From 8fe99f57a42fe2a2f5ed19d3b6c3a7b2b5455edd Mon Sep 17 00:00:00 2001
From: Lukas Markeffsky <@>
Date: Wed, 13 Mar 2024 23:35:24 +0100
Subject: [PATCH] remove unnecessary sized checks

---
 .../rustc_const_eval/src/interpret/eval_context.rs    |  8 ++++----
 compiler/rustc_middle/src/ty/sty.rs                   | 11 +++++++----
 .../rustc_trait_selection/src/solve/assembly/mod.rs   |  2 +-
 .../src/solve/assembly/structural_traits.rs           |  5 +++--
 4 files changed, 15 insertions(+), 11 deletions(-)

diff --git a/compiler/rustc_const_eval/src/interpret/eval_context.rs b/compiler/rustc_const_eval/src/interpret/eval_context.rs
index 09e9cc4b35d3..a9c127af8d53 100644
--- a/compiler/rustc_const_eval/src/interpret/eval_context.rs
+++ b/compiler/rustc_const_eval/src/interpret/eval_context.rs
@@ -1034,8 +1034,7 @@ pub fn storage_live_dyn(
     ) -> InterpResult<'tcx> {
         trace!("{:?} is now live", local);
 
-        // We avoid `ty.is_trivially_sized` since that (a) cannot assume WF, so it recurses through
-        // all fields of a tuple, and (b) does something expensive for ADTs.
+        // We avoid `ty.is_trivially_sized` since that does something expensive for ADTs.
         fn is_very_trivially_sized(ty: Ty<'_>) -> bool {
             match ty.kind() {
                 ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
@@ -1054,9 +1053,10 @@ fn is_very_trivially_sized(ty: Ty<'_>) -> bool {
                 | ty::Closure(..)
                 | ty::CoroutineClosure(..)
                 | ty::Never
-                | ty::Error(_) => true,
+                | ty::Error(_)
+                | ty::Dynamic(_, _, ty::DynStar) => true,
 
-                ty::Str | ty::Slice(_) | ty::Dynamic(..) | ty::Foreign(..) => false,
+                ty::Str | ty::Slice(_) | ty::Dynamic(_, _, ty::Dyn) | ty::Foreign(..) => false,
 
                 ty::Tuple(tys) => tys.last().iter().all(|ty| is_very_trivially_sized(**ty)),
 
diff --git a/compiler/rustc_middle/src/ty/sty.rs b/compiler/rustc_middle/src/ty/sty.rs
index 7ff0b8dac4c8..540936d7d8a6 100644
--- a/compiler/rustc_middle/src/ty/sty.rs
+++ b/compiler/rustc_middle/src/ty/sty.rs
@@ -2484,13 +2484,16 @@ pub fn is_trivially_sized(self, tcx: TyCtxt<'tcx>) -> bool {
             | ty::Closure(..)
             | ty::CoroutineClosure(..)
             | ty::Never
-            | ty::Error(_) => true,
+            | ty::Error(_)
+            | ty::Dynamic(_, _, ty::DynStar) => true,
 
-            ty::Str | ty::Slice(_) | ty::Dynamic(..) | ty::Foreign(..) => false,
+            ty::Str | ty::Slice(_) | ty::Dynamic(_, _, ty::Dyn) | ty::Foreign(..) => false,
 
-            ty::Tuple(tys) => tys.iter().all(|ty| ty.is_trivially_sized(tcx)),
+            ty::Tuple(tys) => tys.last().map_or(true, |ty| ty.is_trivially_sized(tcx)),
 
-            ty::Adt(def, _args) => def.sized_constraint(tcx).is_none(),
+            ty::Adt(def, args) => def
+                .sized_constraint(tcx)
+                .map_or(true, |ty| ty.instantiate(tcx, args).is_trivially_sized(tcx)),
 
             ty::Alias(..) | ty::Param(_) | ty::Placeholder(..) | ty::Bound(..) => false,
 
diff --git a/compiler/rustc_trait_selection/src/solve/assembly/mod.rs b/compiler/rustc_trait_selection/src/solve/assembly/mod.rs
index 9c7fa5216d76..9f33dce2a6df 100644
--- a/compiler/rustc_trait_selection/src/solve/assembly/mod.rs
+++ b/compiler/rustc_trait_selection/src/solve/assembly/mod.rs
@@ -128,7 +128,7 @@ fn consider_trait_alias_candidate(
         goal: Goal<'tcx, Self>,
     ) -> QueryResult<'tcx>;
 
-    /// A type is `Copy` or `Clone` if its components are `Sized`.
+    /// A type is `Sized` if its tail component is `Sized`.
     ///
     /// These components are given by built-in rules from
     /// [`structural_traits::instantiate_constituent_tys_for_sized_trait`].
diff --git a/compiler/rustc_trait_selection/src/solve/assembly/structural_traits.rs b/compiler/rustc_trait_selection/src/solve/assembly/structural_traits.rs
index 8822e55736f7..e7e8ff66e3e8 100644
--- a/compiler/rustc_trait_selection/src/solve/assembly/structural_traits.rs
+++ b/compiler/rustc_trait_selection/src/solve/assembly/structural_traits.rs
@@ -154,8 +154,9 @@ pub(in crate::solve) fn instantiate_constituent_tys_for_sized_trait<'tcx>(
             bug!("unexpected type `{ty}`")
         }
 
-        // impl Sized for (T1, T2, .., Tn) where T1: Sized, T2: Sized, .. Tn: Sized
-        ty::Tuple(tys) => Ok(tys.iter().map(ty::Binder::dummy).collect()),
+        // impl Sized for ()
+        // impl Sized for (T1, T2, .., Tn) where Tn: Sized if n >= 1
+        ty::Tuple(tys) => Ok(tys.last().map_or_else(Vec::new, |&ty| vec![ty::Binder::dummy(ty)])),
 
         // impl Sized for Adt<Args...> where sized_constraint(Adt)<Args...>: Sized
         //   `sized_constraint(Adt)` is the deepest struct trail that can be determined