Evaluate repeat expression lengths as late as possible

src/librustc/mir/mod.rs

@@ -2066,7 +2066,7 @@ pub enum Rvalue<'tcx> {
     Use(Operand<'tcx>),
 
     /// [x; 32]
-    Repeat(Operand<'tcx>, u64),
+    Repeat(Operand<'tcx>, &'tcx ty::Const<'tcx>),
 
     /// &x or &mut x
     Ref(Region<'tcx>, BorrowKind, Place<'tcx>),
@@ -2194,7 +2194,11 @@ impl<'tcx> Debug for Rvalue<'tcx> {
 
         match *self {
             Use(ref place) => write!(fmt, "{:?}", place),
-            Repeat(ref a, ref b) => write!(fmt, "[{:?}; {:?}]", a, b),
+            Repeat(ref a, ref b) => {
+                write!(fmt, "[{:?}; ", a)?;
+                pretty_print_const(b, fmt, false)?;
+                write!(fmt, "]")
+            }
             Len(ref a) => write!(fmt, "Len({:?})", a),
             Cast(ref kind, ref place, ref ty) => {
                 write!(fmt, "{:?} as {:?} ({:?})", place, ty, kind)
@@ -2562,18 +2566,26 @@ impl<'tcx> Debug for Constant<'tcx> {
 
 impl<'tcx> Display for Constant<'tcx> {
     fn fmt(&self, fmt: &mut Formatter<'_>) -> fmt::Result {
-        use crate::ty::print::PrettyPrinter;
         write!(fmt, "const ")?;
-        ty::tls::with(|tcx| {
-            let literal = tcx.lift(&self.literal).unwrap();
-            let mut cx = FmtPrinter::new(tcx, fmt, Namespace::ValueNS);
-            cx.print_alloc_ids = true;
-            cx.pretty_print_const(literal, true)?;
-            Ok(())
-        })
+        pretty_print_const(self.literal, fmt, true)
     }
 }
 
+fn pretty_print_const(
+    c: &ty::Const<'tcx>,
+    fmt: &mut Formatter<'_>,
+    print_types: bool,
+) -> fmt::Result {
+    use crate::ty::print::PrettyPrinter;
+    ty::tls::with(|tcx| {
+        let literal = tcx.lift(&c).unwrap();
+        let mut cx = FmtPrinter::new(tcx, fmt, Namespace::ValueNS);
+        cx.print_alloc_ids = true;
+        cx.pretty_print_const(literal, print_types)?;
+        Ok(())
+    })
+}
+
 impl<'tcx> graph::DirectedGraph for Body<'tcx> {
     type Node = BasicBlock;
 }

src/librustc/mir/tcx.rs

@@ -149,7 +149,9 @@ impl<'tcx> Rvalue<'tcx> {
     {
         match *self {
             Rvalue::Use(ref operand) => operand.ty(local_decls, tcx),
-            Rvalue::Repeat(ref operand, count) => tcx.mk_array(operand.ty(local_decls, tcx), count),
+            Rvalue::Repeat(ref operand, count) => {
+                tcx.mk_ty(ty::Array(operand.ty(local_decls, tcx), count))
+            }
             Rvalue::Ref(reg, bk, ref place) => {
                 let place_ty = place.ty(local_decls, tcx).ty;
                 tcx.mk_ref(reg, ty::TypeAndMut { ty: place_ty, mutbl: bk.to_mutbl_lossy() })

src/librustc/ty/sty.rs

@@ -8,7 +8,7 @@ use self::TyKind::*;
 use crate::infer::canonical::Canonical;
 use crate::middle::region;
 use crate::mir::interpret::ConstValue;
-use crate::mir::interpret::Scalar;
+use crate::mir::interpret::{LitToConstInput, Scalar};
 use crate::mir::Promoted;
 use crate::ty::layout::VariantIdx;
 use crate::ty::subst::{GenericArg, GenericArgKind, InternalSubsts, Subst, SubstsRef};
@@ -2401,7 +2401,75 @@ pub struct Const<'tcx> {
 #[cfg(target_arch = "x86_64")]
 static_assert_size!(Const<'_>, 48);
 
