`todo_edit` — editing task metadata

todo_edit changes the metadata of an existing task: its description, its group, and/or its dependency list. The edit is partial — every field on TodoEdit is Option, and only the fields you provide are touched:

pub fn todo_edit(list: &TodoList, edit: &TodoEdit)
    -> Result<TodoWriteOutput, PlanError>

let edit = TodoEdit {
    id: "task-1".into(),
    description: Some("Design the schema (v2)".into()),  // rewrite
    group: Some("Backend".into()),                       // move to another group
    depends_on: None,                                    // leave unchanged
};

The wrapper method is Plan::todo_edit(&mut self, edit).

What each field does

Field	Effect
`description: Some(text)`	Replaces the description. Empty/whitespace text is rejected (`"Description must be non-empty text."`).
`group: Some(title)`	Moves the task to that group, creating it if needed (new groups start `tests_verified: false`). A group equal to the current one is a no-op.
`depends_on: Some(ids)`	Replaces the whole dependency list.

Fields left None are untouched — that is what makes TodoEdit safe to reuse: one struct can describe a rename, a move, or both without needing to know the task’s current values.

Dependency problems nag (and are still applied)

Like Add, an edited depends_on list is stored as given, and problems only produce nags:

Problem	Nag
`depends_on` contains the task’s own id	`"Dependency 'x' is a self-reference."`
`depends_on` references a nonexistent task	`"Dependency 'x' does not exist in the task list."`
The dependency would create a cycle	`"Dependency 'x' would create a circular dependency chain."`

The cycle check is transitive: it walks the dependency graph from the new dependency to see whether the edited task is already reachable from it (A → B → C, then editing A to depend on C is caught). Despite the nag, the list is updated — the module warns, it does not police.

Errors

Nonexistent task id → "Task 'x' does not exist. Use List to see available tasks."
Empty new description → "Description must be non-empty text."

Both reject the whole edit: nothing changes.

Example

use cosh_tools::plan::{TodoList, TodoWriteAction, TodoEdit, todo_write, todo_edit};

let mut list = TodoList::default();
list = todo_write(&list, &TodoWriteAction::Add {
    group: "Database".into(),
    description: "Design schema".into(),
    depends_on: None,
})?.list;

// Rename the task and move it to the "Backend" group in one edit.
let out = todo_edit(&list, &TodoEdit {
    id: "task-1".into(),
    description: Some("Design schema (v2)".into()),
    group: Some("Backend".into()),
    depends_on: None,
})?;
list = out.list;

// A circular dependency nags — task-1 now depends on task-2, and task-2
// depends on task-1 — but the edit still lands.
let list2 = todo_write(&list, &TodoWriteAction::Add {
    group: "Backend".into(),
    description: "Migrations".into(),
    depends_on: Some(vec!["task-1".into()]),
})?.list;
let out = todo_edit(&list2, &TodoEdit {
    id: "task-1".into(),
    description: None,
    group: None,
    depends_on: Some(vec!["task-2".into()]),
})?;
assert!(out.nags.iter().any(|n| n.message.contains("circular")));

todo_edit — editing task metadata

What each field does

Dependency problems nag (and are still applied)

Errors

Example

`todo_edit` — editing task metadata