Rollup of 6 pull requests

compiler/rustc_ast_lowering/src/block.rs

@@ -1,8 +1,8 @@
 use crate::{ImplTraitContext, ImplTraitPosition, LoweringContext};
-use rustc_ast::{AttrVec, Block, BlockCheckMode, Expr, Local, LocalKind, Stmt, StmtKind};
+use rustc_ast::{Block, BlockCheckMode, Local, LocalKind, Stmt, StmtKind};
 use rustc_hir as hir;
 use rustc_session::parse::feature_err;
-use rustc_span::{sym, DesugaringKind};
+use rustc_span::sym;
 
 use smallvec::SmallVec;
 
@@ -36,21 +36,11 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             match s.kind {
                 StmtKind::Local(ref local) => {
                     let hir_id = self.lower_node_id(s.id);
-                    match &local.kind {
-                        LocalKind::InitElse(init, els) => {
-                            let e = self.lower_let_else(hir_id, local, init, els, tail);
-                            expr = Some(e);
-                            // remaining statements are in let-else expression
-                            break;
-                        }
-                        _ => {
-                            let local = self.lower_local(local);
-                            self.alias_attrs(hir_id, local.hir_id);
-                            let kind = hir::StmtKind::Local(local);
-                            let span = self.lower_span(s.span);
-                            stmts.push(hir::Stmt { hir_id, kind, span });
-                        }
-                    }
+                    let local = self.lower_local(local);
+                    self.alias_attrs(hir_id, local.hir_id);
+                    let kind = hir::StmtKind::Local(local);
+                    let span = self.lower_span(s.span);
+                    stmts.push(hir::Stmt { hir_id, kind, span });
                 }
                 StmtKind::Item(ref it) => {
                     stmts.extend(self.lower_item_ref(it).into_iter().enumerate().map(
@@ -101,10 +91,24 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         let init = l.kind.init().map(|init| self.lower_expr(init));
         let hir_id = self.lower_node_id(l.id);
         let pat = self.lower_pat(&l.pat);
+        let els = if let LocalKind::InitElse(_, els) = &l.kind {
+            if !self.tcx.features().let_else {
+                feature_err(
+                    &self.tcx.sess.parse_sess,
+                    sym::let_else,
+                    l.span,
+                    "`let...else` statements are unstable",
+                )
+                .emit();
+            }
+            Some(self.lower_block(els, false))
+        } else {
+            None
+        };
         let span = self.lower_span(l.span);
         let source = hir::LocalSource::Normal;
         self.lower_attrs(hir_id, &l.attrs);
-        self.arena.alloc(hir::Local { hir_id, ty, pat, init, span, source })
+        self.arena.alloc(hir::Local { hir_id, ty, pat, init, els, span, source })
     }
 
     fn lower_block_check_mode(&mut self, b: &BlockCheckMode) -> hir::BlockCheckMode {
@@ -115,59 +119,4 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             }
         }
     }
-
-    fn lower_let_else(
-        &mut self,
-        stmt_hir_id: hir::HirId,
-        local: &Local,
-        init: &Expr,
-        els: &Block,
-        tail: &[Stmt],
-    ) -> &'hir hir::Expr<'hir> {
-        let ty = local
-            .ty
-            .as_ref()
-            .map(|t| self.lower_ty(t, ImplTraitContext::Disallowed(ImplTraitPosition::Variable)));
-        let span = self.lower_span(local.span);
-        let span = self.mark_span_with_reason(DesugaringKind::LetElse, span, None);
-        let init = self.lower_expr(init);
-        let local_hir_id = self.lower_node_id(local.id);
-        self.lower_attrs(local_hir_id, &local.attrs);
-        let let_expr = {
-            let lex = self.arena.alloc(hir::Let {
-                hir_id: local_hir_id,
-                pat: self.lower_pat(&local.pat),
-                ty,
-                init,
-                span,
-            });
-            self.arena.alloc(self.expr(span, hir::ExprKind::Let(lex), AttrVec::new()))
-        };
-        let then_expr = {
-            let (stmts, expr) = self.lower_stmts(tail);
-            let block = self.block_all(span, stmts, expr);
-            self.arena.alloc(self.expr_block(block, AttrVec::new()))
-        };
-        let else_expr = {
-            let block = self.lower_block(els, false);
-            self.arena.alloc(self.expr_block(block, AttrVec::new()))
-        };
-        self.alias_attrs(let_expr.hir_id, local_hir_id);
-        self.alias_attrs(else_expr.hir_id, local_hir_id);
-        let if_expr = self.arena.alloc(hir::Expr {
-            hir_id: stmt_hir_id,
-            span,
-            kind: hir::ExprKind::If(let_expr, then_expr, Some(else_expr)),
-        });
-        if !self.tcx.features().let_else {
-            feature_err(
-                &self.tcx.sess.parse_sess,
-                sym::let_else,
-                local.span,
-                "`let...else` statements are unstable",
-            )
-            .emit();
-        }
-        if_expr
-    }
 }

