Auto merge of #120196 - matthiaskrgr:rollup-id2zocf, r=matthiaskrgr

Rollup of 8 pull requests

Successful merges:

 - #120005 (Update Readme)
 - #120045 (Un-hide `iter::repeat_n`)
 - #120128 (Make stable_mir::with_tables sound)
 - #120145 (fix: Drop guard was deallocating with the incorrect size)
 - #120158 (`rustc_mir_dataflow`: Restore removed exports)
 - #120167 (Capture the rationale for `-Zallow-features=` in bootstrap.py)
 - #120174 (Warn users about limited review for tier 2 and 3 code)
 - #120180 (Document some alternatives to `Vec::split_off`)

Failed merges:

 - #120171 (Fix assume and assert in jump threading)

r? `@ghost`
`@rustbot` modify labels: rollup

Author: bors

README.md

@@ -15,6 +15,7 @@ If you wish to _contribute_ to the compiler, you should read
 <summary>Table of Contents</summary>
 
 - [Quick Start](#quick-start)
+- [Installing from Source](#installing-from-source)
 - [Getting Help](#getting-help)
 - [Contributing](#contributing)
 - [License](#license)
@@ -29,9 +30,10 @@ Read ["Installation"] from [The Book].
 ["Installation"]: https://doc.rust-lang.org/book/ch01-01-installation.html
 [The Book]: https://doc.rust-lang.org/book/index.html
 
-## Installing from source
+## Installing from Source
 
-If you really want to install from source (though this is not recommended), see [INSTALL.md](INSTALL.md).
+If you really want to install from source (though this is not recommended), see
+[INSTALL.md](INSTALL.md).
 
 ## Getting Help
 

compiler/rustc_arena/src/lib.rs

@@ -484,6 +484,19 @@ impl DroplessArena {
         }
     }
 
+    /// Used by `Lift` to check whether this slice is allocated
+    /// in this arena.
+    #[inline]
+    pub fn contains_slice<T>(&self, slice: &[T]) -> bool {
+        for chunk in self.chunks.borrow_mut().iter_mut() {
+            let ptr = slice.as_ptr().cast::<u8>().cast_mut();
+            if chunk.start() <= ptr && chunk.end() >= ptr {
+                return true;
+            }
+        }
+        false
+    }
+
     /// Allocates a string slice that is copied into the `DroplessArena`, returning a
     /// reference to it. Will panic if passed an empty string.
     ///

compiler/rustc_middle/src/mir/consts.rs

@@ -195,7 +195,7 @@ impl<'tcx> ConstValue<'tcx> {
 /// Constants
 
 #[derive(Clone, Copy, PartialEq, Eq, TyEncodable, TyDecodable, Hash, HashStable, Debug)]
-#[derive(TypeFoldable, TypeVisitable)]
+#[derive(TypeFoldable, TypeVisitable, Lift)]
 pub enum Const<'tcx> {
     /// This constant came from the type system.
     ///
@@ -456,7 +456,7 @@ impl<'tcx> Const<'tcx> {
 
 /// An unevaluated (potentially generic) constant used in MIR.
 #[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord, TyEncodable, TyDecodable)]
-#[derive(Hash, HashStable, TypeFoldable, TypeVisitable)]
+#[derive(Hash, HashStable, TypeFoldable, TypeVisitable, Lift)]
 pub struct UnevaluatedConst<'tcx> {
     pub def: DefId,
     pub args: GenericArgsRef<'tcx>,

compiler/rustc_middle/src/ty/context.rs

@@ -1416,6 +1416,7 @@ nop_lift! {const_; Const<'a> => Const<'tcx>}
 nop_lift! {const_allocation; ConstAllocation<'a> => ConstAllocation<'tcx>}
 nop_lift! {predicate; Predicate<'a> => Predicate<'tcx>}
 nop_lift! {predicate; Clause<'a> => Clause<'tcx>}
+nop_lift! {layout; Layout<'a> => Layout<'tcx>}
 
 nop_list_lift! {type_lists; Ty<'a> => Ty<'tcx>}
 nop_list_lift! {poly_existential_predicates; PolyExistentialPredicate<'a> => PolyExistentialPredicate<'tcx>}
@@ -1424,8 +1425,28 @@ nop_list_lift! {bound_variable_kinds; ty::BoundVariableKind => ty::BoundVariable
 // This is the impl for `&'a GenericArgs<'a>`.
 nop_list_lift! {args; GenericArg<'a> => GenericArg<'tcx>}
 
+macro_rules! nop_slice_lift {
+    ($ty:ty => $lifted:ty) => {
+        impl<'a, 'tcx> Lift<'tcx> for &'a [$ty] {
+            type Lifted = &'tcx [$lifted];
+            fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+                if self.is_empty() {
+                    return Some(&[]);
+                }
+                tcx.interners
+                    .arena
+                    .dropless
+                    .contains_slice(self)
+                    .then(|| unsafe { mem::transmute(self) })
+            }
+        }
+    };
+}
+
+nop_slice_lift! {ty::ValTree<'a> => ty::ValTree<'tcx>}
+
 TrivialLiftImpls! {
-    ImplPolarity,
+    ImplPolarity, Promoted
 }
 
 macro_rules! sty_debug_print {

compiler/rustc_mir_dataflow/src/lib.rs

@@ -16,15 +16,17 @@ extern crate rustc_middle;
 
 use rustc_middle::ty;
 
+// Please change the public `use` directives cautiously, as they might be used by external tools.
+// See issue #120130.
 pub use self::drop_flag_effects::{
     drop_flag_effects_for_function_entry, drop_flag_effects_for_location,
     move_path_children_matching, on_all_children_bits, on_lookup_result_bits,
 };
 pub use self::framework::{
-    fmt, lattice, visit_results, Analysis, AnalysisDomain, Direction, GenKill, GenKillAnalysis,
-    JoinSemiLattice, MaybeReachable, Results, ResultsCursor, ResultsVisitable, ResultsVisitor,
+    fmt, graphviz, lattice, visit_results, Analysis, AnalysisDomain, Backward, Direction, Engine,
+    Forward, GenKill, GenKillAnalysis, JoinSemiLattice, MaybeReachable, Results, ResultsCursor,
+    ResultsVisitable, ResultsVisitor, SwitchIntEdgeEffects,
 };
-use self::framework::{Backward, SwitchIntEdgeEffects};
 use self::move_paths::MoveData;
 
 pub mod debuginfo;