implement while let desugaring

3 files changed, 47 insertions, 14 deletions

diff --git a/crates/ra_hir/src/expr.rs b/crates/ra_hir/src/expr.rs
index b59787a83..f33676655 100644
--- a/crates/ra_hir/src/expr.rs
+++ b/crates/ra_hir/src/expr.rs
@@ -11,17 +11,16 @@ use ra_syntax::{
     },
     AstNode, AstPtr, SyntaxNodePtr,
 };
+use test_utils::tested_by;
 
 use crate::{
     name::{AsName, SELF_PARAM},
+    path::GenericArgs,
+    ty::primitive::{FloatTy, IntTy, UncertainFloatTy, UncertainIntTy},
     type_ref::{Mutability, TypeRef},
     DefWithBody, Either, HasSource, HirDatabase, HirFileId, MacroCallLoc, MacroFileKind, Name,
     Path, Resolver,
 };
-use crate::{
-    path::GenericArgs,
-    ty::primitive::{FloatTy, IntTy, UncertainFloatTy, UncertainIntTy},
-};
 
 pub use self::scope::ExprScopes;
 
@@ -603,17 +602,30 @@ where
                 self.alloc_expr(Expr::Loop { body }, syntax_ptr)
             }
             ast::ExprKind::WhileExpr(e) => {
-                let condition = if let Some(condition) = e.condition() {
-                    if condition.pat().is_none() {
-                        self.collect_expr_opt(condition.expr())
-                    } else {
-                        // FIXME handle while let
-                        return self.alloc_expr(Expr::Missing, syntax_ptr);
-                    }
-                } else {
-                    self.exprs.alloc(Expr::Missing)
-                };
                 let body = self.collect_block_opt(e.loop_body());
+
+                let condition = match e.condition() {
+                    None => self.exprs.alloc(Expr::Missing),
+                    Some(condition) => match condition.pat() {
+                        None => self.collect_expr_opt(condition.expr()),
+                        // if let -- desugar to match
+                        Some(pat) => {
+                            tested_by!(infer_while_let);
+                            let pat = self.collect_pat(pat);
+                            let match_expr = self.collect_expr_opt(condition.expr());
+                            let placeholder_pat = self.pats.alloc(Pat::Missing);
+                            let break_ = self.exprs.alloc(Expr::Break { expr: None });
+                            let arms = vec![
+                                MatchArm { pats: vec![pat], expr: body, guard: None },
+                                MatchArm { pats: vec![placeholder_pat], expr: break_, guard: None },
+                            ];
+                            let match_expr =
+                                self.exprs.alloc(Expr::Match { expr: match_expr, arms });
+                            return self.alloc_expr(Expr::Loop { body: match_expr }, syntax_ptr);
+                        }
+                    },
+                };
+
                 self.alloc_expr(Expr::While { condition, body }, syntax_ptr)
             }
             ast::ExprKind::ForExpr(e) => {
diff --git a/crates/ra_hir/src/marks.rs b/crates/ra_hir/src/marks.rs
index 2d831f0d8..5b15eee90 100644
--- a/crates/ra_hir/src/marks.rs
+++ b/crates/ra_hir/src/marks.rs
@@ -10,4 +10,5 @@ test_utils::marks!(
     std_prelude
     match_ergonomics_ref
     trait_resolution_on_fn_type
+    infer_while_let
 );
diff --git a/crates/ra_hir/src/ty/tests.rs b/crates/ra_hir/src/ty/tests.rs
index 36dea17a3..d5f7a4d25 100644
--- a/crates/ra_hir/src/ty/tests.rs
+++ b/crates/ra_hir/src/ty/tests.rs
@@ -145,6 +145,26 @@ mod collections {
 }
 
 #[test]
+fn infer_while_let() {
+    covers!(infer_while_let);
+    let (db, pos) = MockDatabase::with_position(
+        r#"
+//- /main.rs
+enum Option<T> { Some(T), None }
+
+fn test() {
+    let foo: Option<f32> = None;
+    while let Option::Some(x) = foo {
+        <|>x
+    }
+}
+
+"#,
+    );
+    assert_eq!("f32", type_at_pos(&db, pos));
+}
+
+#[test]
 fn infer_basics() {
     assert_snapshot_matches!(
         infer(r#"