perf: eagerly convert literals to consts, this avoids creating loads on unevaluated consts

which requires a lot of unnecessary work to evaluate them further down the line.

Author: skinnyBat

src/librustc/dep_graph/dep_node.rs

@@ -52,7 +52,7 @@
 use crate::hir::map::DefPathHash;
 use crate::ich::{Fingerprint, StableHashingContext};
 use crate::mir;
-use crate::mir::interpret::GlobalId;
+use crate::mir::interpret::{GlobalId, LitToConstInput};
 use crate::traits;
 use crate::traits::query::{
     CanonicalPredicateGoal, CanonicalProjectionGoal, CanonicalTyGoal,

src/librustc/mir/interpret/mod.rs

@@ -119,7 +119,7 @@ use crate::mir;
 use crate::ty::codec::TyDecoder;
 use crate::ty::layout::{self, Size};
 use crate::ty::subst::GenericArgKind;
-use crate::ty::{self, Instance, TyCtxt};
+use crate::ty::{self, Instance, Ty, TyCtxt};
 use byteorder::{BigEndian, LittleEndian, ReadBytesExt, WriteBytesExt};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::sync::{HashMapExt, Lock};
@@ -131,6 +131,7 @@ use std::fmt;
 use std::io;
 use std::num::NonZeroU32;
 use std::sync::atomic::{AtomicU32, Ordering};
+use syntax::ast::LitKind;
 
 /// Uniquely identifies one of the following:
 /// - A constant
@@ -147,6 +148,24 @@ pub struct GlobalId<'tcx> {
     pub promoted: Option<mir::Promoted>,
 }
 
+/// Input argument for `tcx.lit_to_const`
+#[derive(Copy, Clone, Debug, Eq, PartialEq, Hash, HashStable)]
+pub struct LitToConstInput<'tcx> {
+    /// The absolute value of the resultant constant
+    pub lit: &'tcx LitKind,
+    /// The type of the constant
+    pub ty: Ty<'tcx>,
+    /// If the constant is negative
+    pub neg: bool,
+}
+
+/// Error type for `tcx.lit_to_const`
+#[derive(Copy, Clone, Debug, Eq, PartialEq, HashStable)]
+pub enum LitToConstError {
+    UnparseableFloat,
+    Reported,
+}
+
 #[derive(Copy, Clone, Eq, Hash, Ord, PartialEq, PartialOrd, Debug)]
 pub struct AllocId(pub u64);
 

src/librustc/query/mod.rs

@@ -1,6 +1,6 @@
 use crate::dep_graph::{DepKind, DepNode, RecoverKey, SerializedDepNodeIndex};
 use crate::mir;
-use crate::mir::interpret::GlobalId;
+use crate::mir::interpret::{GlobalId, LitToConstInput};
 use crate::traits;
 use crate::traits::query::{
     CanonicalPredicateGoal, CanonicalProjectionGoal, CanonicalTyGoal,
@@ -509,6 +509,13 @@ rustc_queries! {
             no_force
             desc { "get a &core::panic::Location referring to a span" }
         }
+
+        query lit_to_const(
+            key: LitToConstInput<'tcx>
+        ) -> Result<&'tcx ty::Const<'tcx>, LitToConstError> {
+            no_force
+            desc { "converting literal to const" }
+        }
     }
 
     TypeChecking {

src/librustc/ty/query/keys.rs

@@ -52,6 +52,16 @@ impl<'tcx> Key for mir::interpret::GlobalId<'tcx> {
     }
 }
 
+impl<'tcx> Key for mir::interpret::LitToConstInput<'tcx> {
+    fn query_crate(&self) -> CrateNum {
+        LOCAL_CRATE
+    }
+
+    fn default_span(&self, _tcx: TyCtxt<'_>) -> Span {
+        DUMMY_SP
+    }
+}
+
 impl Key for CrateNum {
     fn query_crate(&self) -> CrateNum {
         *self

