Auto merge of #94876 - b-naber:thir-abstract-const-changes, r=lcnr

Change Thir to lazily create constants

To allow `AbstractConst`s to work with the previous thir changes we made and those we want to make, i.e. to avoid problems due to `ValTree` and `ConstValue` conversions, we instead switch to a thir representation for constants that allows us to lazily create constants.

r? `@oli-obk`

Author: bors

compiler/rustc_middle/src/query/mod.rs

@@ -955,6 +955,7 @@ rustc_queries! {
         desc { "get a &core::panic::Location referring to a span" }
     }
 
+    // FIXME get rid of this with valtrees
     query lit_to_const(
         key: LitToConstInput<'tcx>
     ) -> Result<ty::Const<'tcx>, LitToConstError> {

compiler/rustc_middle/src/thir.rs

@@ -369,7 +369,8 @@ pub enum ExprKind<'tcx> {
     },
     /// An inline `const` block, e.g. `const {}`.
     ConstBlock {
-        value: Const<'tcx>,
+        did: DefId,
+        substs: SubstsRef<'tcx>,
     },
     /// An array literal constructed from one repeated element, e.g. `[1; 5]`.
     Repeat {
@@ -408,13 +409,25 @@ pub enum ExprKind<'tcx> {
     },
     /// A literal.
     Literal {
-        literal: Const<'tcx>,
+        lit: &'tcx hir::Lit,
+        neg: bool,
+    },
+    /// For literals that don't correspond to anything in the HIR
+    NonHirLiteral {
+        lit: ty::ScalarInt,
+        user_ty: Option<Canonical<'tcx, UserType<'tcx>>>,
+    },
+    /// Associated constants and named constants
+    NamedConst {
+        def_id: DefId,
+        substs: SubstsRef<'tcx>,
         user_ty: Option<Canonical<'tcx, UserType<'tcx>>>,
-        /// The `DefId` of the `const` item this literal
-        /// was produced from, if this is not a user-written
-        /// literal value.
-        const_id: Option<DefId>,
     },
+    ConstParam {
+        param: ty::ParamConst,
+        def_id: DefId,
+    },
+    // FIXME improve docs for `StaticRef` by distinguishing it from `NamedConst`
     /// A literal containing the address of a `static`.
     ///
     /// This is only distinguished from `Literal` so that we can register some
@@ -439,6 +452,12 @@ pub enum ExprKind<'tcx> {
     },
 }
 
+impl<'tcx> ExprKind<'tcx> {
+    pub fn zero_sized_literal(user_ty: Option<Canonical<'tcx, UserType<'tcx>>>) -> Self {
+        ExprKind::NonHirLiteral { lit: ty::ScalarInt::ZST, user_ty }
+    }
+}
+
 /// Represents the association of a field identifier and an expression.
 ///
 /// This is used in struct constructors.

compiler/rustc_middle/src/thir/visit.rs

@@ -1,7 +1,6 @@
 use super::{
     Arm, Block, Expr, ExprKind, Guard, InlineAsmOperand, Pat, PatKind, Stmt, StmtKind, Thir,
 };
-use rustc_middle::ty::Const;
 
 pub trait Visitor<'a, 'tcx: 'a>: Sized {
     fn thir(&self) -> &'a Thir<'tcx>;
@@ -25,8 +24,6 @@ pub trait Visitor<'a, 'tcx: 'a>: Sized {
     fn visit_pat(&mut self, pat: &Pat<'tcx>) {
         walk_pat(self, pat);
     }
-
-    fn visit_const(&mut self, _cnst: Const<'tcx>) {}
 }
 
 pub fn walk_expr<'a, 'tcx: 'a, V: Visitor<'a, 'tcx>>(visitor: &mut V, expr: &Expr<'tcx>) {
@@ -93,10 +90,9 @@ pub fn walk_expr<'a, 'tcx: 'a, V: Visitor<'a, 'tcx>>(visitor: &mut V, expr: &Exp
                 visitor.visit_expr(&visitor.thir()[value])
             }
         }
-        ConstBlock { value } => visitor.visit_const(value),
-        Repeat { value, count } => {
+        ConstBlock { did: _, substs: _ } => {}
+        Repeat { value, count: _ } => {
             visitor.visit_expr(&visitor.thir()[value]);
-            visitor.visit_const(count);
         }
         Array { ref fields } | Tuple { ref fields } => {
             for &field in &**fields {
@@ -122,7 +118,10 @@ pub fn walk_expr<'a, 'tcx: 'a, V: Visitor<'a, 'tcx>>(visitor: &mut V, expr: &Exp
             visitor.visit_expr(&visitor.thir()[source])
         }
         Closure { closure_id: _, substs: _, upvars: _, movability: _, fake_reads: _ } => {}
-        Literal { literal, user_ty: _, const_id: _ } => visitor.visit_const(literal),
+        Literal { lit: _, neg: _ } => {}
+        NonHirLiteral { lit: _, user_ty: _ } => {}
+        NamedConst { def_id: _, substs: _, user_ty: _ } => {}
+        ConstParam { param: _, def_id: _ } => {}
         StaticRef { alloc_id: _, ty: _, def_id: _ } => {}
         InlineAsm { ref operands, template: _, options: _, line_spans: _ } => {
             for op in &**operands {
@@ -209,11 +208,8 @@ pub fn walk_pat<'a, 'tcx: 'a, V: Visitor<'a, 'tcx>>(visitor: &mut V, pat: &Pat<'
                 visitor.visit_pat(&subpattern.pattern);
             }
         }
-        Constant { value } => visitor.visit_const(*value),
-        Range(range) => {
-            visitor.visit_const(range.lo);
-            visitor.visit_const(range.hi);
-        }
+        Constant { value: _ } => {}
+        Range(_) => {}
         Slice { prefix, slice, suffix } | Array { prefix, slice, suffix } => {
             for subpattern in prefix {
                 visitor.visit_pat(&subpattern);

