cranelift: use a deferred display wrapper for logging the vcode's IR

cranelift/codegen/src/log.rs

@@ -0,0 +1,39 @@
+//! This module implements deferred display helpers.
+//!
+//! These are particularly useful in logging contexts, where the maximum logging level filter might
+//! be enabled, but we don't want the arguments to be evaluated early:
+//!
+//! ```
+//! log::set_max_level(log::LevelFilter::max());
+//! fn expensive_calculation() -> String {
+//!   "a string that is very slow to generate".into()
+//! }
+//! log::debug!("{}", expensive_calculation());
+//! ```
+//!
+//! If the associated log implementation filters out log debug entries, the expensive calculation
+//! would have been spurious. In this case, we can wrap the expensive computation within an
+//! `DeferredDisplay`, so that the computation only happens when the actual `fmt` function is
+//! called.
+
+use core::fmt;
+
+pub(crate) struct DeferredDisplay<F>(F);
+
+impl<F: Fn() -> T, T: fmt::Display> DeferredDisplay<F> {
+    pub(crate) fn new(f: F) -> Self {
+        Self(f)
+    }
+}
+
+impl<F: Fn() -> T, T: fmt::Display> fmt::Display for DeferredDisplay<F> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.0().fmt(f)
+    }
+}
+
+impl<F: Fn() -> T, T: fmt::Debug> fmt::Debug for DeferredDisplay<F> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.0().fmt(f)
+    }
+}