Log to stderr by default.

docs/cli-logging.md

@@ -0,0 +1,26 @@
+# Logging in the `wasmtime` CLI
+
+Wasmtime's libraries use Rust's [`log`] crate to log diagnostic
+information, and the `wasmtime` CLI executable uses [`pretty_env_logger`]
+by default for logging this information to the console.
+
+Basic logging is controlled by the `RUST_LOG` environment variable. For example,
+To enable logging of WASI system calls, similar to the `strace` command on Linux,
+set `RUST_LOG=wasi_common=trace`.
+
+```sh
+$ RUST_LOG=wasi_common=trace wasmtime hello.wasm
+[...]
+ TRACE wasi_common::hostcalls_impl::fs                       > fd_write(fd=1, iovs_ptr=0x10408, iovs_len=1, nwritten=0x10404)
+Hello, world!
+ TRACE wasi_common::hostcalls_impl::fs                       >      | *nwritten=14
+ TRACE wasi_common::hostcalls                                >      | errno=ESUCCESS (No error occurred. System call completed successfully.)
+ TRACE wasi_common::hostcalls_impl::misc                     > proc_exit(rval=1)
+```
+
+Wasmtime can also redirect the log messages into log files, with the
+`--log-to-files` option. It creates one file per thread within Wasmtime, with
+the files named `wasmtime.dbg.*`.
+
+[`log`]: https://crates.io/crates/log
+[`pretty_env_logger`]: https://crates.io/crates/pretty_env_logger

docs/cli.md

@@ -20,5 +20,5 @@ $ wasmtime --invoke _start foo.wasm
 ```
 
 For more information be sure to check out [how to install the
-CLI](cli-install.md) as well as [the list of options you can
-pass](cli-options.md).
+CLI](cli-install.md), [the list of options you can
+pass](cli-options.md), and [how to enable logging](cli-logging.md).

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")]