compiler/rustc_mir_build/src/build/expr/as_constant.rs

@@ -1,43 +1,141 @@
 //! See docs in build/expr/mod.rs
 
 use crate::build::Builder;
-use rustc_middle::mir::interpret::{ConstValue, Scalar};
+use crate::thir::constant::parse_float;
+use rustc_ast::ast;
+use rustc_hir::def_id::DefId;
+use rustc_middle::mir::interpret::{
+    Allocation, ConstValue, LitToConstError, LitToConstInput, Scalar,
+};
 use rustc_middle::mir::*;
 use rustc_middle::thir::*;
-use rustc_middle::ty::CanonicalUserTypeAnnotation;
+use rustc_middle::ty::subst::SubstsRef;
+use rustc_middle::ty::{self, CanonicalUserTypeAnnotation, Ty, TyCtxt};
+use rustc_target::abi::Size;
 
 impl<'a, 'tcx> Builder<'a, 'tcx> {
     /// Compile `expr`, yielding a compile-time constant. Assumes that
     /// `expr` is a valid compile-time constant!
     crate fn as_constant(&mut self, expr: &Expr<'tcx>) -> Constant<'tcx> {
+        let create_uneval_from_def_id =
+            |tcx: TyCtxt<'tcx>, def_id: DefId, ty: Ty<'tcx>, substs: SubstsRef<'tcx>| {
+                let uneval = ty::Unevaluated::new(ty::WithOptConstParam::unknown(def_id), substs);
+                tcx.mk_const(ty::ConstS { val: ty::ConstKind::Unevaluated(uneval), ty })
+            };
+
         let this = self;
+        let tcx = this.tcx;
         let Expr { ty, temp_lifetime: _, span, ref kind } = *expr;
         match *kind {
             ExprKind::Scope { region_scope: _, lint_level: _, value } => {
                 this.as_constant(&this.thir[value])
             }
-            ExprKind::Literal { literal, user_ty, const_id: _ } => {
+            ExprKind::Literal { lit, neg } => {
+                let literal =
+                    match lit_to_constant(tcx, LitToConstInput { lit: &lit.node, ty, neg }) {
+                        Ok(c) => c,
+                        Err(LitToConstError::Reported) => ConstantKind::Ty(tcx.const_error(ty)),
+                        Err(LitToConstError::TypeError) => {
+                            bug!("encountered type error in `lit_to_constant")
+                        }
+                    };
+
+                Constant { span, user_ty: None, literal: literal.into() }
+            }
+            ExprKind::NonHirLiteral { lit, user_ty } => {
                 let user_ty = user_ty.map(|user_ty| {
                     this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
                         span,
                         user_ty,
                         inferred_ty: ty,
                     })
                 });
-                assert_eq!(literal.ty(), ty);
-                Constant { span, user_ty, literal: literal.into() }
+
+                let literal = ConstantKind::Val(ConstValue::Scalar(Scalar::Int(lit)), ty);
+
+                Constant { span, user_ty: user_ty, literal }
+            }
+            ExprKind::NamedConst { def_id, substs, user_ty } => {
+                let user_ty = user_ty.map(|user_ty| {
+                    this.canonical_user_type_annotations.push(CanonicalUserTypeAnnotation {
+                        span,
+                        user_ty,
+                        inferred_ty: ty,
+                    })
+                });
+                let literal = ConstantKind::Ty(create_uneval_from_def_id(tcx, def_id, ty, substs));
+
+                Constant { user_ty, span, literal }
+            }
+            ExprKind::ConstParam { param, def_id: _ } => {
+                let const_param =
+                    tcx.mk_const(ty::ConstS { val: ty::ConstKind::Param(param), ty: expr.ty });
+                let literal = ConstantKind::Ty(const_param);
+
+                Constant { user_ty: None, span, literal }
+            }
+            ExprKind::ConstBlock { did: def_id, substs } => {
+                let literal = ConstantKind::Ty(create_uneval_from_def_id(tcx, def_id, ty, substs));
+
+                Constant { user_ty: None, span, literal }
             }
             ExprKind::StaticRef { alloc_id, ty, .. } => {
-                let const_val =
-                    ConstValue::Scalar(Scalar::from_pointer(alloc_id.into(), &this.tcx));
+                let const_val = ConstValue::Scalar(Scalar::from_pointer(alloc_id.into(), &tcx));
                 let literal = ConstantKind::Val(const_val, ty);
 
