4 files changed, 136 insertions, 3 deletions

diff --git a/crates/ra_hir/src/diagnostics.rs b/crates/ra_hir/src/diagnostics.rs
index 301109cb8..718345d75 100644
--- a/crates/ra_hir/src/diagnostics.rs
+++ b/crates/ra_hir/src/diagnostics.rs
@@ -143,3 +143,34 @@ impl AstDiagnostic for MissingFields {
         ast::RecordFieldList::cast(node).unwrap()
     }
 }
+
+#[derive(Debug)]
+pub struct MissingOkInTailExpr {
+    pub file: HirFileId,
+    pub expr: AstPtr<ast::Expr>,
+}
+
+impl Diagnostic for MissingOkInTailExpr {
+    fn message(&self) -> String {
+        "wrap return expression in Ok".to_string()
+    }
+    fn file(&self) -> HirFileId {
+        self.file
+    }
+    fn syntax_node_ptr(&self) -> SyntaxNodePtr {
+        self.expr.into()
+    }
+    fn as_any(&self) -> &(dyn Any + Send + 'static) {
+        self
+    }
+}
+
+impl AstDiagnostic for MissingOkInTailExpr {
+    type AST = ast::Expr;
+
+    fn ast(&self, db: &impl HirDatabase) -> Self::AST {
+        let root = db.parse_or_expand(self.file()).unwrap();
+        let node = self.syntax_node_ptr().to_node(&root);
+        ast::Expr::cast(node).unwrap()
+    }
+}
diff --git a/crates/ra_hir/src/expr/validation.rs b/crates/ra_hir/src/expr/validation.rs
index 62f7d41f5..f5e641557 100644
--- a/crates/ra_hir/src/expr/validation.rs
+++ b/crates/ra_hir/src/expr/validation.rs
@@ -6,11 +6,12 @@ use ra_syntax::ast::{AstNode, RecordLit};
 use super::{Expr, ExprId, RecordLitField};
 use crate::{
     adt::AdtDef,
-    diagnostics::{DiagnosticSink, MissingFields},
+    diagnostics::{DiagnosticSink, MissingFields, MissingOkInTailExpr},
     expr::AstPtr,
-    ty::InferenceResult,
+    ty::{InferenceResult, Ty, TypeCtor},
     Function, HasSource, HirDatabase, Name, Path,
 };
+use ra_syntax::ast;
 
 pub(crate) struct ExprValidator<'a, 'b: 'a> {
     func: Function,
@@ -29,11 +30,23 @@ impl<'a, 'b> ExprValidator<'a, 'b> {
 
     pub(crate) fn validate_body(&mut self, db: &impl HirDatabase) {
         let body = self.func.body(db);
+
+        // The final expr in the function body is the whole body,
+        // so the expression being returned is the penultimate expr.
+        let mut penultimate_expr = None;
+        let mut final_expr = None;
+
         for e in body.exprs() {
+            penultimate_expr = final_expr;
+            final_expr = Some(e);
+
             if let (id, Expr::RecordLit { path, fields, spread }) = e {
                 self.validate_record_literal(id, path, fields, *spread, db);
             }
         }
+        if let Some(e) = penultimate_expr {
+            self.validate_results_in_tail_expr(e.0, db);
+        }
     }
 
