add support for explicitly setting syntax highlighter language

src/highlighters/syntect.rs

@@ -82,7 +82,9 @@ impl SyntectHighlighter {
 
     /// Determine syntect SyntaxReference to use for given SourceCode
     fn get_syntax_from_source(&self, source: &dyn SourceCode) -> Option<&syntect::SyntaxReference> {
-        if let Some(name) = source.name() {
+        if let Some(language) = source.language() {
+            self.syntax_set.find_syntax_by_name(language)
+        } else if let Some(name) = source.name() {
             if let Some(ext) = Path::new(name).extension() {
                 self.syntax_set
                     .find_syntax_by_extension(ext.to_string_lossy().as_ref())

src/named_source.rs

@@ -6,13 +6,15 @@ use crate::{MietteError, MietteSpanContents, SourceCode, SpanContents};
 pub struct NamedSource {
     source: Box<dyn SourceCode + 'static>,
     name: String,
+    language: Option<String>,
 }
 
 impl std::fmt::Debug for NamedSource {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         f.debug_struct("NamedSource")
             .field("name", &self.name)
-            .field("source", &"<redacted>");
+            .field("source", &"<redacted>")
+            .field("language", &self.language);
         Ok(())
     }
 }
@@ -24,6 +26,7 @@ impl NamedSource {
         Self {
             source: Box::new(source),
             name: name.as_ref().to_string(),
+            language: None,
         }
     }
 
@@ -32,6 +35,12 @@ impl NamedSource {
     pub fn inner(&self) -> &(dyn SourceCode + 'static) {
         &*self.source
     }
+
+    /// Sets the [`language`](SourceCode::language) for this source code.
+    pub fn with_language(mut self, language: impl Into<String>) -> Self {
+        self.language = Some(language.into());
+        self
+    }
 }
 
 impl SourceCode for NamedSource {
@@ -57,4 +66,11 @@ impl SourceCode for NamedSource {
     fn name(&self) -> Option<&str> {
         Some(&self.name)
     }
+
+    fn language(&self) -> Option<&str> {
+        match &self.language {
+            Some(language) => Some(language),
+            None => self.source.language(),
+        }
+    }
 }

src/protocol.rs

@@ -245,6 +245,15 @@ pub trait SourceCode: Send + Sync {
     fn name(&self) -> Option<&str> {
         None
     }
+
+    /// Optional method. The language name for this source code, if any.
+    /// This is used to drive syntax highlighting.
+    ///
+    /// Examples: Rust, TOML, C
+    ///
+    fn language(&self) -> Option<&str> {
+        None
+    }
 }
 
 /// A labeled [`SourceSpan`].