3 files changed, 128 insertions, 0 deletions

diff --git a/crates/assists/src/handlers/infer_function_return_type.rs b/crates/assists/src/handlers/infer_function_return_type.rs
new file mode 100644
index 000000000..da60ff9de
--- /dev/null
+++ b/crates/assists/src/handlers/infer_function_return_type.rs
@@ -0,0 +1,113 @@
+use hir::HirDisplay;
+use syntax::{ast, AstNode, TextSize};
+use test_utils::mark;
+
+use crate::{AssistContext, AssistId, AssistKind, Assists};
+
+// Assist: infer_function_return_type
+//
+// Adds the return type to a function inferred from its tail expression if it doesn't have a return
+// type specified.
+//
+// ```
+// fn foo() { 4<|>2i32 }
+// ```
+// ->
+// ```
+// fn foo() -> i32 { 42i32 }
+// ```
+pub(crate) fn infer_function_return_type(acc: &mut Assists, ctx: &AssistContext) -> Option<()> {
+    let expr = ctx.find_node_at_offset::<ast::Expr>()?;
+    let func = expr.syntax().ancestors().find_map(ast::Fn::cast)?;
+
+    if func.ret_type().is_some() {
+        mark::hit!(existing_ret_type);
+        return None;
+    }
+    let body = func.body()?;
+    let tail_expr = body.expr()?;
+    // check whether the expr we were at is indeed the tail expression
+    if !tail_expr.syntax().text_range().contains_range(expr.syntax().text_range()) {
+        mark::hit!(not_tail_expr);
+        return None;
+    }
+    let module = ctx.sema.scope(func.syntax()).module()?;
+    let ty = ctx.sema.type_of_expr(&tail_expr)?;
+    let ty = ty.display_source_code(ctx.db(), module.into()).ok()?;
+    let rparen = func.param_list()?.r_paren_token()?;
+
+    acc.add(
+        AssistId("change_return_type_to_result", AssistKind::RefactorRewrite),
+        "Wrap return type in Result",
+        tail_expr.syntax().text_range(),
+        |builder| {
+            let insert_pos = rparen.text_range().end() + TextSize::from(1);
+
+            builder.insert(insert_pos, &format!("-> {} ", ty));
+        },
+    )
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::tests::{check_assist, check_assist_not_applicable};
+
+    use super::*;
+
+    #[test]
+    fn infer_return_type() {
+        check_assist(
+            infer_function_return_type,
+            r#"fn foo() {
+                45<|>
+            }"#,
+            r#"fn foo() -> i32 {
+                45
+            }"#,
+        );
+    }
+
+    #[test]
+    fn infer_return_type_nested() {
+        check_assist(
+            infer_function_return_type,
+            r#"fn foo() {
+                if true {
+                    3<|>
+                } else {
+                    5
+                }
+            }"#,
+            r#"fn foo() -> i32 {
+                if true {
+                    3
+                } else {
+                    5
+                }
+            }"#,
+        );
+    }
+
+    #[test]
+    fn not_applicable_ret_type_specified() {
+        mark::check!(existing_ret_type);
+        check_assist_not_applicable(
+            infer_function_return_type,
+            r#"fn foo() -> i32 {
+                ( 45<|> + 32 ) * 123
+            }"#,
+        );
+    }
+
+    #[test]
+    fn not_applicable_non_tail_expr() {
+        mark::check!(not_tail_expr);
+        check_assist_not_applicable(
+            infer_function_return_type,
+            r#"fn foo() {
+                let x = <|>3;
+                ( 45 + 32 ) * 123
+            }"#,
+        );
+    }
+}
diff --git a/crates/assists/src/lib.rs b/crates/assists/src/lib.rs
index b804e495d..af88b3437 100644
--- a/crates/assists/src/lib.rs
+++ b/crates/assists/src/lib.rs
@@ -143,6 +143,7 @@ mod handlers {
     mod generate_function;
     mod generate_impl;
     mod generate_new;
+    mod infer_function_return_type;
     mod inline_local_variable;
     mod introduce_named_lifetime;
     mod invert_if;
@@ -190,6 +191,7 @@ mod handlers {
             generate_function::generate_function,
             generate_impl::generate_impl,
             generate_new::generate_new,
+            infer_function_return_type::infer_function_return_type,
             inline_local_variable::inline_local_variable,
             introduce_named_lifetime::introduce_named_lifetime,
             invert_if::invert_if,
diff --git a/crates/assists/src/tests/generated.rs b/crates/assists/src/tests/generated.rs
index acbf5b652..168e1626a 100644
--- a/crates/assists/src/tests/generated.rs
+++ b/crates/assists/src/tests/generated.rs
@@ -506,6 +506,19 @@ impl<T: Clone> Ctx<T> {
 }
 
 #[test]
+fn doctest_infer_function_return_type() {
+    check_doc_test(
+        "infer_function_return_type",
+        r#####"
+fn foo() { 4<|>2i32 }
+"#####,
+        r#####"
+fn foo() -> i32 { 42i32 }
+"#####,
+    )
+}
+
+#[test]
 fn doctest_inline_local_variable() {
     check_doc_test(
         "inline_local_variable",