Use Serde to parse YAML input

[dtolnay]

The errors from clap when my YAML isn't perfect are far less helpful than they could be. This makes it difficult to iterate on a YAML input.

Using Serde to read the YAML, errors would be able to provide the line, column, and property path at which the error occurs.

Steps to reproduce the issue

• Introduce a typo in examples/17_yaml.yml (some example below)
• cargo run --features yaml --example 17_yaml

---

diff --git a/examples/17_yaml.yml b/examples/17_yaml.yml
index 4e55891..7226619 100644
--- a/examples/17_yaml.yml
+++ b/examples/17_yaml.yml
@@ -28,7 +28,7 @@ args:
     - flag:
         help: demo flag argument
         short: F
-        multiple: true
+        multiple: tru
         global: true
         # Conflicts, mutual overrides, and requirements can all be defined as a
         # list, where the key is the name of the other argument

Actual clap error

failed to convert YAML String("tru") value to a string

(this seems like a bug)

Serde-style error

args[2].flag.multiple: invalid type: string "tru", expected a boolean at line 31 column 19

---

diff --git a/examples/17_yaml.yml b/examples/17_yaml.yml
index 4e55891..5b6ee43 100644
--- a/examples/17_yaml.yml
+++ b/examples/17_yaml.yml
@@ -9,7 +9,7 @@ settings:
 
 # All Args must be defined in the 'args:' list where the name of the arg, is the
 # key to a Hash object
-args:
+arguments:
     # The name of this argument, is 'opt' which will be used to access the value
     # later in your Rust code
     - opt:

Current clap error

• None, silently ignores everything I wrote

Serde-style error

unknown field `arguments`, expected one of `name`, `version`, `about`, `author`, `settings`, `args`, `subcommands`, `groups` at line 12 column 1

---

diff --git a/examples/17_yaml.yml b/examples/17_yaml.yml
index 4e55891..a1fc412 100644
--- a/examples/17_yaml.yml
+++ b/examples/17_yaml.yml
@@ -67,7 +67,7 @@ args:
 subcommands:
     # The nae of this subcommand will be 'subcmd' which can be accessed in your
     # Rust code later
-    - subcmd:
+    subcmd:
         about: demos subcommands from yaml
         version: "0.1"
         author: Kevin K. <kbknapp@gmail.com>

Current clap error

• None, silently ignores my subcommand

Serde-style error

subcommands: invalid type: map, expected a list of subcommands at line 70 column 5

[kbknapp]

Thanks for the detailed write up! I'm 100% for this if it can be made in a backwards compatible way, if not 3.x shouldn't be too far on the horizon and would accept a PR to the 3x-dev branch.

I'll mark this as "help wanted" simply because my bandwidth is limited right now with work and would be more than willing to take a look at a PR on the matter. Once I knock out a few more issues and my bandwidth opens back up, I'll take a swing at implementing this if it hasn't been done already 😉

[kbknapp]

Blocker: dtolnay/serde-yaml#94

[epage]

#1041 would help with the lifetime blocker

[epage]

We have decided to deprecate the YAML API. For more details, see #3087. If there is interest in a YAML API, it will likely be broken out into a clap_yaml and developed independent from the core of clap.