub(crate) with_cause_chain: Option<bool>,
+    pub(crate) message_formatter: Option<Box<MessageFormatter>>,
 }
 
 impl MietteHandlerOpts {
@@ -164,6 +168,11 @@ impl MietteHandlerOpts {
         self
     }
 
+    pub fn message_formatter<F: MessageFormatter>(mut self, formatter: F) -> Self {
+        self.message_formatter = Some(Box::new(formatter));
+        self
+    }
+
     /// Builds a [`MietteHandler`] from this builder.
     pub fn build(self) -> MietteHandler {
         let graphical = self.is_graphical();
@@ -233,6 +242,9 @@ impl MietteHandlerOpts {
             if let Some(w) = self.tab_width {
                 handler = handler.tab_width(w);
             }
+            if let Some(formatter) = self.message_formatter {
+                handler = handler.with_message_formatter(formatter);
+            }
             MietteHandler {
                 inner: Box::new(handler),
             }

src/handlers/graphical.rs

@@ -6,7 +6,9 @@ use unicode_width::UnicodeWidthChar;
 use crate::diagnostic_chain::DiagnosticChain;
 use crate::handlers::theme::*;
 use crate::protocol::{Diagnostic, Severity};
-use crate::{LabeledSpan, MietteError, ReportHandler, SourceCode, SourceSpan, SpanContents};
+use crate::{
+    LabeledSpan, MessageFormatter, MietteError, ReportHandler, SourceCode, SourceSpan, SpanContents,
+};
 
 /**
 A [`ReportHandler`] that displays a given [`Report`](crate::Report) in a
@@ -30,6 +32,7 @@ pub struct GraphicalReportHandler {
     pub(crate) context_lines: usize,
     pub(crate) tab_width: usize,
     pub(crate) with_cause_chain: bool,
+    pub(crate) message_formatter: Option<Box<MessageFormatter>>,
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -51,6 +54,7 @@ impl GraphicalReportHandler {
             context_lines: 1,
             tab_width: 4,
             with_cause_chain: true,
+            message_formatter: None,
         }
     }
 
@@ -64,6 +68,7 @@ impl GraphicalReportHandler {
             context_lines: 1,
             tab_width: 4,
             with_cause_chain: true,
+            message_formatter: None,
         }
     }
 
@@ -133,6 +138,11 @@ impl GraphicalReportHandler {
         self.context_lines = lines;
         self
     }
+
+    pub fn with_message_formatter<F: MessageFormatter>(mut self, formatter: F) -> Self {
+        self.message_formatter = Some(Box::new(formatter));
+        self
+    }
 }
 
 impl Default for GraphicalReportHandler {
@@ -214,8 +224,13 @@ impl GraphicalReportHandler {
         let opts = textwrap::Options::new(width)
             .initial_indent(&initial_indent)
             .subsequent_indent(&rest_indent);
+        let mut message = diagnostic.to_string();
 
-        writeln!(f, "{}", textwrap::fill(&diagnostic.to_string(), opts))?;
+        if let Some(formatter) = self.message_formatter {
+            message = formatter(message);
+        }
+
+        writeln!(f, "{}", textwrap::fill(&message, opts))?;
 
         if !self.with_cause_chain {
             return Ok(());