Make env_logger an optional dependency (#1226)

Now the built-in `env_logger` is guarded behind a Cargo feature
"builtin_env_logger". It is a default feature, but can be disabled in
Cargo.toml by setting `dependencies.mmtk.default-features = false`. In
this way, VM bindings that want to implement its own logger can remove
the `env_logger` crate from its dependencies.

Fixes: #744

Cargo.toml

@@ -28,7 +28,7 @@ crossbeam = "0.8.1"
 delegate = "0.12.0"
 downcast-rs = "1.1.1"
 enum-map = "2.4.2"
-env_logger = "0.11.3"
+env_logger = { version = "0.11.3", optional = true }
 is-terminal = "0.4.7"
 itertools = "0.12.0"
 jemalloc-sys = { version = "0.5.3", features = ["disable_initial_exec_tls"], optional = true }
@@ -66,7 +66,12 @@ name = "main"
 harness = false
 
 [features]
-default = []
+default = ["builtin_env_logger"]
+
+# Built-in env_logger.  This feature is enabled by default.
+# The user can disable this default feature to remove `env_logger` from the dependencies.
+# See `crate::util::logger` for more details.
+builtin_env_logger = ["dep:env_logger"]
 
 # This feature is only supported on x86-64 for now
 # It's manually added to CI scripts

src/memory_manager.rs

@@ -38,7 +38,9 @@ use crate::vm::VMBinding;
 ///    supported. Currently we assume a binding will only need one MMTk instance. Note that GC is enabled by default and the binding should
 ///    implement `VMCollection::is_collection_enabled()` if it requires that the GC should be disabled at a particular time.
 ///
-/// Note that this method will attempt to initialize a logger. If the VM would like to use its own logger, it should initialize the logger before calling this method.
+/// This method will attempt to initialize the built-in `env_logger` if the Cargo feature "builtin_env_logger" is enabled (by default).
+/// If the VM would like to use its own logger, it should disable the default feature "builtin_env_logger" in `Cargo.toml`.
+///
 /// Note that, to allow MMTk to do GC properly, `initialize_collection()` needs to be called after this call when
 /// the VM's thread system is ready to spawn GC workers.
 ///
@@ -51,12 +53,7 @@ use crate::vm::VMBinding;
 /// Arguments:
 /// * `builder`: The reference to a MMTk builder.
 pub fn mmtk_init<VM: VMBinding>(builder: &MMTKBuilder) -> Box<MMTK<VM>> {
-    match crate::util::logger::try_init() {
-        Ok(_) => debug!("MMTk initialized the logger."),
-        Err(_) => debug!(
-            "MMTk failed to initialize the logger. Possibly a logger has been initialized by user."
-        ),
-    }
+    crate::util::logger::try_init();
     #[cfg(all(feature = "perf_counter", target_os = "linux"))]
     {
         use std::fs::File;

src/util/logger.rs

@@ -1,9 +1,38 @@
-use log::{self, SetLoggerError};
+//! This module provides a built-in logger implementation.
+//!
+//! The built-in logger implementation uses the `env_logger` crate.  It is enabled by the Cargo
+//! feature "builtin_env_logger" which is enabled by default.  When enabled, it will be initialized
+//! in [`crate::memory_manager::mmtk_init`] and will show logs of levels INFO or lower (the lower,
+//! the more important).
+//!
+//! This provides convenient out-of-the-box experience for binding developers so that they can see
+//! logs when using MMTk without configuration, and can easily configure log levels from environment
+//! variables.  Some bindings may wish to choose a different implementation, or implement their own
+//! logging implementations to integrate with the existing logging frameworks of their VMs.  In such
+//! cases, the binding can disable the Cargo feature "builtin_env_logger" and register their own
+//! implementations with the `log` crate.
 
 /// Attempt to init a env_logger for MMTk.
-pub fn try_init() -> Result<(), SetLoggerError> {
-    env_logger::try_init_from_env(
-        // By default, use info level logging.
-        env_logger::Env::default().filter_or(env_logger::DEFAULT_FILTER_ENV, "info"),
-    )
+/// Does nothing if the "builtin_env_logger" feature is disabled.
+pub(crate) fn try_init() {
+    cfg_if::cfg_if! {
+        if #[cfg(feature = "builtin_env_logger")] {
+            let result = env_logger::try_init_from_env(
+                // By default, show info level logging.
+                env_logger::Env::default().filter_or(env_logger::DEFAULT_FILTER_ENV, "info"),
+            );
+
+            match result {
+                Ok(()) => {
+                    debug!("MMTk initialized the logger.");
+                }
+                Err(e) => {
+                    // Currently `log::SetLoggerError` can only be raised for one reason: the logger has already been initialized.
+                    debug!("MMTk failed to initialize the built-in env_logger: {e}");
+                }
+            }
+        } else {
+            debug!("MMTk didn't initialize the built-in env_logger.  The Cargo feature \"builtin_env_logger\" is not enabled.");
+        }
+    }
 }