                 Constant { span, user_ty: None, literal }
             }
-            ExprKind::ConstBlock { value } => {
-                Constant { span: span, user_ty: None, literal: value.into() }
-            }
             _ => span_bug!(span, "expression is not a valid constant {:?}", kind),
         }
     }
 }
+
+crate fn lit_to_constant<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    lit_input: LitToConstInput<'tcx>,
+) -> Result<ConstantKind<'tcx>, LitToConstError> {
+    let LitToConstInput { lit, ty, neg } = lit_input;
+    let trunc = |n| {
+        let param_ty = ty::ParamEnv::reveal_all().and(ty);
+        let width = tcx.layout_of(param_ty).map_err(|_| LitToConstError::Reported)?.size;
+        trace!("trunc {} with size {} and shift {}", n, width.bits(), 128 - width.bits());
+        let result = width.truncate(n);
+        trace!("trunc result: {}", result);
+        Ok(ConstValue::Scalar(Scalar::from_uint(result, width)))
+    };
+
+    let value = match (lit, &ty.kind()) {
+        (ast::LitKind::Str(s, _), ty::Ref(_, inner_ty, _)) if inner_ty.is_str() => {
+            let s = s.as_str();
+            let allocation = Allocation::from_bytes_byte_aligned_immutable(s.as_bytes());
+            let allocation = tcx.intern_const_alloc(allocation);
+            ConstValue::Slice { data: allocation, start: 0, end: s.len() }
+        }
+        (ast::LitKind::ByteStr(data), ty::Ref(_, inner_ty, _))
+            if matches!(inner_ty.kind(), ty::Slice(_)) =>
+        {
+            let allocation = Allocation::from_bytes_byte_aligned_immutable(data as &[u8]);
+            let allocation = tcx.intern_const_alloc(allocation);
+            ConstValue::Slice { data: allocation, start: 0, end: data.len() }
+        }
+        (ast::LitKind::ByteStr(data), ty::Ref(_, inner_ty, _)) if inner_ty.is_array() => {
+            let id = tcx.allocate_bytes(data);
+            ConstValue::Scalar(Scalar::from_pointer(id.into(), &tcx))
+        }
+        (ast::LitKind::Byte(n), ty::Uint(ty::UintTy::U8)) => {
+            ConstValue::Scalar(Scalar::from_uint(*n, Size::from_bytes(1)))
+        }
+        (ast::LitKind::Int(n, _), ty::Uint(_)) | (ast::LitKind::Int(n, _), ty::Int(_)) => {
+            trunc(if neg { (*n as i128).overflowing_neg().0 as u128 } else { *n })?
+        }
+        (ast::LitKind::Float(n, _), ty::Float(fty)) => {
+            parse_float(*n, *fty, neg).ok_or(LitToConstError::Reported)?
+        }
+        (ast::LitKind::Bool(b), ty::Bool) => ConstValue::Scalar(Scalar::from_bool(*b)),
+        (ast::LitKind::Char(c), ty::Char) => ConstValue::Scalar(Scalar::from_char(*c)),
+        (ast::LitKind::Err(_), _) => return Err(LitToConstError::Reported),
+        _ => return Err(LitToConstError::TypeError),
+    };
+
+    Ok(ConstantKind::Val(value, ty))
+}

compiler/rustc_mir_build/src/build/expr/as_place.rs

@@ -566,6 +566,9 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             | ExprKind::Continue { .. }
             | ExprKind::Return { .. }
             | ExprKind::Literal { .. }
+            | ExprKind::NamedConst { .. }
+            | ExprKind::NonHirLiteral { .. }
+            | ExprKind::ConstParam { .. }
             | ExprKind::ConstBlock { .. }
             | ExprKind::StaticRef { .. }
             | ExprKind::InlineAsm { .. }

compiler/rustc_mir_build/src/build/expr/as_rvalue.rs

@@ -327,6 +327,9 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             }
             ExprKind::Yield { .. }
             | ExprKind::Literal { .. }
+            | ExprKind::NamedConst { .. }
+            | ExprKind::NonHirLiteral { .. }
+            | ExprKind::ConstParam { .. }
             | ExprKind::ConstBlock { .. }
             | ExprKind::StaticRef { .. }
             | ExprKind::Block { .. }

compiler/rustc_mir_build/src/build/expr/as_temp.rs

@@ -70,7 +70,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     local_decl.local_info =
                         Some(Box::new(LocalInfo::StaticRef { def_id, is_thread_local: true }));
                 }
-                ExprKind::Literal { const_id: Some(def_id), .. } => {
+                ExprKind::NamedConst { def_id, .. } | ExprKind::ConstParam { def_id, .. } => {
                     local_decl.local_info = Some(Box::new(LocalInfo::ConstRef { def_id }));
                 }
                 _ => {}

compiler/rustc_mir_build/src/build/expr/category.rs

@@ -69,9 +69,12 @@ impl Category {
             | ExprKind::AssignOp { .. }
             | ExprKind::ThreadLocalRef(_) => Some(Category::Rvalue(RvalueFunc::AsRvalue)),
 
