Don't show runnable suggestions for other files

It't be actually great to have these once we have run anything dialog,
but for run the thing at point it makes sense to show a limited set.

crates/ide/src/runnables.rs

@@ -9,6 +9,7 @@ use syntax::{
     ast::{self, AstNode, AttrsOwner},
     match_ast, SyntaxNode,
 };
+use test_utils::mark;
 
 use crate::{
     display::{ToNav, TryToNav},
@@ -96,7 +97,7 @@ impl Runnable {
 pub(crate) fn runnables(db: &RootDatabase, file_id: FileId) -> Vec<Runnable> {
     let sema = Semantics::new(db);
     let module = match sema.to_module_def(file_id) {
-        None => return vec![],
+        None => return Vec::new(),
         Some(it) => it,
     };
 
@@ -128,8 +129,14 @@ fn runnables_mod(sema: &Semantics<RootDatabase>, module: hir::Module) -> Vec<Run
     ));
 
     res.extend(module.declarations(sema.db).into_iter().flat_map(|def| match def {
-        hir::ModuleDef::Module(it) => runnables_mod(sema, it),
-        _ => vec![],
+        hir::ModuleDef::Module(submodule) => match submodule.definition_source(sema.db).value {
+            hir::ModuleSource::SourceFile(_) => {
+                mark::hit!(dont_recurse_in_outline_submodules);
+                Vec::new()
+            }
+            hir::ModuleSource::Module(_) => runnables_mod(sema, submodule),
+        },
+        _ => Vec::new(),
     }));
 
     res
@@ -326,6 +333,7 @@ fn has_test_function_or_multiple_test_submodules(
 #[cfg(test)]
 mod tests {
     use expect_test::{expect, Expect};
+    use test_utils::mark;
 
     use crate::fixture;
 
@@ -1050,4 +1058,25 @@ mod tests {
             "#]],
         );
     }
+
+    #[test]
+    fn dont_recurse_in_outline_submodules() {
+        mark::check!(dont_recurse_in_outline_submodules);
+        check(
+            r#"
+//- /lib.rs
+$0
+mod m;
+//- /m.rs
+mod tests {
+    #[test]
+    fn t() {}
+}
+"#,
+            &[],
+            expect![[r#"
+                []
+            "#]],
+        );
+    }
 }

docs/dev/style.md

@@ -280,6 +280,9 @@ Prefer `Default` even it has to be implemented manually.
 
 **Rationale:** less typing in the common case, uniformity.
 
+Use `Vec::new` rather than `vec![]`. **Rationale:** uniformity, strength
+reduction.
+
 ## Functions Over Objects
 
 Avoid creating "doer" objects.