Add a comment on is_trivially_sized about obviously !Sized types

rust-lang · Dylan-DPC-zz · Jul 19, 2020 · Aug 13, 2020 · Sep 5, 2020 · Sep 6, 2020
commit 4f0047ed108889ea97ad7656307a8a829dd56636
diff --git a/compiler/rustc_middle/src/ty/sty.rs b/compiler/rustc_middle/src/ty/sty.rs
@@ -2280,6 +2280,12 @@ impl<'tcx> TyS<'tcx> {
     ///
     /// Returning true means the type is known to be sized. Returning
     /// `false` means nothing -- could be sized, might not be.
+    ///
+    /// Note that we could never rely on the fact that a type such as `[_]` is
+    /// trivially `!Sized` because we could be in a type environment with a
+    /// bound such as `[_]: Copy`. A function with such a bound obviously never
+    /// can be called, but that doesn't mean it shouldn't typecheck. This is why
+    /// this method doesn't return `Option<bool>`.
     pub fn is_trivially_sized(&self, tcx: TyCtxt<'tcx>) -> bool {
         match self.kind() {
             ty::Infer(ty::IntVar(_) | ty::FloatVar(_))