-            ExprKind::ConstBlock { .. } | ExprKind::Literal { .. } | ExprKind::StaticRef { .. } => {
-                Some(Category::Constant)
-            }
+            ExprKind::ConstBlock { .. }
+            | ExprKind::Literal { .. }
+            | ExprKind::NonHirLiteral { .. }
+            | ExprKind::ConstParam { .. }
+            | ExprKind::StaticRef { .. }
+            | ExprKind::NamedConst { .. } => Some(Category::Constant),
 
             ExprKind::Loop { .. }
             | ExprKind::Block { .. }

compiler/rustc_mir_build/src/build/expr/into.rs

@@ -533,6 +533,9 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             | ExprKind::Closure { .. }
             | ExprKind::ConstBlock { .. }
             | ExprKind::Literal { .. }
+            | ExprKind::NamedConst { .. }
+            | ExprKind::NonHirLiteral { .. }
+            | ExprKind::ConstParam { .. }
             | ExprKind::ThreadLocalRef(_)
             | ExprKind::StaticRef { .. } => {
                 debug_assert!(match Category::of(&expr.kind).unwrap() {

compiler/rustc_mir_build/src/build/expr/mod.rs

@@ -60,7 +60,7 @@
 //! basically the point where the "by value" operations are bridged
 //! over to the "by reference" mode (`as_place`).
 
-mod as_constant;
+crate mod as_constant;
 mod as_operand;
 pub mod as_place;
 mod as_rvalue;

compiler/rustc_mir_build/src/check_unsafety.rs

@@ -303,6 +303,9 @@ impl<'a, 'tcx> Visitor<'a, 'tcx> for UnsafetyVisitor<'a, 'tcx> {
             | ExprKind::Block { .. }
             | ExprKind::Borrow { .. }
             | ExprKind::Literal { .. }
+            | ExprKind::NamedConst { .. }
+            | ExprKind::NonHirLiteral { .. }
+            | ExprKind::ConstParam { .. }
             | ExprKind::ConstBlock { .. }
             | ExprKind::Deref { .. }
             | ExprKind::Index { .. }

compiler/rustc_mir_build/src/thir/constant.rs

@@ -7,6 +7,7 @@ use rustc_middle::ty::{self, ParamEnv, TyCtxt};
 use rustc_span::symbol::Symbol;
 use rustc_target::abi::Size;
 
+// FIXME Once valtrees are available, get rid of this function and the query
 crate fn lit_to_const<'tcx>(
     tcx: TyCtxt<'tcx>,
     lit_input: LitToConstInput<'tcx>,
@@ -57,7 +58,12 @@ crate fn lit_to_const<'tcx>(
     Ok(ty::Const::from_value(tcx, lit, ty))
 }
 
-fn parse_float<'tcx>(num: Symbol, fty: ty::FloatTy, neg: bool) -> Option<ConstValue<'tcx>> {
+// FIXME move this to rustc_mir_build::build
+pub(crate) fn parse_float<'tcx>(
+    num: Symbol,
+    fty: ty::FloatTy,
+    neg: bool,
+) -> Option<ConstValue<'tcx>> {
     let num = num.as_str();
     use rustc_apfloat::ieee::{Double, Single};
     let scalar = match fty {

compiler/rustc_mir_build/src/thir/cx/expr.rs

@@ -14,7 +14,9 @@ use rustc_middle::ty::adjustment::{
     Adjust, Adjustment, AutoBorrow, AutoBorrowMutability, PointerCast,
 };
 use rustc_middle::ty::subst::{InternalSubsts, SubstsRef};
-use rustc_middle::ty::{self, AdtKind, Ty, UpvarSubsts, UserType};
+use rustc_middle::ty::{
+    self, AdtKind, InlineConstSubsts, InlineConstSubstsParts, ScalarInt, Ty, UpvarSubsts, UserType,
+};
 use rustc_span::def_id::DefId;
 use rustc_span::Span;
 use rustc_target::abi::VariantIdx;
@@ -290,11 +292,7 @@ impl<'tcx> Cx<'tcx> {
                 }
             }
 