src/librustc/ty/query/mod.rs

@@ -15,6 +15,7 @@ use crate::middle::stability::{self, DeprecationEntry};
 use crate::mir;
 use crate::mir::interpret::GlobalId;
 use crate::mir::interpret::{ConstEvalRawResult, ConstEvalResult};
+use crate::mir::interpret::{LitToConstError, LitToConstInput};
 use crate::mir::mono::CodegenUnit;
 use crate::session::config::{EntryFnType, OptLevel, OutputFilenames, SymbolManglingVersion};
 use crate::session::CrateDisambiguator;

src/librustc_mir/hair/constant.rs

@@ -1,21 +1,15 @@
-use rustc::mir::interpret::{ConstValue, Scalar};
-use rustc::ty::{self, layout::Size, ParamEnv, Ty, TyCtxt};
+use rustc::mir::interpret::{
+    truncate, Allocation, ConstValue, LitToConstError, LitToConstInput, Scalar,
+};
+use rustc::ty::{self, layout::Size, ParamEnv, TyCtxt};
 use rustc_span::symbol::Symbol;
 use syntax::ast;
 
-#[derive(PartialEq)]
-crate enum LitToConstError {
-    UnparseableFloat,
-    Reported,
-}
-
 crate fn lit_to_const<'tcx>(
-    lit: &'tcx ast::LitKind,
     tcx: TyCtxt<'tcx>,
-    ty: Ty<'tcx>,
-    neg: bool,
+    lit_input: LitToConstInput<'tcx>,
 ) -> Result<&'tcx ty::Const<'tcx>, LitToConstError> {
-    use syntax::ast::*;
+    let LitToConstInput { lit, ty, neg } = lit_input;
 
     let trunc = |n| {
         let param_ty = ParamEnv::reveal_all().and(ty);
@@ -26,35 +20,50 @@ crate fn lit_to_const<'tcx>(
         Ok(ConstValue::Scalar(Scalar::from_uint(result, width)))
     };
 
-    use rustc::mir::interpret::*;
     let lit = match *lit {
-        LitKind::Str(ref s, _) => {
+        ast::LitKind::Str(ref s, _) => {
             let s = s.as_str();
             let allocation = Allocation::from_byte_aligned_bytes(s.as_bytes());
             let allocation = tcx.intern_const_alloc(allocation);
             ConstValue::Slice { data: allocation, start: 0, end: s.len() }
         }
-        LitKind::ByteStr(ref data) => {
-            let id = tcx.allocate_bytes(data);
-            ConstValue::Scalar(Scalar::Ptr(id.into()))
+        ast::LitKind::ByteStr(ref data) => {
+            if let ty::Ref(_, ref_ty, _) = ty.kind {
+                match ref_ty.kind {
+                    ty::Slice(_) => {
+                        let allocation = Allocation::from_byte_aligned_bytes(data as &Vec<u8>);
+                        let allocation = tcx.intern_const_alloc(allocation);
+                        ConstValue::Slice { data: allocation, start: 0, end: data.len() }
+                    }
+                    ty::Array(_, _) => {
+                        let id = tcx.allocate_bytes(data);
+                        ConstValue::Scalar(Scalar::Ptr(id.into()))
+                    }
+                    _ => {
+                        bug!("bytestring should have type of either &[u8] or &[u8; _], not {}", ty)
+                    }
+                }
+            } else {
+                bug!("bytestring should have type of either &[u8] or &[u8; _], not {}", ty)
+            }
         }
-        LitKind::Byte(n) => ConstValue::Scalar(Scalar::from_uint(n, Size::from_bytes(1))),
-        LitKind::Int(n, _) if neg => {
+        ast::LitKind::Byte(n) => ConstValue::Scalar(Scalar::from_uint(n, Size::from_bytes(1))),
+        ast::LitKind::Int(n, _) if neg => {
             let n = n as i128;
             let n = n.overflowing_neg().0;
             trunc(n as u128)?
         }
-        LitKind::Int(n, _) => trunc(n)?,
-        LitKind::Float(n, _) => {
+        ast::LitKind::Int(n, _) => trunc(n)?,
+        ast::LitKind::Float(n, _) => {
             let fty = match ty.kind {
                 ty::Float(fty) => fty,
                 _ => bug!(),
             };
             parse_float(n, fty, neg).map_err(|_| LitToConstError::UnparseableFloat)?
         }
-        LitKind::Bool(b) => ConstValue::Scalar(Scalar::from_bool(b)),
-        LitKind::Char(c) => ConstValue::Scalar(Scalar::from_char(c)),
-        LitKind::Err(_) => unreachable!(),
+        ast::LitKind::Bool(b) => ConstValue::Scalar(Scalar::from_bool(b)),
+        ast::LitKind::Char(c) => ConstValue::Scalar(Scalar::from_char(c)),
+        ast::LitKind::Err(_) => return Err(LitToConstError::Reported),
     };
     Ok(tcx.mk_const(ty::Const { val: ty::ConstKind::Value(lit), ty }))
 }

