Add basic support for array lengths in types

This recognizes `let a = [1u8, 2, 3]` as having type `[u8; 3]` instead
of the previous `[u8; _]`. Byte strings and `[0u8; 2]` kinds of range
array declarations are unsupported as before.

I don't know why a bunch of our rustc tests had single quotes inside
strings un-escaped by `UPDATE_EXPECT=1 cargo t`, but I don't think it's
bad? Maybe something in a nightly?

crates/hir_ty/src/consts.rs

@@ -0,0 +1,20 @@
+//! Handling of concrete const values
+
+/// A concrete constant value
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub enum ConstScalar {
+    // for now, we only support the trivial case of constant evaluating the length of an array
+    Usize(usize),
+
+    /// Case of an unknown value that rustc might know but we don't
+    Unknown,
+}
+
+impl std::fmt::Display for ConstScalar {
+    fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> Result<(), std::fmt::Error> {
+        match self {
+            ConstScalar::Usize(us) => write!(fmt, "{}", us),
+            ConstScalar::Unknown => write!(fmt, "_"),
+        }
+    }
+}

crates/hir_ty/src/display.rs

@@ -308,7 +308,7 @@ impl HirDisplay for Const {
                 let param_data = &generics.params.consts[id.local_id];
                 write!(f, "{}", param_data.name)
             }
-            ConstValue::Concrete(_) => write!(f, "_"),
+            ConstValue::Concrete(c) => write!(f, "{}", c.interned),
         }
     }
 }

crates/hir_ty/src/infer/expr.rs

@@ -3,7 +3,7 @@
 use std::iter::{repeat, repeat_with};
 use std::{mem, sync::Arc};
 
-use chalk_ir::{cast::Cast, fold::Shift, Mutability, TyVariableKind};
+use chalk_ir::{cast::Cast, fold::Shift, ConstData, Mutability, TyVariableKind};
 use hir_def::{
     expr::{Array, BinaryOp, Expr, ExprId, Literal, Statement, UnaryOp},
     path::{GenericArg, GenericArgs},
@@ -15,15 +15,17 @@ use stdx::always;
 use syntax::ast::RangeOp;
 
 use crate::{
-    autoderef, dummy_usize_const,
+    autoderef,
+    consts::ConstScalar,
+    dummy_usize_const,
     lower::lower_to_chalk_mutability,
     mapping::from_chalk,
     method_resolution, op,
     primitive::{self, UintTy},
     static_lifetime, to_chalk_trait_id,
     traits::FnTrait,
     utils::{generics, Generics},
-    AdtId, Binders, CallableDefId, FnPointer, FnSig, FnSubst, InEnvironment, Interner,
+    AdtId, Binders, CallableDefId, ConstValue, FnPointer, FnSig, FnSubst, InEnvironment, Interner,
     ProjectionTyExt, Rawness, Scalar, Substitution, TraitRef, Ty, TyBuilder, TyExt, TyKind,
 };
 
@@ -717,11 +719,12 @@ impl<'a> InferenceContext<'a> {
                     _ => self.table.new_type_var(),
                 };
 
-                match array {
+                let len = match array {
                     Array::ElementList(items) => {
                         for expr in items.iter() {
                             self.infer_expr_coerce(*expr, &Expectation::has_type(elem_ty.clone()));
                         }
+                        Some(items.len())
                     }
                     Array::Repeat { initializer, repeat } => {
                         self.infer_expr_coerce(
@@ -734,10 +737,20 @@ impl<'a> InferenceContext<'a> {
                                 TyKind::Scalar(Scalar::Uint(UintTy::Usize)).intern(&Interner),
                             ),
                         );
+                        // FIXME: support length for Repeat array expressions
+                        None
                     }
-                }
+                };
 
-                TyKind::Array(elem_ty, dummy_usize_const()).intern(&Interner)
+                let cd = ConstData {
+                    ty: TyKind::Scalar(Scalar::Uint(UintTy::Usize)).intern(&Interner),
+                    value: ConstValue::Concrete(chalk_ir::ConcreteConst {
+                        interned: len
+                            .map(|len| ConstScalar::Usize(len))
+                            .unwrap_or(ConstScalar::Unknown),
+                    }),
+                };
+                TyKind::Array(elem_ty, cd.intern(&Interner)).intern(&Interner)
             }
             Expr::Literal(lit) => match lit {
                 Literal::Bool(..) => TyKind::Scalar(Scalar::Bool).intern(&Interner),
@@ -747,6 +760,7 @@ impl<'a> InferenceContext<'a> {
                 }
                 Literal::ByteString(..) => {
                     let byte_type = TyKind::Scalar(Scalar::Uint(UintTy::U8)).intern(&Interner);
+
                     let array_type =
                         TyKind::Array(byte_type, dummy_usize_const()).intern(&Interner);
                     TyKind::Ref(Mutability::Not, static_lifetime(), array_type).intern(&Interner)

crates/hir_ty/src/interner.rs

@@ -1,7 +1,7 @@
 //! Implementation of the Chalk `Interner` trait, which allows customizing the
 //! representation of the various objects Chalk deals with (types, goals etc.).
 
