Auto merge of #12778 - Logarithmus:feature/fix-negative-const-generics, r=flodiebold

Support negative, `char` & `bool` const generics

Before:
![Before](https://user-images.githubusercontent.com/29541480/179379832-0c3b2a74-fef6-427e-b89f-7e31d9c37b3d.png)

After:
![After](https://user-images.githubusercontent.com/29541480/179379863-b62475dd-e7bf-41f2-b437-08dfe55951af.png)

I tried to implement stuff like `Const<{NUM1 + 3 + NUM2}>` by using already existing constant evaluation mechanism for ordinary constants, but turned out to be harder than I thought, maybe because I've never ever tinkered with compilers before

crates/hir-ty/src/consteval.rs

@@ -347,17 +347,6 @@ pub fn eval_const(
     }
 }
 
-pub fn eval_usize(expr: Idx<Expr>, mut ctx: ConstEvalCtx<'_>) -> Option<u64> {
-    if let Ok(ce) = eval_const(expr, &mut ctx) {
-        match ce {
-            ComputedExpr::Literal(Literal::Int(x, _)) => return x.try_into().ok(),
-            ComputedExpr::Literal(Literal::Uint(x, _)) => return x.try_into().ok(),
-            _ => {}
-        }
-    }
-    None
-}
-
 pub(crate) fn path_to_const(
     db: &dyn HirDatabase,
     resolver: &Resolver,
@@ -406,17 +395,14 @@ pub fn unknown_const_as_generic(ty: Ty) -> GenericArg {
 }
 
 /// Interns a constant scalar with the given type
-pub fn intern_scalar_const(value: ConstScalar, ty: Ty) -> Const {
+pub fn intern_const_scalar(value: ConstScalar, ty: Ty) -> Const {
     ConstData { ty, value: ConstValue::Concrete(chalk_ir::ConcreteConst { interned: value }) }
         .intern(Interner)
 }
 
 /// Interns a possibly-unknown target usize
-pub fn usize_const(value: Option<u64>) -> Const {
-    intern_scalar_const(
-        value.map(ConstScalar::Usize).unwrap_or(ConstScalar::Unknown),
-        TyBuilder::usize(),
-    )
+pub fn usize_const(value: Option<u128>) -> Const {
+    intern_const_scalar(value.map_or(ConstScalar::Unknown, ConstScalar::UInt), TyBuilder::usize())
 }
 
 pub(crate) fn const_eval_recover(
@@ -463,7 +449,7 @@ pub(crate) fn eval_to_const<'a>(
         }
     }
     let body = ctx.body.clone();
-    let ctx = ConstEvalCtx {
+    let mut ctx = ConstEvalCtx {
         db: ctx.db,
         owner: ctx.owner,
         exprs: &body.exprs,
@@ -471,7 +457,12 @@ pub(crate) fn eval_to_const<'a>(
         local_data: HashMap::default(),
         infer: &ctx.result,
     };
-    usize_const(eval_usize(expr, ctx))
+    let computed_expr = eval_const(expr, &mut ctx);
+    let const_scalar = match computed_expr {
+        Ok(ComputedExpr::Literal(literal)) => literal.into(),
+        _ => ConstScalar::Unknown,
+    };
+    intern_const_scalar(const_scalar, TyBuilder::usize())
 }
 
 #[cfg(test)]

crates/hir-ty/src/infer.rs

@@ -605,10 +605,10 @@ impl<'a> InferenceContext<'a> {
         let data = c.data(Interner);
         match data.value {
             ConstValue::Concrete(cc) => match cc.interned {
-                hir_def::type_ref::ConstScalar::Usize(_) => c,
                 hir_def::type_ref::ConstScalar::Unknown => {
                     self.table.new_const_var(data.ty.clone())
                 }
+                _ => c,
             },
             _ => c,
         }