compiler/rustc_ast_lowering/src/lib.rs

@@ -2146,7 +2146,15 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             debug_assert!(!a.is_empty());
             self.attrs.insert(hir_id.local_id, a);
         }
-        let local = hir::Local { hir_id, init, pat, source, span: self.lower_span(span), ty: None };
+        let local = hir::Local {
+            hir_id,
+            init,
+            pat,
+            els: None,
+            source,
+            span: self.lower_span(span),
+            ty: None,
+        };
         self.stmt(span, hir::StmtKind::Local(self.arena.alloc(local)))
     }
 

compiler/rustc_attr/src/builtin.rs

@@ -856,7 +856,6 @@ pub enum ReprAttr {
     ReprSimd,
     ReprTransparent,
     ReprAlign(u32),
-    ReprNoNiche,
 }
 
 #[derive(Eq, PartialEq, Debug, Copy, Clone)]
@@ -904,7 +903,6 @@ pub fn parse_repr_attr(sess: &Session, attr: &Attribute) -> Vec<ReprAttr> {
                     sym::packed => Some(ReprPacked(1)),
                     sym::simd => Some(ReprSimd),
                     sym::transparent => Some(ReprTransparent),
-                    sym::no_niche => Some(ReprNoNiche),
                     sym::align => {
                         let mut err = struct_span_err!(
                             diagnostic,
@@ -943,7 +941,7 @@ pub fn parse_repr_attr(sess: &Session, attr: &Attribute) -> Vec<ReprAttr> {
                         Ok(literal) => acc.push(ReprPacked(literal)),
                         Err(message) => literal_error = Some(message),
                     };
-                } else if matches!(name, sym::C | sym::simd | sym::transparent | sym::no_niche)
+                } else if matches!(name, sym::C | sym::simd | sym::transparent)
                     || int_type_of_word(name).is_some()
                 {
                     recognised = true;
@@ -1001,7 +999,7 @@ pub fn parse_repr_attr(sess: &Session, attr: &Attribute) -> Vec<ReprAttr> {
                     } else {
                         if matches!(
                             meta_item.name_or_empty(),
-                            sym::C | sym::simd | sym::transparent | sym::no_niche
+                            sym::C | sym::simd | sym::transparent
                         ) || int_type_of_word(meta_item.name_or_empty()).is_some()
                         {
                             recognised = true;
@@ -1039,7 +1037,7 @@ pub fn parse_repr_attr(sess: &Session, attr: &Attribute) -> Vec<ReprAttr> {
                         .emit();
                     } else if matches!(
                         meta_item.name_or_empty(),
-                        sym::C | sym::simd | sym::transparent | sym::no_niche
+                        sym::C | sym::simd | sym::transparent
                     ) || int_type_of_word(meta_item.name_or_empty()).is_some()
                     {
                         recognised = true;

compiler/rustc_const_eval/src/interpret/intern.rs

@@ -217,7 +217,7 @@ impl<'rt, 'mir, 'tcx: 'mir, M: CompileTimeMachine<'mir, 'tcx, const_eval::Memory
         }
 
         if let Some(def) = mplace.layout.ty.ty_adt_def() {
-            if Some(def.did()) == self.ecx.tcx.lang_items().unsafe_cell_type() {
+            if def.is_unsafe_cell() {
                 // We are crossing over an `UnsafeCell`, we can mutate again. This means that
                 // References we encounter inside here are interned as pointing to mutable
                 // allocations.

