Change PreProcessor::supports_renderer to return a Result

This changes `PreProcessor::supports_renderer` to return a `Result` in
preparation to allow preprocessors to be optional when the command
fails.

crates/mdbook-driver/src/builtin_preprocessors/cmd.rs

@@ -114,24 +114,14 @@ impl Preprocessor for CmdPreprocessor {
         })
     }
 
-    fn supports_renderer(&self, renderer: &str) -> bool {
+    fn supports_renderer(&self, renderer: &str) -> Result<bool> {
         debug!(
             "Checking if the \"{}\" preprocessor supports \"{}\"",
             self.name(),
             renderer
         );
 
-        let mut cmd = match crate::compose_command(&self.cmd, &self.root) {
+        let mut cmd = crate::compose_command(&self.cmd, &self.root)?;
-            Ok(c) => c,
-            Err(e) => {
-                warn!(
-                    "Unable to create the command for the \"{}\" preprocessor, {}",
-                    self.name(),
-                    e
-                );
-                return false;
-            }
-        };
 
         let outcome = cmd
             .arg("supports")
@@ -153,7 +143,7 @@ impl Preprocessor for CmdPreprocessor {
             }
         }
 
-        outcome.unwrap_or(false)
+        Ok(outcome.unwrap_or(false))
     }
 }
 

crates/mdbook-driver/src/mdbook.rs

@@ -542,7 +542,7 @@ fn preprocessor_should_run(
 ) -> Result<bool> {
     // default preprocessors should be run by default (if supported)
     if cfg.build.use_default_preprocessors && is_default_preprocessor(preprocessor) {
-        return Ok(preprocessor.supports_renderer(renderer.name()));
+        return preprocessor.supports_renderer(renderer.name());
     }
 
     let key = format!("preprocessor.{}.renderers", preprocessor.name());
@@ -552,7 +552,7 @@ fn preprocessor_should_run(
         Ok(Some(explicit_renderers)) => {
             Ok(explicit_renderers.iter().any(|name| name == renderer_name))
         }
-        Ok(None) => Ok(preprocessor.supports_renderer(renderer_name)),
+        Ok(None) => preprocessor.supports_renderer(renderer_name),
         Err(e) => bail!("failed to get `{key}`: {e}"),
     }
 }

crates/mdbook-driver/src/mdbook/tests.rs

@@ -247,8 +247,8 @@ impl Preprocessor for BoolPreprocessor {
         unimplemented!()
     }
 
-    fn supports_renderer(&self, _renderer: &str) -> bool {
+    fn supports_renderer(&self, _renderer: &str) -> Result<bool> {
-        self.0
+        Ok(self.0)
     }
 }
 

crates/mdbook-preprocessor/src/lib.rs

@@ -39,8 +39,8 @@ pub trait Preprocessor {
     /// particular renderer.
     ///
     /// By default, always returns `true`.
-    fn supports_renderer(&self, _renderer: &str) -> bool {
+    fn supports_renderer(&self, _renderer: &str) -> Result<bool> {
-        true
+        Ok(true)
     }
 }
 

examples/nop-preprocessor.rs

@@ -59,7 +59,7 @@ fn handle_supports(pre: &dyn Preprocessor, sub_args: &ArgMatches) -> ! {
     let renderer = sub_args
         .get_one::<String>("renderer")
         .expect("Required argument");
-    let supported = pre.supports_renderer(renderer);
+    let supported = pre.supports_renderer(renderer).unwrap();
 
     // Signal whether the renderer is supported by exiting with 1 or 0.
     if supported {
@@ -105,8 +105,8 @@ mod nop_lib {
             Ok(book)
         }
 
-        fn supports_renderer(&self, renderer: &str) -> bool {
+        fn supports_renderer(&self, renderer: &str) -> Result<bool> {
-            renderer != "not-supported"
+            Ok(renderer != "not-supported")
         }
     }
 

tests/testsuite/preprocessor.rs

@@ -83,7 +83,7 @@ fn example() -> CmdPreprocessor {
 fn example_supports_whatever() {
     let cmd = example();
 
-    let got = cmd.supports_renderer("whatever");
+    let got = cmd.supports_renderer("whatever").unwrap();
 
     assert_eq!(got, true);
 }
@@ -92,7 +92,7 @@ fn example_supports_whatever() {
 fn example_doesnt_support_not_supported() {
     let cmd = example();
 
-    let got = cmd.supports_renderer("not-supported");
+    let got = cmd.supports_renderer("not-supported").unwrap();
 
     assert_eq!(got, false);
 }