-use crate::{chalk_db, tls, GenericArg};
+use crate::{chalk_db, consts::ConstScalar, tls, GenericArg};
 use base_db::salsa::InternId;
 use chalk_ir::{Goal, GoalData};
 use hir_def::{
@@ -31,6 +31,7 @@ impl_internable!(
     InternedWrapper<chalk_ir::TyData<Interner>>,
     InternedWrapper<chalk_ir::LifetimeData<Interner>>,
     InternedWrapper<chalk_ir::ConstData<Interner>>,
+    InternedWrapper<ConstScalar>,
     InternedWrapper<Vec<chalk_ir::CanonicalVarKind<Interner>>>,
     InternedWrapper<Vec<chalk_ir::ProgramClause<Interner>>>,
     InternedWrapper<Vec<chalk_ir::QuantifiedWhereClause<Interner>>>,
@@ -41,7 +42,7 @@ impl chalk_ir::interner::Interner for Interner {
     type InternedType = Interned<InternedWrapper<chalk_ir::TyData<Interner>>>;
     type InternedLifetime = Interned<InternedWrapper<chalk_ir::LifetimeData<Self>>>;
     type InternedConst = Interned<InternedWrapper<chalk_ir::ConstData<Self>>>;
-    type InternedConcreteConst = ();
+    type InternedConcreteConst = ConstScalar;
     type InternedGenericArg = chalk_ir::GenericArgData<Self>;
     type InternedGoal = Arc<GoalData<Self>>;
     type InternedGoals = Vec<Goal<Self>>;
@@ -245,10 +246,15 @@ impl chalk_ir::interner::Interner for Interner {
     fn const_eq(
         &self,
         _ty: &Self::InternedType,
-        _c1: &Self::InternedConcreteConst,
-        _c2: &Self::InternedConcreteConst,
+        c1: &Self::InternedConcreteConst,
+        c2: &Self::InternedConcreteConst,
     ) -> bool {
-        true
+        match (c1, c2) {
+            (&ConstScalar::Usize(a), &ConstScalar::Usize(b)) => a == b,
+            // we were previously assuming this to be true, I'm not whether true or false on
+            // unknown values is safer.
+            (_, _) => true,
+        }
     }
 
     fn intern_generic_arg(

crates/hir_ty/src/lib.rs

@@ -12,6 +12,7 @@ mod chalk_db;
 mod chalk_ext;
 mod infer;
 mod interner;
+mod consts;
 mod lower;
 mod mapping;
 mod op;
@@ -39,7 +40,7 @@ use chalk_ir::{
 };
 use hir_def::{expr::ExprId, type_ref::Rawness, TypeParamId};
 
-use crate::{db::HirDatabase, display::HirDisplay, utils::generics};
+use crate::{consts::ConstScalar, db::HirDatabase, display::HirDisplay, utils::generics};
 
 pub use autoderef::autoderef;
 pub use builder::TyBuilder;
@@ -250,7 +251,9 @@ pub fn dummy_usize_const() -> Const {
     let usize_ty = chalk_ir::TyKind::Scalar(Scalar::Uint(UintTy::Usize)).intern(&Interner);
     chalk_ir::ConstData {
         ty: usize_ty,
-        value: chalk_ir::ConstValue::Concrete(chalk_ir::ConcreteConst { interned: () }),
+        value: chalk_ir::ConstValue::Concrete(chalk_ir::ConcreteConst {
+            interned: ConstScalar::Unknown,
+        }),
     }
     .intern(&Interner)
 }

crates/hir_ty/src/lower.rs

@@ -174,6 +174,8 @@ impl<'a> TyLoweringContext<'a> {
             }
             TypeRef::Array(inner) => {
                 let inner_ty = self.lower_ty(inner);
+                // FIXME: we don't have length info here because we don't store an expression for
+                // the length
                 TyKind::Array(inner_ty, dummy_usize_const()).intern(&Interner)
             }
             TypeRef::Slice(inner) => {