add default task feature

[gutenfries]

feat(cli): Add default task behavior (#15241)

when deno task is run, this is the updated behavior;
for a deno.json file that looks like this:

{
  "tasks": {
    "boolean_logic": "sleep 0.1 && echo 3 && echo 4 & echo 1 && echo 2 || echo NOPE",
    "echo": "echo 1",
    "deno_echo": "deno eval 'console.log(5)'",
    "strings": "deno run main.ts && deno eval \"console.log(\\\"test\\\")\"",
    "piped": "echo 12345 | (deno eval 'const b = new Uint8Array(1);Deno.stdin.readSync(b);console.log(b)' && deno eval 'const b = new Uint8Array(1);Deno.stdin.readSync(b);console.log(b)')",
    "exit_code_5": "echo $(echo 10 ; exit 2) && exit 5",
    "echo_cwd": "echo $(pwd)",
    "default": "echo 1 && echo 2",
    "main": "echo \"~Main task~\""
  }
}

---

task 'main'
Command:

deno task main

Output:

Warning deno task is unstable and may drastically change in the future
~Main task~

---

no args
Command:

deno task

Output:

Warning deno task is unstable and may drastically change in the future
Warning No task specified; attempting to run a default task. To prevent this behavior, pass `--no-default-task`. Run `deno task --help` for more info.
Running default task: default
1
2

---

no args with --no-default-task
Command:

deno task --no-default-task

Output:

Warning deno task is unstable and may drastically change in the future
No task specified
Available tasks:
- boolean_logic
    sleep 0.1 && echo 3 && echo 4 & echo 1 && echo 2 || echo NOPE
- default
    echo 1 && echo 2
- deno_echo
    deno eval 'console.log(5)'
- echo
    echo 1
- echo_cwd
    echo $(pwd)
- exit_code_5
    echo $(echo 10 ; exit 2) && exit 5
- main
    echo "~Main task~"
- piped
    echo 12345 | (deno eval 'const b = new Uint8Array(1);Deno.stdin.readSync(b);console.log(b)' && deno eval 'const b = new Uint8Array(1);Deno.stdin.readSync(b);console.log(b)')
- strings
    deno run main.ts && deno eval "console.log(\"test\")"

---

Non-existent task
Command:

deno task dinosaur

Output:

Warning deno task is unstable and may drastically change in the future
Task not found: dinosaur
Available tasks:
- boolean_logic
    sleep 0.1 && echo 3 && echo 4 & echo 1 && echo 2 || echo NOPE
- default
    echo 1 && echo 2
- deno_echo
    deno eval 'console.log(5)'
- echo
    echo 1
- echo_cwd
    echo $(pwd)
- exit_code_5
    echo $(echo 10 ; exit 2) && exit 5
- main
    echo "~Main task~"
- piped
    echo 12345 | (deno eval 'const b = new Uint8Array(1);Deno.stdin.readSync(b);console.log(b)' && deno eval 'const b = new Uint8Array(1);Deno.stdin.readSync(b);console.log(b)')
- strings
    deno run main.ts && deno eval "console.log(\"test\")"

---

Edge cases concerning flag combinations, injection, etc were accounted for.

Please see line ~2677 in cli/args/flags.rs.
There is some temporary code that I added (in addition to the existing temporary code), but note that this will need to be changed when this code is updated in relationship to clap-rs/clap#1538

Let me know if there are issues or enhancements, or if there is any insight into the PR.

Thanks!

[CLAassistant]

All committers have signed the CLA.

[dsherret]

Sorry you put the work in, but as mentioned in the issue, this hasn't been agreed upon. We will most likely not do this behaviour.

[gutenfries]

okay, sounds good!

[dsherret]

Again, sorry you worked on this, but it was never agreed upon in the issue to implement this behaviour. Going to close this one as it's not planned to do it.