update comments

Author: RalfJung

Diff for: compiler/rustc_target/src/target_features.rs

@@ -108,21 +108,19 @@ impl Stability {
 // per-function level, since we would then allow safe calls from functions with `+soft-float` to
 // functions without that feature!
 //
-// It is important for soundness that features allowed here do *not* change the function call ABI.
-// For example, disabling the `x87` feature on x86 changes how scalar floats are passed as
-// arguments, so enabling toggling that feature would be unsound. In fact, since `-Ctarget-feature`
-// will just allow unknown features (with a warning), we have to explicitly list features that change
-// the ABI as `Forbidden` to ensure using them causes an error. Note that this is only effective if
-// such features can never be toggled via `-Ctarget-cpu`! If that is ever a possibility, we will need
-// extra checks ensuring that the LLVM-computed target features for a CPU did not (un)set a
-// `Forbidden` feature. See https://github.com/rust-lang/rust/issues/116344 for some more context.
-// FIXME: add such "forbidden" features for non-x86 targets.
+// It is important for soundness to consider the interaction of targets features and the function
+// call ABI. For example, disabling the `x87` feature on x86 changes how scalar floats are passed as
+// arguments, so letting people toggle that feature would be unsound. To this end, the
+// `abi_required_features` function computes which target features must and must not be enabled for
+// any given target, and individual features can also be marked as `Forbidden`.
+// See https://github.com/rust-lang/rust/issues/116344 for some more context.
 //
 // The one exception to features that change the ABI is features that enable larger vector
-// registers. Those are permitted to be listed here. This is currently unsound (see
-// https://github.com/rust-lang/rust/issues/116558); in the future we will have to ensure that
-// functions can only use such vectors as arguments/return types if the corresponding target feature
-// is enabled.
+// registers. Those are permitted to be listed here. The `*_FOR_CORRECT_VECTOR_ABI` arrays store
+// information about which target feature is ABI-required for which vector size; this is used to
+// ensure that vectors can only be passed via `extern "C"` when the right feature is enabled. (For
+// the "Rust" ABI we generally pass vectors by-ref exactly to avoid these issues.)
+// Also see https://github.com/rust-lang/rust/issues/116558.
 //
 // Stabilizing a target feature requires t-lang approval.