src/librustc_mir/hair/cx/mod.rs

@@ -5,9 +5,9 @@
 use crate::hair::util::UserAnnotatedTyHelpers;
 use crate::hair::*;
 
-use crate::hair::constant::{lit_to_const, LitToConstError};
 use rustc::infer::InferCtxt;
 use rustc::middle::region;
+use rustc::mir::interpret::{LitToConstError, LitToConstInput};
 use rustc::ty::layout::VariantIdx;
 use rustc::ty::subst::Subst;
 use rustc::ty::subst::{GenericArg, InternalSubsts};
@@ -136,7 +136,7 @@ impl<'a, 'tcx> Cx<'a, 'tcx> {
     ) -> &'tcx ty::Const<'tcx> {
         trace!("const_eval_literal: {:#?}, {:?}, {:?}, {:?}", lit, ty, sp, neg);
 
-        match lit_to_const(lit, self.tcx, ty, neg) {
+        match self.tcx.at(sp).lit_to_const(LitToConstInput { lit, ty, neg }) {
             Ok(c) => c,
             Err(LitToConstError::UnparseableFloat) => {
                 // FIXME(#31407) this is only necessary because float parsing is buggy

src/librustc_mir/hair/mod.rs

@@ -16,7 +16,7 @@ use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_span::Span;
 
-mod constant;
+pub(crate) mod constant;
 pub mod cx;
 
 pub mod pattern;

src/librustc_mir/hair/pattern/mod.rs

@@ -6,10 +6,11 @@ mod const_to_pat;
 
 pub(crate) use self::check_match::check_match;
 
-use crate::hair::constant::*;
 use crate::hair::util::UserAnnotatedTyHelpers;
 
