Warn if external formatter is not found in validate command output

src/formatters/mod.rs

@@ -88,6 +88,15 @@ impl Formatter {
         };
         res.map_err(Error::Io)
     }
+
+    pub fn is_available(&self) -> bool {
+        match self {
+            Self::PgFormatter(f) => f.check(),
+            Self::SqlFormatRs(_) => true,
+            Self::SqlFormatter(f) => f.check(),
+            Self::SqlFluff(f) => f.check(),
+        }
+    }
 }
 
 /// Returns an ordered vec of formatters discovered on the system.

src/metadata.rs

@@ -207,6 +207,18 @@ impl Metadata {
             }
         }
 
+        // Warn if the provided formatter is not found or installed on
+        // the system
+        if let Some(formatter) = self.formatter.as_ref() {
+            if !formatter.is_available() {
+                // @SAFE use of unwrap because if executable() returns
+                // None, it means the formatter is internal and hence
+                // will always be available.
+                let exec_path = formatter.executable().unwrap().display();
+                warn!("Executable for external formatter not found: {exec_path}");
+            }
+        }
+
         Ok(())
     }