Get the wasm-bindgen version and display it in the error message

src/bindgen.rs

@@ -204,7 +204,7 @@ pub fn wasm_bindgen_build(
         Target::Bundler => "--browser",
     };
     let bindgen_path = bindgen.binary("wasm-bindgen")?;
-    let mut cmd = Command::new(bindgen_path);
+    let mut cmd = Command::new(&bindgen_path);
     cmd.arg(&wasm_path)
         .arg("--out-dir")
         .arg(out_dir)
@@ -229,42 +229,47 @@ pub fn wasm_bindgen_build(
     let result = child::run(cmd, "wasm-bindgen");
     let result: Result<(), failure::Error> = match result {
         Ok(r) => Ok(r),
-        Err(e) => process_error(e),
+        Err(e) => process_error(&bindgen_path, e),
     };
     result.context("Running the wasm-bindgen CLI")?;
     Ok(())
 }
 
-fn process_error(e: child::CommandError) -> Result<(), failure::Error> {
+fn process_error(bindgen_path: &PathBuf, e: child::CommandError) -> Result<(), failure::Error> {
     match &e.stderr {
-        Some(err) if err.trim().starts_with("Unknown flag: '--web'") =>
-            bail!("Failed to execute `wasm-bindgen`: --web is not supported. Upgrade wasm-bindgen to a more recent version."),
+        Some(err) if err.trim().starts_with("Unknown flag: '--web'") => {
+            let v = wasm_bindgen_get_version(bindgen_path).unwrap_or(String::from("unknown"));
+            bail!("Failed to execute `wasm-bindgen`: --web is not supported in version '{}'. Upgrade the wasm-bindgen dependency in Cargo.toml to version 0.2.39 or later.", v)
+        }
         Some(err) => {
             eprintln!("{}", err);
             bail!("{}", e.to_string())
-        },
+        }
         _ => bail!("{}", e.to_string()),
     }
 }
 
 /// Check if the `wasm-bindgen` dependency is locally satisfied.
 fn wasm_bindgen_version_check(bindgen_path: &PathBuf, dep_version: &str) -> bool {
+    wasm_bindgen_get_version(bindgen_path)
+        .map(|v| {
+            info!(
+                "Checking installed `wasm-bindgen` version == expected version: {} == {}",
+                v, dep_version
+            );
+            v == dep_version
+        })
+        .unwrap_or(false)
+}
+
+/// Get the `wasm-bindgen` version
+fn wasm_bindgen_get_version(bindgen_path: &PathBuf) -> Option<String> {
     let mut cmd = Command::new(bindgen_path);
     cmd.arg("--version");
     child::run_capture_stdout(cmd, "wasm-bindgen")
-        .map(|stdout| {
-            stdout
-                .trim()
-                .split_whitespace()
-                .nth(1)
-                .map(|v| {
-                    info!(
-                        "Checking installed `wasm-bindgen` version == expected version: {} == {}",
-                        v, dep_version
-                    );
-                    v == dep_version
-                })
-                .unwrap_or(false)
+        .map(|stdout| match stdout.trim().split_whitespace().nth(1) {
+            Some(v) => return Some(v.to_owned()),
+            None => return None,
         })
-        .unwrap_or(false)
+        .unwrap_or(None)
 }