tasks: Add experimental support for user-defined task variables (#13699)

Context:
@bennetbo spotted a regression in handling of `cargo run` task in zed
repo following a merge of #13658. We've started invoking `cargo run`
from the folder of an active file whereas previously we did it from the
workspace root. We brainstormed few solutions that involved adding a
separate task that gets invoked at a workspace level, but I realized
that a cleaner solution may be to finally add user-configured task
variables. This way, we can choose which crate to run by default at a
workspace level.

This has been originally brought up in the context of javascript tasks
in
https://github.com/zed-industries/zed/pull/12118#issuecomment-2129232114

Note that this is intended for internal use only for the time being.
/cc @RemcoSmitsDev we should be unblocked on having runner-dependant
tasks now.

Release notes:

- N/A

crates/project/src/task_inventory.rs

@@ -15,7 +15,7 @@ use futures::{
 };
 use gpui::{AppContext, Context, Model, ModelContext, Task};
 use itertools::Itertools;
-use language::{ContextProvider, Language, Location};
+use language::{ContextProvider, File, Language, Location};
 use task::{
     static_source::StaticSource, ResolvedTask, TaskContext, TaskId, TaskTemplate, TaskTemplates,
     TaskVariables, VariableName,
@@ -155,14 +155,16 @@ impl Inventory {
     /// returns all task templates with their source kinds, in no specific order.
     pub fn list_tasks(
         &self,
+        file: Option<Arc<dyn File>>,
         language: Option<Arc<Language>>,
         worktree: Option<WorktreeId>,
+        cx: &AppContext,
     ) -> Vec<(TaskSourceKind, TaskTemplate)> {
         let task_source_kind = language.as_ref().map(|language| TaskSourceKind::Language {
             name: language.name(),
         });
         let language_tasks = language
-            .and_then(|language| language.context_provider()?.associated_tasks())
+            .and_then(|language| language.context_provider()?.associated_tasks(file, cx))
             .into_iter()
             .flat_map(|tasks| tasks.0.into_iter())
             .flat_map(|task| Some((task_source_kind.as_ref()?, task)));
@@ -207,8 +209,11 @@ impl Inventory {
         let task_source_kind = language.as_ref().map(|language| TaskSourceKind::Language {
             name: language.name(),
         });
+        let file = location
+            .as_ref()
+            .and_then(|location| location.buffer.read(cx).file().cloned());
         let language_tasks = language
-            .and_then(|language| language.context_provider()?.associated_tasks())
+            .and_then(|language| language.context_provider()?.associated_tasks(file, cx))
             .into_iter()
             .flat_map(|tasks| tasks.0.into_iter())
             .flat_map(|task| Some((task_source_kind.as_ref()?, task)));
@@ -471,9 +476,9 @@ mod test_inventory {
         worktree: Option<WorktreeId>,
         cx: &mut TestAppContext,
     ) -> Vec<String> {
-        inventory.update(cx, |inventory, _| {
+        inventory.update(cx, |inventory, cx| {
             inventory
-                .list_tasks(None, worktree)
+                .list_tasks(None, None, worktree, cx)
                 .into_iter()
                 .map(|(_, task)| task.label)
                 .sorted()
@@ -486,9 +491,9 @@ mod test_inventory {
         task_name: &str,
         cx: &mut TestAppContext,
     ) {
-        inventory.update(cx, |inventory, _| {
+        inventory.update(cx, |inventory, cx| {
             let (task_source_kind, task) = inventory
-                .list_tasks(None, None)
+                .list_tasks(None, None, None, cx)
                 .into_iter()
                 .find(|(_, task)| task.label == task_name)
                 .unwrap_or_else(|| panic!("Failed to find task with name {task_name}"));
@@ -639,7 +644,11 @@ impl ContextProviderWithTasks {
 }
 
 impl ContextProvider for ContextProviderWithTasks {
-    fn associated_tasks(&self) -> Option<TaskTemplates> {
+    fn associated_tasks(
+        &self,
+        _: Option<Arc<dyn language::File>>,
+        _: &AppContext,
+    ) -> Option<TaskTemplates> {
         Some(self.templates.clone())
     }
 }