-            hir::ExprKind::Lit(ref lit) => ExprKind::Literal {
-                literal: self.const_eval_literal(&lit.node, expr_ty, lit.span, false),
-                user_ty: None,
-                const_id: None,
-            },
+            hir::ExprKind::Lit(ref lit) => ExprKind::Literal { lit, neg: false },
 
             hir::ExprKind::Binary(op, ref lhs, ref rhs) => {
                 if self.typeck_results().is_method_call(expr) {
@@ -359,11 +357,7 @@ impl<'tcx> Cx<'tcx> {
                     let arg = self.mirror_expr(arg);
                     self.overloaded_operator(expr, Box::new([arg]))
                 } else if let hir::ExprKind::Lit(ref lit) = arg.kind {
-                    ExprKind::Literal {
-                        literal: self.const_eval_literal(&lit.node, expr_ty, lit.span, true),
-                        user_ty: None,
-                        const_id: None,
-                    }
+                    ExprKind::Literal { lit, neg: true }
                 } else {
                     ExprKind::Unary { op: UnOp::Neg, arg: self.mirror_expr(arg) }
                 }
@@ -524,11 +518,7 @@ impl<'tcx> Cx<'tcx> {
                                                 ty,
                                                 temp_lifetime,
                                                 span: expr.span,
-                                                kind: ExprKind::Literal {
-                                                    literal: ty::Const::zero_sized(self.tcx, ty),
-                                                    user_ty,
-                                                    const_id: None,
-                                                },
+                                                kind: ExprKind::zero_sized_literal(user_ty),
                                             }),
                                         }
                                     }
@@ -550,11 +540,7 @@ impl<'tcx> Cx<'tcx> {
                                                 ty,
                                                 temp_lifetime,
                                                 span: expr.span,
-                                                kind: ExprKind::Literal {
-                                                    literal: ty::Const::zero_sized(self.tcx, ty),
-                                                    user_ty: None,
-                                                    const_id: None,
-                                                },
+                                                kind: ExprKind::zero_sized_literal(None),
                                             }),
                                         }
                                     }
@@ -568,13 +554,21 @@ impl<'tcx> Cx<'tcx> {
             },
 
             hir::ExprKind::ConstBlock(ref anon_const) => {
-                let anon_const_def_id = self.tcx.hir().local_def_id(anon_const.hir_id);
-
-                // FIXME Do we want to use `from_inline_const` once valtrees
-                // are introduced? This would create `ValTree`s that will never be used...
-                let value = ty::Const::from_inline_const(self.tcx, anon_const_def_id);
-
-                ExprKind::ConstBlock { value }
+                let tcx = self.tcx;
+                let local_def_id = tcx.hir().local_def_id(anon_const.hir_id);
+                let anon_const_def_id = local_def_id.to_def_id();
+
+                // Need to include the parent substs
+                let hir_id = tcx.hir().local_def_id_to_hir_id(local_def_id);
+                let ty = tcx.typeck(local_def_id).node_type(hir_id);
+                let typeck_root_def_id = tcx.typeck_root_def_id(anon_const_def_id);
+                let parent_substs =
+                    tcx.erase_regions(InternalSubsts::identity_for_item(tcx, typeck_root_def_id));
+                let substs =
+                    InlineConstSubsts::new(tcx, InlineConstSubstsParts { parent_substs, ty })
+                        .substs;
+
+                ExprKind::ConstBlock { did: anon_const_def_id, substs }
             }
             // Now comes the rote stuff:
             hir::ExprKind::Repeat(ref v, _) => {
@@ -692,32 +686,36 @@ impl<'tcx> Cx<'tcx> {
                     };
 
                     let source = if let Some((did, offset, var_ty)) = var {
-                        let mk_const = |literal| Expr {
+                        let param_env_ty = self.param_env.and(var_ty);
+                        let size = self
+                            .tcx
+                            .layout_of(param_env_ty)
+                            .unwrap_or_else(|e| {
+                                panic!("could not compute layout for {:?}: {:?}", param_env_ty, e)
+                            })
+                            .size;
+                        let lit = ScalarInt::try_from_uint(offset as u128, size).unwrap();
+                        let kind = ExprKind::NonHirLiteral { lit, user_ty: None };
+                        let offset = self.thir.exprs.push(Expr {
                             temp_lifetime,
                             ty: var_ty,
                             span: expr.span,
-                            kind: ExprKind::Literal { literal, user_ty: None, const_id: None },
-                        };
-                        let offset = self.thir.exprs.push(mk_const(ty::Const::from_bits(
-                            self.tcx,
-                            offset as u128,
-                            self.param_env.and(var_ty),
-                        )));
+                            kind,
+                        });
                         match did {
                             Some(did) => {
                                 // in case we are offsetting from a computed discriminant
                                 // and not the beginning of discriminants (which is always `0`)
                                 let substs = InternalSubsts::identity_for_item(self.tcx(), did);
-                                let lhs = ty::ConstS {
-                                    val: ty::ConstKind::Unevaluated(ty::Unevaluated::new(
-                                        ty::WithOptConstParam::unknown(did),
-                                        substs,
-                                    )),
+                                let kind =
+                                    ExprKind::NamedConst { def_id: did, substs, user_ty: None };
+                                let lhs = self.thir.exprs.push(Expr {
+                                    temp_lifetime,
                                     ty: var_ty,
-                                };
-                                let lhs = self.thir.exprs.push(mk_const(self.tcx().mk_const(lhs)));
-                                let bin =
-                                    ExprKind::Binary { op: BinOp::Add, lhs: lhs, rhs: offset };
+                                    span: expr.span,
+                                    kind,
+                                });
+                                let bin = ExprKind::Binary { op: BinOp::Add, lhs, rhs: offset };
                                 self.thir.exprs.push(Expr {
                                     temp_lifetime,
                                     ty: var_ty,
@@ -832,16 +830,7 @@ impl<'tcx> Cx<'tcx> {
             }
         };
         let ty = self.tcx().mk_fn_def(def_id, substs);
-        Expr {
-            temp_lifetime,
-            ty,
-            span,
-            kind: ExprKind::Literal {
-                literal: ty::Const::zero_sized(self.tcx(), ty),
-                user_ty,
-                const_id: None,
-            },
-        }
+        Expr { temp_lifetime, ty, span, kind: ExprKind::zero_sized_literal(user_ty) }
     }
 
