4 files changed, 140 insertions, 2 deletions

diff --git a/crates/ra_hir/src/ty.rs b/crates/ra_hir/src/ty.rs
index 8c320a705..b685259d7 100644
--- a/crates/ra_hir/src/ty.rs
+++ b/crates/ra_hir/src/ty.rs
@@ -26,7 +26,7 @@ use ena::unify::{InPlaceUnificationTable, UnifyKey, UnifyValue, NoError};
 
 use ra_db::{LocalSyntaxPtr, Cancelable};
 use ra_syntax::{
-    ast::{self, AstNode, LoopBodyOwner, ArgListOwner, PrefixOp},
+    ast::{self, AstNode, LoopBodyOwner, ArgListOwner, PrefixOp, BinOp},
     SyntaxNodeRef
 };
 
@@ -527,6 +527,20 @@ struct InferenceContext<'a, D: HirDatabase> {
     return_ty: Ty,
 }
 
+// helper function that determines whether a binary operator
+// always returns a boolean
+fn is_boolean_operator(op: BinOp) -> bool {
+    match op {
+        BinOp::BooleanOr
+        | BinOp::BooleanAnd
+        | BinOp::EqualityTest
+        | BinOp::LesserEqualTest
+        | BinOp::GreaterEqualTest
+        | BinOp::LesserTest
+        | BinOp::GreaterTest => true,
+    }
+}
+
 impl<'a, D: HirDatabase> InferenceContext<'a, D> {
     fn new(
         db: &'a D,
@@ -899,7 +913,24 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 }
             }
             ast::Expr::RangeExpr(_e) => Ty::Unknown,
-            ast::Expr::BinExpr(_e) => Ty::Unknown,
+            ast::Expr::BinExpr(e) => match e.op() {
+                Some(op) => {
+                    let subtype_expectation = match op {
+                        BinOp::BooleanAnd | BinOp::BooleanOr => Expectation::has_type(Ty::Bool),
+                        _ => Expectation::none(),
+                    };
+                    let (lhs, rhs) = e.sub_exprs();
+                    let _lhs_ty = self.infer_expr_opt(lhs, &subtype_expectation)?;
+                    let _rhs_ty = self.infer_expr_opt(rhs, &subtype_expectation)?;
+
+                    if is_boolean_operator(op) {
+                        Ty::Bool
+                    } else {
+                        Ty::Unknown
+                    }
+                }
+                _ => Ty::Unknown,
+            },
             ast::Expr::Literal(_e) => Ty::Unknown,
         };
         // use a new type variable if we got Ty::Unknown here
diff --git a/crates/ra_hir/src/ty/tests.rs b/crates/ra_hir/src/ty/tests.rs
index 515c66e85..25a354947 100644
--- a/crates/ra_hir/src/ty/tests.rs
+++ b/crates/ra_hir/src/ty/tests.rs
@@ -156,6 +156,30 @@ impl S {
     );
 }
 
+#[test]
+fn infer_boolean_op() {
+    check_inference(
+        r#"
+fn f(x: bool) -> i32 {
+    0i32
+}
+
+fn test() {
+    let x = a && b;
+    let y = true || false;
+    let z = x == y;
+    let h = CONST_1 <= CONST_2;
+    let c = f(z || y) + 5;
+    let d = b;
+    let e = 3i32 && "hello world";
+
+    10 < 3
+}
+"#,
+        "0008_boolean_op.txt",
+    );
+}
+
 fn infer(content: &str) -> String {
     let (db, _, file_id) = MockDatabase::with_single_file(content);
     let source_file = db.source_file(file_id);
diff --git a/crates/ra_hir/src/ty/tests/data/0008_boolean_op.txt b/crates/ra_hir/src/ty/tests/data/0008_boolean_op.txt
new file mode 100644
index 000000000..ca01ad159
--- /dev/null
+++ b/crates/ra_hir/src/ty/tests/data/0008_boolean_op.txt
@@ -0,0 +1,31 @@
+[28; 32) '0i32': i32
+[22; 34) '{     0i32 }': i32
+[6; 7) 'x': [unknown]
+[127; 134) 'CONST_1': [unknown]
+[201; 205) '3i32': bool
+[76; 77) 'y': bool
+[65; 66) 'b': bool
+[60; 66) 'a && b': bool
+[127; 145) 'CONST_...ONST_2': bool
+[182; 183) 'd': [unknown]
+[229; 231) '10': [unknown]
+[209; 222) '"hello world"': bool
+[229; 235) '10 < 3': bool
+[186; 187) 'b': [unknown]
+[159; 172) 'f(z || y) + 5': [unknown]
+[56; 57) 'x': bool
+[112; 113) 'y': bool
+[201; 222) '3i32 &...world"': bool
+[234; 235) '3': [unknown]
+[138; 145) 'CONST_2': [unknown]
+[80; 93) 'true || false': bool
+[46; 237) '{     ... < 3 }': bool
+[197; 198) 'e': bool
+[107; 113) 'x == y': bool
+[88; 93) 'false': bool
+[80; 84) 'true': bool
+[123; 124) 'h': bool
+[155; 156) 'c': [unknown]
+[103; 104) 'z': bool
+[60; 61) 'a': bool
+[107; 108) 'x': bool
diff --git a/crates/ra_syntax/src/ast.rs b/crates/ra_syntax/src/ast.rs
index c10169d90..9df8ec663 100644
--- a/crates/ra_syntax/src/ast.rs
+++ b/crates/ra_syntax/src/ast.rs
@@ -489,6 +489,58 @@ impl<'a> PrefixExpr<'a> {
 }
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+pub enum BinOp {
+    /// The `||` operator for boolean OR
+    BooleanOr,
+    /// The `&&` operator for boolean AND
+    BooleanAnd,
+    /// The `==` operator for equality testing
+    EqualityTest,
+    /// The `<=` operator for lesser-equal testing
+    LesserEqualTest,
+    /// The `>=` operator for greater-equal testing
+    GreaterEqualTest,
+    /// The `<` operator for comparison
+    LesserTest,
+    /// The `>` operator for comparison
+    GreaterTest,
+    // TODO: lots of others
+}
+
+impl<'a> BinExpr<'a> {
+    pub fn op(&self) -> Option<BinOp> {
+        self.syntax()
+            .children()
+            .filter_map(|c| match c.kind() {
+                PIPEPIPE => Some(BinOp::BooleanOr),
+                AMPAMP => Some(BinOp::BooleanAnd),
+                EQEQ => Some(BinOp::EqualityTest),
+                LTEQ => Some(BinOp::LesserEqualTest),
+                GTEQ => Some(BinOp::GreaterEqualTest),
+                L_ANGLE => Some(BinOp::LesserTest),
+                R_ANGLE => Some(BinOp::GreaterTest),
+                _ => None,
+            })
+            .next()
+    }
+
+    pub fn lhs(self) -> Option<Expr<'a>> {
+        children(self).nth(0)
+    }
+
+    pub fn rhs(self) -> Option<Expr<'a>> {
+        children(self).nth(1)
+    }
+
+    pub fn sub_exprs(self) -> (Option<Expr<'a>>, Option<Expr<'a>>) {
+        let mut children = children(self);
+        let first = children.next();
+        let second = children.next();
+        (first, second)
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
 pub enum SelfParamFlavor {
     /// self
     Owned,