Add support for lengths in array repeats, if they are literals

Now we will get the type of `[0u8; 4]`.

2 files changed, 17 insertions, 7 deletions

diff --git a/crates/hir_ty/src/infer/expr.rs b/crates/hir_ty/src/infer/expr.rs
index 46e4777a4..0b36ac861 100644
--- a/crates/hir_ty/src/infer/expr.rs
+++ b/crates/hir_ty/src/infer/expr.rs
@@ -1,10 +1,14 @@
 //! Type inference for expressions.
 
-use std::iter::{repeat, repeat_with};
+use std::{
+    convert::TryInto,
+    iter::{repeat, repeat_with},
+};
 use std::{mem, sync::Arc};
 
 use chalk_ir::{cast::Cast, fold::Shift, ConstData, Mutability, TyVariableKind};
 use hir_def::{
+    builtin_type::BuiltinUint,
     expr::{Array, BinaryOp, Expr, ExprId, Literal, Statement, UnaryOp},
     path::{GenericArg, GenericArgs},
     resolver::resolver_for_expr,
@@ -724,7 +728,7 @@ impl<'a> InferenceContext<'a> {
                         for expr in items.iter() {
                             self.infer_expr_coerce(*expr, &Expectation::has_type(elem_ty.clone()));
                         }
-                        Some(items.len())
+                        Some(items.len() as u64)
                     }
                     Array::Repeat { initializer, repeat } => {
                         self.infer_expr_coerce(
@@ -737,9 +741,15 @@ impl<'a> InferenceContext<'a> {
                                 TyKind::Scalar(Scalar::Uint(UintTy::Usize)).intern(&Interner),
                             ),
                         );
-                        // FIXME: we don't know the length here because hir Exprs don't actually
-                        // get the value out of the AST, even though it is there.
-                        None
+
+                        let repeat_expr = &self.body.exprs[*repeat];
+                        match repeat_expr {
+                            Expr::Literal(Literal::Uint(v, None))
+                            | Expr::Literal(Literal::Uint(v, Some(BuiltinUint::Usize))) => {
+                                (*v).try_into().ok()
+                            }
+                            _ => None,
+                        }
                     }
                 };
 
@@ -747,7 +757,7 @@ impl<'a> InferenceContext<'a> {
                     ty: TyKind::Scalar(Scalar::Uint(UintTy::Usize)).intern(&Interner),
                     value: ConstValue::Concrete(chalk_ir::ConcreteConst {
                         interned: len
-                            .map(|len| ConstScalar::Usize(len as u64))
+                            .map(|len| ConstScalar::Usize(len))
                             .unwrap_or(ConstScalar::Unknown),
                     }),
                 };
diff --git a/crates/hir_ty/src/tests/coercion.rs b/crates/hir_ty/src/tests/coercion.rs
index c2afaf6ec..190471069 100644
--- a/crates/hir_ty/src/tests/coercion.rs
+++ b/crates/hir_ty/src/tests/coercion.rs
@@ -93,7 +93,7 @@ fn coerce_places() {
             340..345 '[arr]': [&[u8]; 1]
             341..344 'arr': &[u8; 1]
             355..356 'f': [&[u8]; 2]
-            370..378 '[arr; 2]': [&[u8]; _]
+            370..378 '[arr; 2]': [&[u8]; 2]
             371..374 'arr': &[u8; 1]
             376..377 '2': usize
             388..389 'g': (&[u8], &[u8])