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/language/src/language_settings.rs

@@ -120,6 +120,8 @@ pub struct LanguageSettings {
     pub code_actions_on_format: HashMap<String, bool>,
     /// Whether to perform linked edits
     pub linked_edits: bool,
+    /// Task configuration for this language.
+    pub tasks: LanguageTaskConfig,
 }
 
 impl LanguageSettings {
@@ -340,6 +342,10 @@ pub struct LanguageSettingsContent {
     ///
     /// Default: true
     pub linked_edits: Option<bool>,
+    /// Task configuration for this language.
+    ///
+    /// Default: {}
+    pub tasks: Option<LanguageTaskConfig>,
 }
 
 /// The contents of the inline completion settings.
@@ -546,6 +552,13 @@ fn scroll_debounce_ms() -> u64 {
     50
 }
 
+/// The task settings for a particular language.
+#[derive(Debug, Clone, Deserialize, PartialEq, Serialize, JsonSchema)]
+pub struct LanguageTaskConfig {
+    /// Extra task variables to set for a particular language.
+    pub variables: HashMap<String, String>,
+}
+
 impl InlayHintSettings {
     /// Returns the kinds of inlay hints that are enabled based on the settings.
     pub fn enabled_inlay_hint_kinds(&self) -> HashSet<Option<InlayHintKind>> {
@@ -823,6 +836,7 @@ fn merge_settings(settings: &mut LanguageSettings, src: &LanguageSettingsContent
         src.code_actions_on_format.clone(),
     );
     merge(&mut settings.linked_edits, src.linked_edits);
+    merge(&mut settings.tasks, src.tasks.clone());
 
     merge(
         &mut settings.preferred_line_length,

crates/language/src/task_context.rs

@@ -1,4 +1,4 @@
-use std::ops::Range;
+use std::{ops::Range, sync::Arc};
 
 use crate::{Location, Runnable};
 
@@ -31,7 +31,11 @@ pub trait ContextProvider: Send + Sync {
     }
 
     /// Provides all tasks, associated with the current language.
-    fn associated_tasks(&self) -> Option<TaskTemplates> {
+    fn associated_tasks(
+        &self,
+        _: Option<Arc<dyn crate::File>>,
+        _cx: &AppContext,
+    ) -> Option<TaskTemplates> {
         None
     }
 }