crates/hir-ty/src/infer/expr.rs

@@ -729,7 +729,7 @@ impl<'a> InferenceContext<'a> {
                             let cur_elem_ty = self.infer_expr_inner(expr, &expected);
                             coerce.coerce(self, Some(expr), &cur_elem_ty);
                         }
-                        consteval::usize_const(Some(items.len() as u64))
+                        consteval::usize_const(Some(items.len() as u128))
                     }
                     &Array::Repeat { initializer, repeat } => {
                         self.infer_expr_coerce(initializer, &Expectation::has_type(elem_ty));
@@ -766,7 +766,7 @@ impl<'a> InferenceContext<'a> {
                 Literal::ByteString(bs) => {
                     let byte_type = TyKind::Scalar(Scalar::Uint(UintTy::U8)).intern(Interner);
 
-                    let len = consteval::usize_const(Some(bs.len() as u64));
+                    let len = consteval::usize_const(Some(bs.len() as u128));
 
                     let array_type = TyKind::Array(byte_type, len).intern(Interner);
                     TyKind::Ref(Mutability::Not, static_lifetime(), array_type).intern(Interner)

crates/hir-ty/src/infer/pat.rs

@@ -11,6 +11,7 @@ use hir_def::{
 use hir_expand::name::Name;
 
 use crate::{
+    consteval::intern_const_scalar,
     infer::{BindingMode, Expectation, InferenceContext, TypeMismatch},
     lower::lower_to_chalk_mutability,
     static_lifetime, ConcreteConst, ConstValue, Interner, Substitution, Ty, TyBuilder, TyExt,
@@ -262,13 +263,19 @@ impl<'a> InferenceContext<'a> {
                 if let &Some(slice_pat_id) = slice {
                     let rest_pat_ty = match expected.kind(Interner) {
                         TyKind::Array(_, length) => {
-                            let length = match length.data(Interner).value {
+                            let len = match length.data(Interner).value {
                                 ConstValue::Concrete(ConcreteConst {
-                                    interned: ConstScalar::Usize(length),
-                                }) => length.checked_sub((prefix.len() + suffix.len()) as u64),
+                                    interned: ConstScalar::UInt(len),
+                                }) => len.checked_sub((prefix.len() + suffix.len()) as u128),
                                 _ => None,
                             };
-                            TyKind::Array(elem_ty.clone(), crate::consteval::usize_const(length))
+                            TyKind::Array(
+                                elem_ty.clone(),
+                                intern_const_scalar(
+                                    len.map_or(ConstScalar::Unknown, |len| ConstScalar::UInt(len)),
+                                    TyBuilder::usize(),
+                                ),
+                            )
                         }
                         _ => TyKind::Slice(elem_ty.clone()),
                     }

crates/hir-ty/src/interner.rs

@@ -257,12 +257,7 @@ impl chalk_ir::interner::Interner for Interner {
         c1: &Self::InternedConcreteConst,
         c2: &Self::InternedConcreteConst,
     ) -> bool {
-        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,
-        }
+        (c1 == &ConstScalar::Unknown) || (c2 == &ConstScalar::Unknown) || (c1 == c2)
     }
 
     fn intern_generic_arg(

crates/hir-ty/src/lower.rs

@@ -44,7 +44,7 @@ use syntax::{ast, SmolStr};
 
 use crate::{
     all_super_traits,
-    consteval::{intern_scalar_const, path_to_const, unknown_const, unknown_const_as_generic},
+    consteval::{intern_const_scalar, path_to_const, unknown_const, unknown_const_as_generic},
     db::HirDatabase,
     make_binders,
     mapping::ToChalk,
@@ -1743,7 +1743,7 @@ pub(crate) fn const_or_path_to_chalk(
     debruijn: DebruijnIndex,
 ) -> Const {
     match value {
-        ConstScalarOrPath::Scalar(s) => intern_scalar_const(s.clone(), expected_ty),
+        ConstScalarOrPath::Scalar(s) => intern_const_scalar(s.clone(), expected_ty),
         ConstScalarOrPath::Path(n) => {
             let path = ModPath::from_segments(PathKind::Plain, Some(n.clone()));
             path_to_const(db, resolver, &path, mode, args, debruijn)