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Author: RalfJung

library/core/src/fmt/rt.rs

@@ -80,6 +80,17 @@ macro_rules! argument_new {
                 // To avoid this crash, we use a helper function when CFI is enabled. To avoid the
                 // cost of this helper function (mainly code-size) when it is not needed, we
                 // transmute the function pointer otherwise.
+                //
+                // This is is similar to what the Rust compiler does internally with vtables when
+                // CFI is enabled, where it generates trampoline functions that only serve to adjust
+                // the expected type of the argument. `ArgumentType::Placeholder` is a bit like a
+                // manually constructed trait object, so it is not surprising that the same approach
+                // has to be applied here as well.
+                //
+                // It is still considered problematic (from the Rust side) that CFI rejects entirely
+                // legal Rust programs, so we do not consider anything done here a stable guarantee,
+                // but meanwhile we carry this work-around to keep Rust compatible with CFI
+                // runtimes.
                 #[cfg(not(any(sanitize = "cfi", sanitize = "kcfi")))]
                 formatter: {
                     let f: fn(&$t, &mut Formatter<'_>) -> Result = $f;