Log to stderr by default. (#1266)

Change the default from file-per-thread-logger to pretty-env-logger,
which is more common in Rust projects, and change the option from `-d`
to `--log-to-files`.

src/commands/run.rs

@@ -89,11 +89,11 @@ pub struct RunCommand {
 impl RunCommand {
     /// Executes the command.
     pub fn execute(&self) -> Result<()> {
-        if self.common.debug {
-            pretty_env_logger::init();
-        } else {
+        if self.common.log_to_files {
             let prefix = "wasmtime.dbg.";
             init_file_per_thread_logger(prefix);
+        } else {
+            pretty_env_logger::init();
         }
 
         let config = self.common.config()?;

src/commands/wasm2obj.rs

@@ -54,11 +54,11 @@ impl WasmToObjCommand {
     }
 
     fn handle_module(&self) -> Result<()> {
-        if self.common.debug {
-            pretty_env_logger::init();
-        } else {
+        if self.common.log_to_files {
             let prefix = "wasm2obj.dbg.";
             init_file_per_thread_logger(prefix);
+        } else {
+            pretty_env_logger::init();
         }
 
         let cache_config = if self.common.disable_cache {

src/commands/wast.rs

@@ -26,11 +26,11 @@ pub struct WastCommand {
 impl WastCommand {
     /// Executes the command.
     pub fn execute(&self) -> Result<()> {
-        if self.common.debug {
-            pretty_env_logger::init();
-        } else {
+        if self.common.log_to_files {
             let prefix = "wast.dbg.";
             init_file_per_thread_logger(prefix);
+        } else {
+            pretty_env_logger::init();
         }
 
         let config = self.common.config()?;

src/lib.rs

@@ -88,9 +88,9 @@ struct CommonOptions {
     #[structopt(long, conflicts_with = "lightbeam")]
     cranelift: bool,
 
-    /// Enable debug output
-    #[structopt(short, long)]
-    debug: bool,
+    /// Log to per-thread log files instead of stderr.
+    #[structopt(long)]
+    log_to_files: bool,
 
     /// Generate debug information
     #[structopt(short = "g")]