+/// Returns the `DefId` of the constant parameter that the provided expression is a path to.
+fn const_param_def_id(expr: &hir::Expr<'_>) -> Option<DefId> {
+    // Unwrap a block, so that e.g. `{ P }` is recognised as a parameter. Const arguments
+    // currently have to be wrapped in curly brackets, so it's necessary to special-case.
+    let expr = match &expr.kind {
+        hir::ExprKind::Block(block, _) if block.stmts.is_empty() && block.expr.is_some() => {
+            block.expr.as_ref().unwrap()
+        }
+        _ => expr,
+    };
+
+    match &expr.kind {
+        hir::ExprKind::Path(hir::QPath::Resolved(_, path)) => match path.res {
+            hir::def::Res::Def(hir::def::DefKind::ConstParam, did) => Some(did),
+            _ => None,
+        },
+        _ => None,
+    }
+}
+
 impl<'tcx> Const<'tcx> {
+    pub fn from_hir_anon_const(
+        tcx: TyCtxt<'tcx>,
+        ast_const: &hir::AnonConst,
+        ty: Ty<'tcx>,
+    ) -> &'tcx Self {
+        debug!("Const::from_hir_anon_const(id={:?}, ast_const={:?})", ast_const.hir_id, ast_const);
+
+        let def_id = tcx.hir().local_def_id(ast_const.hir_id);
+
+        let expr = &tcx.hir().body(ast_const.body).value;
+
+        let lit_input = match expr.kind {
+            hir::ExprKind::Lit(ref lit) => Some(LitToConstInput { lit: &lit.node, ty, neg: false }),
+            hir::ExprKind::Unary(hir::UnOp::UnNeg, ref expr) => match expr.kind {
+                hir::ExprKind::Lit(ref lit) => {
+                    Some(LitToConstInput { lit: &lit.node, ty, neg: true })
+                }
+                _ => None,
+            },
+            _ => None,
+        };
+
+        if let Some(lit_input) = lit_input {
+            // If an error occurred, ignore that it's a literal and leave reporting the error up to
+            // mir.
+            if let Ok(c) = tcx.at(expr.span).lit_to_const(lit_input) {
+                return c;
+            } else {
+                tcx.sess.delay_span_bug(expr.span, "ast_const_to_const: couldn't lit_to_const");
+            }
+        }
+
+        let kind = if let Some(def_id) = const_param_def_id(expr) {
+            // Find the name and index of the const parameter by indexing the generics of the
+            // parent item and construct a `ParamConst`.
+            let hir_id = tcx.hir().as_local_hir_id(def_id).unwrap();
+            let item_id = tcx.hir().get_parent_node(hir_id);
+            let item_def_id = tcx.hir().local_def_id(item_id);
+            let generics = tcx.generics_of(item_def_id);
+            let index = generics.param_def_id_to_index[&tcx.hir().local_def_id(hir_id)];
+            let name = tcx.hir().name(hir_id);
+            ty::ConstKind::Param(ty::ParamConst::new(index, name))
+        } else {
+            ty::ConstKind::Unevaluated(def_id, InternalSubsts::identity_for_item(tcx, def_id), None)
+        };
+        tcx.mk_const(ty::Const { val: kind, ty })
+    }
+
     #[inline]
     pub fn from_value(tcx: TyCtxt<'tcx>, val: ConstValue<'tcx>, ty: Ty<'tcx>) -> &'tcx Self {
         tcx.mk_const(Self { val: ConstKind::Value(val), ty })

src/librustc_codegen_ssa/mir/rvalue.rs

@@ -106,6 +106,9 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     }
                 }
 
+                let count =
+                    self.monomorphize(&count).eval_usize(bx.cx().tcx(), ty::ParamEnv::reveal_all());
+
                 bx.write_operand_repeatedly(cg_elem, count, dest)
             }
 

src/librustc_mir/borrow_check/type_check/mod.rs

@@ -1986,7 +1986,11 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             }
 
             Rvalue::Repeat(operand, len) => {
-                if *len > 1 {
+                // If the length cannot be evaluated we must assume that the length can be larger
+                // than 1.
+                // If the length is larger than 1, the repeat expression will need to copy the
+                // element, so we require the `Copy` trait.
+                if len.try_eval_usize(tcx, self.param_env).map_or(true, |len| len > 1) {
                     if let Operand::Move(_) = operand {
                         // While this is located in `nll::typeck` this error is not an NLL error, it's
                         // a required check to make sure that repeated elements implement `Copy`.

src/librustc_mir_build/hair/cx/expr.rs

@@ -3,7 +3,7 @@ use crate::hair::cx::to_ref::ToRef;
 use crate::hair::cx::Cx;
 use crate::hair::util::UserAnnotatedTyHelpers;
 use crate::hair::*;
-use rustc::mir::interpret::{ErrorHandled, Scalar};
+use rustc::mir::interpret::Scalar;
 use rustc::mir::BorrowKind;
 use rustc::ty::adjustment::{Adjust, Adjustment, AutoBorrow, AutoBorrowMutability, PointerCast};
 use rustc::ty::subst::{InternalSubsts, SubstsRef};
@@ -406,34 +406,7 @@ fn make_mirror_unadjusted<'a, 'tcx>(
 