     fn convert_arm(&mut self, arm: &'tcx hir::Arm<'tcx>) -> ArmId {
@@ -868,17 +857,9 @@ impl<'tcx> Cx<'tcx> {
             Res::Def(DefKind::Fn, _)
             | Res::Def(DefKind::AssocFn, _)
             | Res::Def(DefKind::Ctor(_, CtorKind::Fn), _)
-            | Res::SelfCtor(..) => {
+            | Res::SelfCtor(_) => {
                 let user_ty = self.user_substs_applied_to_res(expr.hir_id, res);
-                debug!("convert_path_expr: user_ty={:?}", user_ty);
-                ExprKind::Literal {
-                    literal: ty::Const::zero_sized(
-                        self.tcx,
-                        self.typeck_results().node_type(expr.hir_id),
-                    ),
-                    user_ty,
-                    const_id: None,
-                }
+                ExprKind::zero_sized_literal(user_ty)
             }
 
             Res::Def(DefKind::ConstParam, def_id) => {
@@ -888,31 +869,14 @@ impl<'tcx> Cx<'tcx> {
                 let generics = self.tcx.generics_of(item_def_id);
                 let index = generics.param_def_id_to_index[&def_id];
                 let name = self.tcx.hir().name(hir_id);
-                let val = ty::ConstKind::Param(ty::ParamConst::new(index, name));
-                ExprKind::Literal {
-                    literal: self.tcx.mk_const(ty::ConstS {
-                        val,
-                        ty: self.typeck_results().node_type(expr.hir_id),
-                    }),
-                    user_ty: None,
-                    const_id: Some(def_id),
-                }
+                let param = ty::ParamConst::new(index, name);
+
+                ExprKind::ConstParam { param, def_id }
             }
 
             Res::Def(DefKind::Const, def_id) | Res::Def(DefKind::AssocConst, def_id) => {
                 let user_ty = self.user_substs_applied_to_res(expr.hir_id, res);
-                debug!("convert_path_expr: (const) user_ty={:?}", user_ty);
-                ExprKind::Literal {
-                    literal: self.tcx.mk_const(ty::ConstS {
-                        val: ty::ConstKind::Unevaluated(ty::Unevaluated::new(
-                            ty::WithOptConstParam::unknown(def_id),
-                            substs,
-                        )),
-                        ty: self.typeck_results().node_type(expr.hir_id),
-                    }),
-                    user_ty,
-                    const_id: Some(def_id),
-                }
+                ExprKind::NamedConst { def_id, substs, user_ty: user_ty }
             }
 
             Res::Def(DefKind::Ctor(_, CtorKind::Const), def_id) => {

compiler/rustc_mir_build/src/thir/cx/mod.rs

@@ -5,17 +5,15 @@
 use crate::thir::pattern::pat_from_hir;
 use crate::thir::util::UserAnnotatedTyHelpers;
 
-use rustc_ast as ast;
 use rustc_data_structures::steal::Steal;
 use rustc_errors::ErrorGuaranteed;
 use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::HirId;
 use rustc_hir::Node;
 use rustc_middle::middle::region;
-use rustc_middle::mir::interpret::{LitToConstError, LitToConstInput};
 use rustc_middle::thir::*;
-use rustc_middle::ty::{self, Ty, TyCtxt};
+use rustc_middle::ty::{self, TyCtxt};
 use rustc_span::Span;
 
 crate fn thir_body<'tcx>(
@@ -77,25 +75,6 @@ impl<'tcx> Cx<'tcx> {
         }
     }
 
-    crate fn const_eval_literal(
-        &mut self,
-        lit: &'tcx ast::LitKind,
-        ty: Ty<'tcx>,
-        sp: Span,
-        neg: bool,
-    ) -> ty::Const<'tcx> {
-        trace!("const_eval_literal: {:#?}, {:?}, {:?}, {:?}", lit, ty, sp, neg);
-
-        match self.tcx.at(sp).lit_to_const(LitToConstInput { lit, ty, neg }) {
-            Ok(c) => c,
-            Err(LitToConstError::Reported) => {
-                // create a dummy value and continue compiling
-                self.tcx.const_error(ty)
-            }
-            Err(LitToConstError::TypeError) => bug!("const_eval_literal: had type error"),
-        }
-    }
-
     crate fn pattern_from_hir(&mut self, p: &hir::Pat<'_>) -> Pat<'tcx> {
         let p = match self.tcx.hir().get(p.hir_id) {
             Node::Pat(p) | Node::Binding(p) => p,

