[ty] Add diagnosis for function with no return statement but with return type annotation

crates/ty_python_semantic/resources/mdtest/function/return_type.md

@@ -278,6 +278,20 @@ def f(cond: bool) -> int:
         return 2
 ```
 
+## Invalid implicit return type always None
+
+<!-- snapshot-diagnostics -->
+
+If the function has no `return` statement or if it has only bare `return` statement (no variable in
+the return statement), then we show a diagnostic hint that the return annotation should be `-> None`
+or a `return` statement should be added.
+
+```py
+# error: [invalid-return-type]
+def f() -> int:
+    print("hello")
+```
+
 ## NotImplemented
 
 ### Default Python version

crates/ty_python_semantic/resources/mdtest/snapshots/return_type.md_-_Function_return_type_-_Diagnostics_for_`inv…_(35563a74094b14d5).snap

@@ -24,13 +24,14 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/function/return_type.md
 # Diagnostics
 
 ```
-error[invalid-return-type]: Function can implicitly return `None`, which is not assignable to return type `str`
+error[invalid-return-type]: Function always implicitly returns `None`, which is not assignable to return type `str`
  --> src/mdtest_snippet.py:7:25
   |
 6 | class Concrete(Abstract):
 7 |     def method(self) -> str: ...  # error: [invalid-return-type]
   |                         ^^^
   |
+info: Consider changing the return annotation to `-> None` or adding a `return` statement
 info: Only functions in stub files, methods on protocol classes, or methods with `@abstractmethod` are permitted to have empty bodies
 info: Class `Concrete` has `typing.Protocol` in its MRO, but it is not a protocol class
 info: Only classes that directly inherit from `typing.Protocol` or `typing_extensions.Protocol` are considered protocol classes

crates/ty_python_semantic/resources/mdtest/snapshots/return_type.md_-_Function_return_type_-_Invalid_implicit_ret…_(393cb38bf7119649).snap

@@ -71,7 +71,7 @@ info: rule `invalid-return-type` is enabled by default
 ```
 
 ```
-error[invalid-return-type]: Function can implicitly return `None`, which is not assignable to return type `int`
+error[invalid-return-type]: Function always implicitly returns `None`, which is not assignable to return type `int`
   --> src/mdtest_snippet.py:12:22
    |
 11 | # error: [invalid-return-type]
@@ -80,6 +80,7 @@ error[invalid-return-type]: Function can implicitly return `None`, which is not
 13 |     if cond:
 14 |         raise ValueError()
    |
+info: Consider changing the return annotation to `-> None` or adding a `return` statement
 info: rule `invalid-return-type` is enabled by default
 
 ```

crates/ty_python_semantic/resources/mdtest/snapshots/return_type.md_-_Function_return_type_-_Invalid_implicit_ret…_(3d2d19aa49b28f1c).snap

@@ -0,0 +1,34 @@
+---
+source: crates/ty_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: return_type.md - Function return type - Invalid implicit return type always None
+mdtest path: crates/ty_python_semantic/resources/mdtest/function/return_type.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | # error: [invalid-return-type]
+2 | def f() -> int:
+3 |     print("hello")
+```
+
+# Diagnostics
+
+```
+error[invalid-return-type]: Function always implicitly returns `None`, which is not assignable to return type `int`
+ --> src/mdtest_snippet.py:2:12
+  |
+1 | # error: [invalid-return-type]
+2 | def f() -> int:
+  |            ^^^
+3 |     print("hello")
+  |
+info: Consider changing the return annotation to `-> None` or adding a `return` statement
+info: rule `invalid-return-type` is enabled by default
+
+```

crates/ty_python_semantic/resources/mdtest/snapshots/return_type.md_-_Function_return_type_-_Invalid_return_type_(a91e0c67519cd77f).snap

@@ -35,14 +35,15 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/function/return_type.md
 # Diagnostics
 