         // Now comes the rote stuff:
         hir::ExprKind::Repeat(ref v, ref count) => {
-            let def_id = cx.tcx.hir().local_def_id(count.hir_id);
-            let substs = InternalSubsts::identity_for_item(cx.tcx, def_id);
-            let span = cx.tcx.def_span(def_id);
-            let count = match cx.tcx.const_eval_resolve(
-                ty::ParamEnv::reveal_all(),
-                def_id,
-                substs,
-                None,
-                Some(span),
-            ) {
-                Ok(cv) => {
-                    if let Some(count) = cv.try_to_bits_for_ty(
-                        cx.tcx,
-                        ty::ParamEnv::reveal_all(),
-                        cx.tcx.types.usize,
-                    ) {
-                        count as u64
-                    } else {
-                        bug!("repeat count constant value can't be converted to usize");
-                    }
-                }
-                Err(ErrorHandled::Reported) => 0,
-                Err(ErrorHandled::TooGeneric) => {
-                    let span = cx.tcx.def_span(def_id);
-                    cx.tcx.sess.span_err(span, "array lengths can't depend on generic parameters");
-                    0
-                }
-            };
+            let count = ty::Const::from_hir_anon_const(cx.tcx, count, cx.tcx.types.usize);
 
             ExprKind::Repeat { value: v.to_ref(), count }
         }

src/librustc_mir_build/hair/mod.rs

@@ -229,7 +229,7 @@ crate enum ExprKind<'tcx> {
     },
     Repeat {
         value: ExprRef<'tcx>,
-        count: u64,
+        count: &'tcx Const<'tcx>,
     },
     Array {
         fields: Vec<ExprRef<'tcx>>,

src/librustc_typeck/astconv.rs

@@ -22,7 +22,7 @@ use rustc_hir::def::{CtorOf, DefKind, Namespace, Res};
 use rustc_hir::def_id::DefId;
 use rustc_hir::intravisit::Visitor;
 use rustc_hir::print;
-use rustc_hir::{Constness, ExprKind, GenericArg, GenericArgs};
+use rustc_hir::{Constness, GenericArg, GenericArgs};
 use rustc_session::lint::builtin::{AMBIGUOUS_ASSOCIATED_ITEMS, LATE_BOUND_LIFETIME_ARGUMENTS};
 use rustc_session::parse::feature_err;
 use rustc_session::Session;
@@ -39,8 +39,6 @@ use std::collections::BTreeSet;
 use std::iter;
 use std::slice;
 
-use rustc::mir::interpret::LitToConstInput;
-
 #[derive(Debug)]
 pub struct PathSeg(pub DefId, pub usize);
 
@@ -782,7 +780,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                     }
                 }
                 (GenericParamDefKind::Const, GenericArg::Const(ct)) => {
-                    self.ast_const_to_const(&ct.value, tcx.type_of(param.def_id)).into()
+                    ty::Const::from_hir_anon_const(tcx, &ct.value, tcx.type_of(param.def_id)).into()
                 }
                 _ => unreachable!(),
             },
@@ -2766,7 +2764,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                     .unwrap_or(tcx.types.err)
             }
             hir::TyKind::Array(ref ty, ref length) => {
-                let length = self.ast_const_to_const(length, tcx.types.usize);
+                let length = ty::Const::from_hir_anon_const(tcx, length, tcx.types.usize);
                 let array_ty = tcx.mk_ty(ty::Array(self.ast_ty_to_ty(&ty), length));
                 self.normalize_ty(ast_ty.span, array_ty)
             }
@@ -2798,75 +2796,6 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         result_ty
     }
 