     fn validate_record_literal(
@@ -87,4 +100,43 @@ impl<'a, 'b> ExprValidator<'a, 'b> {
             })
         }
     }
+
+    fn validate_results_in_tail_expr(&mut self, id: ExprId, db: &impl HirDatabase) {
+        let expr_ty = &self.infer[id];
+        let func_ty = self.func.ty(db);
+        let func_sig = func_ty.callable_sig(db).unwrap();
+        let ret = func_sig.ret();
+        let ret = match ret {
+            Ty::Apply(t) => t,
+            _ => return,
+        };
+        let ret_enum = match ret.ctor {
+            TypeCtor::Adt(AdtDef::Enum(e)) => e,
+            _ => return,
+        };
+        let enum_name = ret_enum.name(db);
+        if enum_name.is_none() || enum_name.unwrap().to_string() != "Result" {
+            return;
+        }
+        let params = &ret.parameters;
+        if params.len() == 2 && &params[0] == expr_ty {
+            let source_map = self.func.body_source_map(db);
+            let file_id = self.func.source(db).file_id;
+            let parse = db.parse(file_id.original_file(db));
+            let source_file = parse.tree();
+            let expr_syntax = source_map.expr_syntax(id);
+            if expr_syntax.is_none() {
+                return;
+            }
+            let expr_syntax = expr_syntax.unwrap();
+            let node = expr_syntax.to_node(source_file.syntax());
+            let ast = ast::Expr::cast(node);
+            if ast.is_none() {
+                return;
+            }
+            let ast = ast.unwrap();
+
+            self.sink.push(MissingOkInTailExpr { file: file_id, expr: AstPtr::new(&ast) });
+        }
+    }
 }
diff --git a/crates/ra_hir/src/ty.rs b/crates/ra_hir/src/ty.rs
index b54c80318..0efd94cef 100644
--- a/crates/ra_hir/src/ty.rs
+++ b/crates/ra_hir/src/ty.rs
@@ -516,7 +516,7 @@ impl Ty {
         }
     }
 
-    fn callable_sig(&self, db: &impl HirDatabase) -> Option<FnSig> {
+    pub fn callable_sig(&self, db: &impl HirDatabase) -> Option<FnSig> {
         match self {
             Ty::Apply(a_ty) => match a_ty.ctor {
                 TypeCtor::FnPtr { .. } => Some(FnSig::from_fn_ptr_substs(&a_ty.parameters)),
diff --git a/crates/ra_ide_api/src/diagnostics.rs b/crates/ra_ide_api/src/diagnostics.rs
index c2b959cb3..be5197767 100644
--- a/crates/ra_ide_api/src/diagnostics.rs
+++ b/crates/ra_ide_api/src/diagnostics.rs
@@ -75,6 +75,19 @@ pub(crate) fn diagnostics(db: &RootDatabase, file_id: FileId) -> Vec<Diagnostic>
             severity: Severity::Error,
             fix: Some(fix),
         })
+    })
+    .on::<hir::diagnostics::MissingOkInTailExpr, _>(|d| {
+        let node = d.ast(db);
+        let mut builder = TextEditBuilder::default();
+        let replacement = format!("Ok({})", node.syntax().text());
+        builder.replace(node.syntax().text_range(), replacement);
+        let fix = SourceChange::source_file_edit_from("wrap with ok", file_id, builder.finish());
+        res.borrow_mut().push(Diagnostic {
+            range: d.highlight_range(),
+            message: d.message(),
+            severity: Severity::Error,
+            fix: Some(fix),
+        })
     });
     if let Some(m) = source_binder::module_from_file_id(db, file_id) {
         m.diagnostics(db, &mut sink);
@@ -219,6 +232,43 @@ mod tests {
     }
 
     #[test]
+    fn test_wrap_return_type() {
+        let before = r#"
+            enum Result<T, E> { Ok(T), Err(E) }
+            struct String { }
+
+            fn div(x: i32, y: i32) -> Result<i32, String> {
+                if y == 0 {
+                    return Err("div by zero".into());
+                }
+                x / y
+            }
+        "#;
+        let after = r#"
+            enum Result<T, E> { Ok(T), Err(E) }
+            struct String { }
+
+            fn div(x: i32, y: i32) -> Result<i32, String> {
+                if y == 0 {
+                    return Err("div by zero".into());
+                }
+                Ok(x / y)
+            }
+        "#;
+        check_apply_diagnostic_fix(before, after);
+    }
+
+    #[test]
+    fn test_wrap_return_type_not_applicable() {
+        let content = r#"
+            fn foo() -> Result<String, i32> {
+                0
+            }
+        "#;
+        check_no_diagnostic(content);
+    }
+
+    #[test]
     fn test_fill_struct_fields_empty() {
         let before = r"
             struct TestStruct {