-use rustc::mir::interpret::{get_slice_bytes, sign_extend, ConstValue, ErrorHandled};
+use rustc::mir::interpret::{
+    get_slice_bytes, sign_extend, ConstValue, ErrorHandled, LitToConstError, LitToConstInput,
+};
 use rustc::mir::UserTypeProjection;
 use rustc::mir::{BorrowKind, Field, Mutability};
 use rustc::ty::layout::VariantIdx;
@@ -803,35 +804,30 @@ impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
     /// which would overflow if we tried to evaluate `128_i8` and then negate
     /// afterwards.
     fn lower_lit(&mut self, expr: &'tcx hir::Expr<'tcx>) -> PatKind<'tcx> {
-        match expr.kind {
-            hir::ExprKind::Lit(ref lit) => {
-                let ty = self.tables.expr_ty(expr);
-                match lit_to_const(&lit.node, self.tcx, ty, false) {
-                    Ok(val) => *self.const_to_pat(val, expr.hir_id, lit.span).kind,
-                    Err(LitToConstError::UnparseableFloat) => {
-                        self.errors.push(PatternError::FloatBug);
-                        PatKind::Wild
-                    }
-                    Err(LitToConstError::Reported) => PatKind::Wild,
+        if let hir::ExprKind::Path(ref qpath) = expr.kind {
+            *self.lower_path(qpath, expr.hir_id, expr.span).kind
+        } else {
+            let (lit, neg) = match expr.kind {
+                hir::ExprKind::Lit(ref lit) => (lit, false),
+                hir::ExprKind::Unary(hir::UnOp::UnNeg, ref expr) => {
+                    let lit = match expr.kind {
+                        hir::ExprKind::Lit(ref lit) => lit,
+                        _ => span_bug!(expr.span, "not a literal: {:?}", expr),
+                    };
+                    (lit, true)
                 }
-            }
-            hir::ExprKind::Path(ref qpath) => *self.lower_path(qpath, expr.hir_id, expr.span).kind,
-            hir::ExprKind::Unary(hir::UnOp::UnNeg, ref expr) => {
-                let ty = self.tables.expr_ty(expr);
-                let lit = match expr.kind {
-                    hir::ExprKind::Lit(ref lit) => lit,
-                    _ => span_bug!(expr.span, "not a literal: {:?}", expr),
-                };
-                match lit_to_const(&lit.node, self.tcx, ty, true) {
-                    Ok(val) => *self.const_to_pat(val, expr.hir_id, lit.span).kind,
-                    Err(LitToConstError::UnparseableFloat) => {
-                        self.errors.push(PatternError::FloatBug);
-                        PatKind::Wild
-                    }
-                    Err(LitToConstError::Reported) => PatKind::Wild,
+                _ => span_bug!(expr.span, "not a literal: {:?}", expr),
+            };
+
+            let lit_input = LitToConstInput { lit: &lit.node, ty: self.tables.expr_ty(expr), neg };
+            match self.tcx.at(expr.span).lit_to_const(lit_input) {
+                Ok(val) => *self.const_to_pat(val, expr.hir_id, lit.span).kind,
+                Err(LitToConstError::UnparseableFloat) => {
+                    self.errors.push(PatternError::FloatBug);
+                    PatKind::Wild
                 }
+                Err(LitToConstError::Reported) => PatKind::Wild,
             }
-            _ => span_bug!(expr.span, "not a literal: {:?}", expr),
         }
     }
 }

src/librustc_mir/lib.rs

@@ -64,4 +64,5 @@ pub fn provide(providers: &mut Providers<'_>) {
         let (param_env, (value, field)) = param_env_and_value.into_parts();
         const_eval::const_field(tcx, param_env, None, field, value)
     };
+    providers.lit_to_const = hair::constant::lit_to_const;
 }

src/librustc_typeck/astconv.rs

@@ -1,3 +1,4 @@
+// ignore-tidy-filelength FIXME(#67418) Split up this file
 //! Conversion from AST representation of types to the `ty.rs` representation.
 //! The main routine here is `ast_ty_to_ty()`; each use is parameterized by an
 //! instance of `AstConv`.
@@ -38,6 +39,7 @@ use std::collections::BTreeSet;
 use std::iter;
 use std::slice;
 
+use rustc::mir::interpret::LitToConstInput;
 use rustc_error_codes::*;
 
 #[derive(Debug)]
@@ -2696,13 +2698,28 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         let tcx = self.tcx();
         let def_id = tcx.hir().local_def_id(ast_const.hir_id);
 
-        let mut const_ = ty::Const {
-            val: ty::ConstKind::Unevaluated(def_id, InternalSubsts::identity_for_item(tcx, def_id)),
-            ty,
+        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,
         };
 
-        let expr = &tcx.hir().body(ast_const.body).value;
-        if let Some(def_id) = self.const_param_def_id(expr) {
+        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;
+            }
+        }
+
+        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();
@@ -2711,10 +2728,11 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             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);
-            const_.val = ty::ConstKind::Param(ty::ParamConst::new(index, name));
-        }
-
-        tcx.mk_const(const_)
+            ty::ConstKind::Param(ty::ParamConst::new(index, name))
+        } else {
+            ty::ConstKind::Unevaluated(def_id, InternalSubsts::identity_for_item(tcx, def_id))
+        };
+        tcx.mk_const(ty::Const { val: kind, ty })
     }
 
     pub fn impl_trait_ty_to_ty(