compiler/rustc_trait_selection/src/traits/const_evaluatable.rs

@@ -14,7 +14,9 @@ use rustc_hir::def::DefKind;
 use rustc_index::vec::IndexVec;
 use rustc_infer::infer::InferCtxt;
 use rustc_middle::mir;
-use rustc_middle::mir::interpret::ErrorHandled;
+use rustc_middle::mir::interpret::{
+    ConstValue, ErrorHandled, LitToConstError, LitToConstInput, Scalar,
+};
 use rustc_middle::thir;
 use rustc_middle::thir::abstract_const::{self, Node, NodeId, NotConstEvaluatable};
 use rustc_middle::ty::subst::{Subst, SubstsRef};
@@ -28,13 +30,13 @@ use std::iter;
 use std::ops::ControlFlow;
 
 /// Check if a given constant can be evaluated.
+#[instrument(skip(infcx), level = "debug")]
 pub fn is_const_evaluatable<'cx, 'tcx>(
     infcx: &InferCtxt<'cx, 'tcx>,
     uv: ty::Unevaluated<'tcx, ()>,
     param_env: ty::ParamEnv<'tcx>,
     span: Span,
 ) -> Result<(), NotConstEvaluatable> {
-    debug!("is_const_evaluatable({:?})", uv);
     let tcx = infcx.tcx;
 
     if tcx.features().generic_const_exprs {
@@ -304,6 +306,7 @@ impl<'a, 'tcx> AbstractConstBuilder<'a, 'tcx> {
         Err(reported)
     }
 
+    #[instrument(skip(tcx, body, body_id), level = "debug")]
     fn new(
         tcx: TyCtxt<'tcx>,
         (body, body_id): (&'a thir::Thir<'tcx>, thir::ExprId),
@@ -315,29 +318,61 @@ impl<'a, 'tcx> AbstractConstBuilder<'a, 'tcx> {
             thir: &'a thir::Thir<'tcx>,
         }
 
+        use crate::rustc_middle::thir::visit::Visitor;
         use thir::visit;
-        impl<'a, 'tcx: 'a> visit::Visitor<'a, 'tcx> for IsThirPolymorphic<'a, 'tcx> {
+
+        impl<'a, 'tcx> IsThirPolymorphic<'a, 'tcx> {
+            fn expr_is_poly(&mut self, expr: &thir::Expr<'tcx>) -> bool {
+                if expr.ty.has_param_types_or_consts() {
+                    return true;
+                }
+
+                match expr.kind {
+                    thir::ExprKind::NamedConst { substs, .. } => substs.has_param_types_or_consts(),
+                    thir::ExprKind::ConstParam { .. } => true,
+                    thir::ExprKind::Repeat { value, count } => {
+                        self.visit_expr(&self.thir()[value]);
+                        count.has_param_types_or_consts()
+                    }
+                    _ => false,
+                }
+            }
+
+            fn pat_is_poly(&mut self, pat: &thir::Pat<'tcx>) -> bool {
+                if pat.ty.has_param_types_or_consts() {
+                    return true;
+                }
+
+                match pat.kind.as_ref() {
+                    thir::PatKind::Constant { value } => value.has_param_types_or_consts(),
+                    thir::PatKind::Range(thir::PatRange { lo, hi, .. }) => {
+                        lo.has_param_types_or_consts() || hi.has_param_types_or_consts()
+                    }
+                    _ => false,
+                }
+            }
+        }
+
+        impl<'a, 'tcx> visit::Visitor<'a, 'tcx> for IsThirPolymorphic<'a, 'tcx> {
             fn thir(&self) -> &'a thir::Thir<'tcx> {
                 &self.thir
             }
 
+            #[instrument(skip(self), level = "debug")]
             fn visit_expr(&mut self, expr: &thir::Expr<'tcx>) {
-                self.is_poly |= expr.ty.has_param_types_or_consts();
+                self.is_poly |= self.expr_is_poly(expr);
                 if !self.is_poly {
                     visit::walk_expr(self, expr)
                 }
             }
 
+            #[instrument(skip(self), level = "debug")]
             fn visit_pat(&mut self, pat: &thir::Pat<'tcx>) {
-                self.is_poly |= pat.ty.has_param_types_or_consts();
+                self.is_poly |= self.pat_is_poly(pat);
                 if !self.is_poly {
                     visit::walk_pat(self, pat);
                 }
             }
-
-            fn visit_const(&mut self, ct: ty::Const<'tcx>) {
-                self.is_poly |= ct.has_param_types_or_consts();
-            }
         }
 