compiler/rustc_const_eval/src/interpret/validity.rs

@@ -821,7 +821,7 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValueVisitor<'mir, 'tcx, M>
         // Special check preventing `UnsafeCell` in the inner part of constants
         if let Some(def) = op.layout.ty.ty_adt_def() {
             if matches!(self.ctfe_mode, Some(CtfeValidationMode::Const { inner: true, .. }))
-                && Some(def.did()) == self.ecx.tcx.lang_items().unsafe_cell_type()
+                && def.is_unsafe_cell()
             {
                 throw_validation_failure!(self.path, { "`UnsafeCell` in a `const`" });
             }

compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs

@@ -96,13 +96,13 @@ impl Qualif for HasMutInterior {
     }
 
     fn in_adt_inherently<'tcx>(
-        cx: &ConstCx<'_, 'tcx>,
+        _cx: &ConstCx<'_, 'tcx>,
         adt: AdtDef<'tcx>,
         _: SubstsRef<'tcx>,
     ) -> bool {
         // Exactly one type, `UnsafeCell`, has the `HasMutInterior` qualif inherently.
         // It arises structurally for all other types.
-        Some(adt.did()) == cx.tcx.lang_items().unsafe_cell_type()
+        adt.is_unsafe_cell()
     }
 }
 

compiler/rustc_feature/src/active.rs

@@ -156,9 +156,6 @@ declare_features! (
     (active, intrinsics, "1.0.0", None, None),
     /// Allows using `#[lang = ".."]` attribute for linking items to special compiler logic.
     (active, lang_items, "1.0.0", None, None),
-    /// Allows `#[repr(no_niche)]` (an implementation detail of `rustc`,
-    /// it is not on path for eventual stabilization).
-    (active, no_niche, "1.42.0", None, None),
     /// Allows using `#[omit_gdb_pretty_printer_section]`.
     (active, omit_gdb_pretty_printer_section, "1.5.0", None, None),
     /// Allows using `#[prelude_import]` on glob `use` items.

compiler/rustc_hir/src/hir.rs

@@ -1316,6 +1316,8 @@ pub struct Local<'hir> {
     pub ty: Option<&'hir Ty<'hir>>,
     /// Initializer expression to set the value, if any.
     pub init: Option<&'hir Expr<'hir>>,
+    /// Else block for a `let...else` binding.
+    pub els: Option<&'hir Block<'hir>>,
     pub hir_id: HirId,
     pub span: Span,
     /// Can be `ForLoopDesugar` if the `let` statement is part of a `for` loop

compiler/rustc_hir/src/intravisit.rs

@@ -472,6 +472,9 @@ pub fn walk_local<'v, V: Visitor<'v>>(visitor: &mut V, local: &'v Local<'v>) {
     walk_list!(visitor, visit_expr, &local.init);
     visitor.visit_id(local.hir_id);
     visitor.visit_pat(&local.pat);
+    if let Some(els) = local.els {
+        visitor.visit_block(els);
+    }
     walk_list!(visitor, visit_ty, &local.ty);
 }
 

compiler/rustc_hir_pretty/src/lib.rs

@@ -883,7 +883,12 @@ impl<'a> State<'a> {
         self.ann.post(self, AnnNode::SubItem(ii.hir_id()))
     }
 