-    /// Returns the `DefId` of the constant parameter that the provided expression is a path to.
-    pub fn const_param_def_id(&self, expr: &hir::Expr<'_>) -> Option<DefId> {
-        // Unwrap a block, so that e.g. `{ P }` is recognised as a parameter. Const arguments
-        // currently have to be wrapped in curly brackets, so it's necessary to special-case.
-        let expr = match &expr.kind {
-            ExprKind::Block(block, _) if block.stmts.is_empty() && block.expr.is_some() => {
-                block.expr.as_ref().unwrap()
-            }
-            _ => expr,
-        };
-
-        match &expr.kind {
-            ExprKind::Path(hir::QPath::Resolved(_, path)) => match path.res {
-                Res::Def(DefKind::ConstParam, did) => Some(did),
-                _ => None,
-            },
-            _ => None,
-        }
-    }
-
-    pub fn ast_const_to_const(
-        &self,
-        ast_const: &hir::AnonConst,
-        ty: Ty<'tcx>,
-    ) -> &'tcx ty::Const<'tcx> {
-        debug!("ast_const_to_const(id={:?}, ast_const={:?})", ast_const.hir_id, ast_const);
-
-        let tcx = self.tcx();
-        let def_id = tcx.hir().local_def_id(ast_const.hir_id);
-
-        let expr = &tcx.hir().body(ast_const.body).value;
-
-        let lit_input = match expr.kind {
-            hir::ExprKind::Lit(ref lit) => Some(LitToConstInput { lit: &lit.node, ty, neg: false }),
-            hir::ExprKind::Unary(hir::UnOp::UnNeg, ref expr) => match expr.kind {
-                hir::ExprKind::Lit(ref lit) => {
-                    Some(LitToConstInput { lit: &lit.node, ty, neg: true })
-                }
-                _ => None,
-            },
-            _ => None,
-        };
-
-        if let Some(lit_input) = lit_input {
-            // If an error occurred, ignore that it's a literal and leave reporting the error up to
-            // mir.
-            if let Ok(c) = tcx.at(expr.span).lit_to_const(lit_input) {
-                return c;
-            } else {
-                tcx.sess.delay_span_bug(expr.span, "ast_const_to_const: couldn't lit_to_const");
-            }
-        }
-
-        let kind = if let Some(def_id) = self.const_param_def_id(expr) {
-            // Find the name and index of the const parameter by indexing the generics of the
-            // parent item and construct a `ParamConst`.
-            let hir_id = tcx.hir().as_local_hir_id(def_id).unwrap();
-            let item_id = tcx.hir().get_parent_node(hir_id);
-            let item_def_id = tcx.hir().local_def_id(item_id);
-            let generics = tcx.generics_of(item_def_id);
-            let index = generics.param_def_id_to_index[&tcx.hir().local_def_id(hir_id)];
-            let name = tcx.hir().name(hir_id);
-            ty::ConstKind::Param(ty::ParamConst::new(index, name))
-        } else {
-            ty::ConstKind::Unevaluated(def_id, InternalSubsts::identity_for_item(tcx, def_id), None)
-        };
-        tcx.mk_const(ty::Const { val: kind, ty })
-    }
-
     pub fn impl_trait_ty_to_ty(
         &self,
         def_id: DefId,

src/librustc_typeck/check/expr.rs

@@ -18,7 +18,6 @@ use crate::type_error_struct;
 use crate::util::common::ErrorReported;
 
 use rustc::middle::lang_items;
-use rustc::mir::interpret::ErrorHandled;
 use rustc::ty;
 use rustc::ty::adjustment::{Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability};
 use rustc::ty::Ty;
@@ -1009,12 +1008,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     ) -> Ty<'tcx> {
         let tcx = self.tcx;
         let count_def_id = tcx.hir().local_def_id(count.hir_id);
-        let count = if self.const_param_def_id(count).is_some() {
-            Ok(self.to_const(count, tcx.type_of(count_def_id)))
-        } else {
-            tcx.const_eval_poly(count_def_id)
-                .map(|val| ty::Const::from_value(tcx, val, tcx.type_of(count_def_id)))
-        };
+        let count = self.to_const(count, tcx.type_of(count_def_id));
 
         let uty = match expected {
             ExpectHasType(uty) => match uty.kind {
@@ -1042,17 +1036,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         if element_ty.references_error() {
             return tcx.types.err;
         }
-        match count {
-            Ok(count) => tcx.mk_ty(ty::Array(t, count)),
-            Err(ErrorHandled::TooGeneric) => {
-                self.tcx.sess.span_err(
-                    tcx.def_span(count_def_id),
-                    "array lengths can't depend on generic parameters",
-                );
-                tcx.types.err
-            }
-            Err(ErrorHandled::Reported) => tcx.types.err,
-        }
+
+        tcx.mk_ty(ty::Array(t, count))
     }
 
     fn check_expr_tuple(

src/librustc_typeck/check/mod.rs

@@ -3279,13 +3279,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         ty
     }
 
-    /// Returns the `DefId` of the constant parameter that the provided expression is a path to.
-    pub fn const_param_def_id(&self, hir_c: &hir::AnonConst) -> Option<DefId> {
-        AstConv::const_param_def_id(self, &self.tcx.hir().body(hir_c.body).value)
-    }
-
     pub fn to_const(&self, ast_c: &hir::AnonConst, ty: Ty<'tcx>) -> &'tcx ty::Const<'tcx> {
-        AstConv::ast_const_to_const(self, ast_c, ty)
+        ty::Const::from_hir_anon_const(self.tcx, ast_c, ty)
     }
 
     // If the type given by the user has free regions, save it for later, since