         let mut is_poly_vis = IsThirPolymorphic { is_poly: false, thir: body };
@@ -393,16 +428,49 @@ impl<'a, 'tcx> AbstractConstBuilder<'a, 'tcx> {
     fn recurse_build(&mut self, node: thir::ExprId) -> Result<NodeId, ErrorGuaranteed> {
         use thir::ExprKind;
         let node = &self.body.exprs[node];
-        debug!("recurse_build: node={:?}", node);
         Ok(match &node.kind {
             // I dont know if handling of these 3 is correct
             &ExprKind::Scope { value, .. } => self.recurse_build(value)?,
             &ExprKind::PlaceTypeAscription { source, .. }
             | &ExprKind::ValueTypeAscription { source, .. } => self.recurse_build(source)?,
+            &ExprKind::Literal { lit, neg} => {
+                let sp = node.span;
+                let constant =
+                    match self.tcx.at(sp).lit_to_const(LitToConstInput { lit: &lit.node, ty: node.ty, neg }) {
+                        Ok(c) => c,
+                        Err(LitToConstError::Reported) => {
+                            self.tcx.const_error(node.ty)
+                        }
+                        Err(LitToConstError::TypeError) => {
+                            bug!("encountered type error in lit_to_const")
+                        }
+                    };
+
+                self.nodes.push(Node::Leaf(constant))
+            }
+            &ExprKind::NonHirLiteral { lit , user_ty: _} => {
+                // FIXME Construct a Valtree from this ScalarInt when introducing Valtrees
+                let const_value = ConstValue::Scalar(Scalar::Int(lit));
+                self.nodes.push(Node::Leaf(ty::Const::from_value(self.tcx, const_value, node.ty)))
+            }
+            &ExprKind::NamedConst { def_id, substs, user_ty: _ } => {
+                let uneval = ty::Unevaluated::new(ty::WithOptConstParam::unknown(def_id), substs);
+
+                let constant = self.tcx.mk_const(ty::ConstS {
+                                val: ty::ConstKind::Unevaluated(uneval),
+                                ty: node.ty,
+                            });
 
-            // subtle: associated consts are literals this arm handles
-            // `<T as Trait>::ASSOC` as well as `12`
-            &ExprKind::Literal { literal, .. } => self.nodes.push(Node::Leaf(literal)),
+                self.nodes.push(Node::Leaf(constant))
+            }
+
+            ExprKind::ConstParam {param, ..} => {
+                let const_param = self.tcx.mk_const(ty::ConstS {
+                        val: ty::ConstKind::Param(*param),
+                        ty: node.ty,
+                    });
+                self.nodes.push(Node::Leaf(const_param))
+            }
 
             ExprKind::Call { fun, args, .. } => {
                 let fun = self.recurse_build(*fun)?;
@@ -585,6 +653,7 @@ pub(super) fn try_unify_abstract_consts<'tcx>(
     // on `ErrorGuaranteed`.
 }
 
+#[instrument(skip(tcx, f), level = "debug")]
 pub fn walk_abstract_const<'tcx, R, F>(
     tcx: TyCtxt<'tcx>,
     ct: AbstractConst<'tcx>,
@@ -593,13 +662,15 @@ pub fn walk_abstract_const<'tcx, R, F>(
 where
     F: FnMut(AbstractConst<'tcx>) -> ControlFlow<R>,
 {
+    #[instrument(skip(tcx, f), level = "debug")]
     fn recurse<'tcx, R>(
         tcx: TyCtxt<'tcx>,
         ct: AbstractConst<'tcx>,
         f: &mut dyn FnMut(AbstractConst<'tcx>) -> ControlFlow<R>,
     ) -> ControlFlow<R> {
         f(ct)?;
         let root = ct.root(tcx);
+        debug!(?root);
         match root {
             Node::Leaf(_) => ControlFlow::CONTINUE,
             Node::Binop(_, l, r) => {

src/test/ui/mir/thir-constparam-temp.rs

@@ -0,0 +1,20 @@
+// build-pass
+
+#![feature(adt_const_params)]
+#![allow(incomplete_features)]
+
+#[derive(PartialEq, Eq)]
+struct Yikes;
+
+impl Yikes {
+    fn mut_self(&mut self) {}
+}
+
+fn foo<const YIKES: Yikes>() {
+    YIKES.mut_self()
+    //~^ WARNING taking a mutable reference
+}
+
+fn main() {
+    foo::<{ Yikes }>()
+}

src/test/ui/mir/thir-constparam-temp.stderr

@@ -0,0 +1,22 @@
+warning: taking a mutable reference to a `const` item
+  --> $DIR/thir-constparam-temp.rs:14:5
+   |
+LL |     YIKES.mut_self()
+   |     ^^^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(const_item_mutation)]` on by default
+   = note: each usage of a `const` item creates a new temporary
+   = note: the mutable reference will refer to this temporary, not the original `const` item
+note: mutable reference created due to call to this method
+  --> $DIR/thir-constparam-temp.rs:10:5
+   |
+LL |     fn mut_self(&mut self) {}
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+note: `const` item defined here
+  --> $DIR/thir-constparam-temp.rs:13:14
+   |
+LL | fn foo<const YIKES: Yikes>() {
+   |              ^^^^^
+
+warning: 1 warning emitted
+