-    pub fn print_local(&mut self, init: Option<&hir::Expr<'_>>, decl: impl Fn(&mut Self)) {
+    pub fn print_local(
+        &mut self,
+        init: Option<&hir::Expr<'_>>,
+        els: Option<&hir::Block<'_>>,
+        decl: impl Fn(&mut Self),
+    ) {
         self.space_if_not_bol();
         self.ibox(INDENT_UNIT);
         self.word_nbsp("let");
@@ -897,14 +902,21 @@ impl<'a> State<'a> {
             self.word_space("=");
             self.print_expr(init);
         }
+
+        if let Some(els) = els {
+            self.nbsp();
+            self.word_space("else");
+            self.print_block(els);
+        }
+
         self.end()
     }
 
     pub fn print_stmt(&mut self, st: &hir::Stmt<'_>) {
         self.maybe_print_comment(st.span.lo());
         match st.kind {
             hir::StmtKind::Local(loc) => {
-                self.print_local(loc.init, |this| this.print_local_decl(loc));
+                self.print_local(loc.init, loc.els, |this| this.print_local_decl(loc));
             }
             hir::StmtKind::Item(item) => self.ann.nested(self, Nested::Item(item)),
             hir::StmtKind::Expr(expr) => {
@@ -1404,7 +1416,7 @@ impl<'a> State<'a> {
 
                 // Print `let _t = $init;`:
                 let temp = Ident::from_str("_t");
-                self.print_local(Some(init), |this| this.print_ident(temp));
+                self.print_local(Some(init), None, |this| this.print_ident(temp));
                 self.word(";");
 
                 // Print `_t`:

compiler/rustc_lint/src/types.rs

@@ -703,9 +703,8 @@ fn ty_is_known_nonnull<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>, mode: CItemKi
                 return true;
             }
 
-            // Types with a `#[repr(no_niche)]` attribute have their niche hidden.
-            // The attribute is used by the UnsafeCell for example (the only use so far).
-            if def.repr().hide_niche() {
+            // `UnsafeCell` has its niche hidden.
+            if def.is_unsafe_cell() {
                 return false;
             }
 

compiler/rustc_middle/src/thir.rs

@@ -182,6 +182,9 @@ pub enum StmtKind<'tcx> {
         /// `let pat: ty = <INIT>`
         initializer: Option<ExprId>,
 
+        /// `let pat: ty = <INIT> else { <ELSE> }
+        else_block: Option<Block>,
+
         /// The lint level for this `let` statement.
         lint_level: LintLevel,
     },

compiler/rustc_middle/src/thir/visit.rs

@@ -167,11 +167,15 @@ pub fn walk_stmt<'a, 'tcx: 'a, V: Visitor<'a, 'tcx>>(visitor: &mut V, stmt: &Stm
             init_scope: _,
             ref pattern,
             lint_level: _,
+            else_block,
         } => {
             if let Some(init) = initializer {
                 visitor.visit_expr(&visitor.thir()[*init]);
             }
             visitor.visit_pat(pattern);
+            if let Some(block) = else_block {
+                visitor.visit_block(block)
+            }
         }
     }
 }

compiler/rustc_middle/src/ty/adt.rs

@@ -52,6 +52,8 @@ bitflags! {
         /// Indicates whether the variant list of this ADT is `#[non_exhaustive]`.
         /// (i.e., this flag is never set unless this ADT is an enum).
         const IS_VARIANT_LIST_NON_EXHAUSTIVE = 1 << 8;
+        /// Indicates whether the type is `UnsafeCell`.
+        const IS_UNSAFE_CELL              = 1 << 9;
     }
 }
 
@@ -247,6 +249,9 @@ impl AdtDefData {
         if Some(did) == tcx.lang_items().manually_drop() {
             flags |= AdtFlags::IS_MANUALLY_DROP;
         }
+        if Some(did) == tcx.lang_items().unsafe_cell_type() {
+            flags |= AdtFlags::IS_UNSAFE_CELL;
+        }
 
         AdtDefData { did, variants, flags, repr }
     }
@@ -333,6 +338,12 @@ impl<'tcx> AdtDef<'tcx> {
         self.flags().contains(AdtFlags::IS_BOX)
     }
 
+    /// Returns `true` if this is UnsafeCell<T>.
+    #[inline]
+    pub fn is_unsafe_cell(self) -> bool {
+        self.flags().contains(AdtFlags::IS_UNSAFE_CELL)
+    }
+
     /// Returns `true` if this is `ManuallyDrop<T>`.
     #[inline]
     pub fn is_manually_drop(self) -> bool {