 ```
-error[invalid-return-type]: Function can implicitly return `None`, which is not assignable to return type `int`
+error[invalid-return-type]: Function always implicitly returns `None`, which is not assignable to return type `int`
  --> src/mdtest_snippet.py:2:12
   |
 1 | # error: [invalid-return-type]
 2 | def f() -> int:
   |            ^^^
 3 |     1
   |
+info: Consider changing the return annotation to `-> None` or adding a `return` statement
 info: rule `invalid-return-type` is enabled by default
 
 ```
@@ -84,13 +85,14 @@ info: rule `invalid-return-type` is enabled by default
 ```
 
 ```
-error[invalid-return-type]: Function can implicitly return `None`, which is not assignable to return type `T`
+error[invalid-return-type]: Function always implicitly returns `None`, which is not assignable to return type `T`
   --> src/mdtest_snippet.py:18:16
    |
 17 | # error: [invalid-return-type]
 18 | def m(x: T) -> T: ...
    |                ^
    |
+info: Consider changing the return annotation to `-> None` or adding a `return` statement
 info: rule `invalid-return-type` is enabled by default
 
 ```

crates/ty_python_semantic/resources/mdtest/snapshots/return_type.md_-_Function_return_type_-_Invalid_return_type_…_(c3a523878447af6b).snap

@@ -46,7 +46,7 @@ info: rule `invalid-return-type` is enabled by default
 ```
 
 ```
-error[invalid-return-type]: Function can implicitly return `None`, which is not assignable to return type `int`
+error[invalid-return-type]: Function always implicitly returns `None`, which is not assignable to return type `int`
  --> src/mdtest_snippet.pyi:6:14
   |
 5 | # error: [invalid-return-type]
@@ -55,12 +55,13 @@ error[invalid-return-type]: Function can implicitly return `None`, which is not
 7 |     print("...")
 8 |     ...
   |
+info: Consider changing the return annotation to `-> None` or adding a `return` statement
 info: rule `invalid-return-type` is enabled by default
 
 ```
 
 ```
-error[invalid-return-type]: Function can implicitly return `None`, which is not assignable to return type `int`
+error[invalid-return-type]: Function always implicitly returns `None`, which is not assignable to return type `int`
   --> src/mdtest_snippet.pyi:11:14
    |
 10 | # error: [invalid-return-type]
@@ -69,6 +70,7 @@ error[invalid-return-type]: Function can implicitly return `None`, which is not
 12 |     f"""{foo} is a function that ..."""
 13 |     ...
    |
+info: Consider changing the return annotation to `-> None` or adding a `return` statement
 info: rule `invalid-return-type` is enabled by default
 
 ```

crates/ty_python_semantic/src/types/diagnostic.rs

@@ -1684,15 +1684,29 @@ pub(super) fn report_implicit_return_type(
     expected_ty: Type,
     has_empty_body: bool,
     enclosing_class_of_method: Option<ClassLiteral>,
+    no_return: bool,
 ) {
     let Some(builder) = context.report_lint(&INVALID_RETURN_TYPE, range) else {
         return;
     };
     let db = context.db();
-    let mut diagnostic = builder.into_diagnostic(format_args!(
-        "Function can implicitly return `None`, which is not assignable to return type `{}`",
-        expected_ty.display(db)
-    ));
+
+    // If no return statement is defined in the function, then the function always returns `None`
+    let mut diagnostic = if no_return {
+        let mut diag = builder.into_diagnostic(format_args!(
+            "Function always implicitly returns `None`, which is not assignable to return type `{}`",
+            expected_ty.display(db),
+        ));
+        diag.info(
+            "Consider changing the return annotation to `-> None` or adding a `return` statement",
+        );
+        diag
+    } else {
+        builder.into_diagnostic(format_args!(
+            "Function can implicitly return `None`, which is not assignable to return type `{}`",
+            expected_ty.display(db),
+        ))
+    };
     if !has_empty_body {
         return;
     }

crates/ty_python_semantic/src/types/infer.rs

@@ -1853,12 +1853,14 @@ impl<'db> TypeInferenceBuilder<'db> {
             if use_def.can_implicit_return(self.db())
                 && !Type::none(self.db()).is_assignable_to(self.db(), declared_ty)
             {
+                let no_return = self.return_types_and_ranges.is_empty();
                 report_implicit_return_type(
                     &self.context,
                     returns.range(),
                     declared_ty,
                     has_empty_body,
                     enclosing_class_context,
+                    no_return,
                 );
             }
         }