Fix error when no arguments are parsed

When cooking the span, it used to give us a (0,1) span even
though the byte slice of the input was empty. Now it does (0,0).

Closes #1

Author: fasterthanlime

src/deserialize.rs

@@ -253,6 +253,17 @@ impl<'input, F: Format<SpanType = Raw, Input<'input> = [&'input str]>> ToCooked<
 {
     #[inline]
     fn to_cooked(self, _format: &F, input: &'input [&'input str]) -> Span<Cooked> {
+        trace!(
+            "cooking span, self.start = {}, input.len = {}",
+            self.start,
+            input.len()
+        );
+
+        if self.start == 0 && input.is_empty() {
+            // empty input
+            return Span::<Cooked>::new(0, 0);
+        }
+
         if self.start >= input.len() {
             // start points past the end of the args;
             // use byte offset = total length of whole input minus 1, len = 1

src/deserialize/error.rs

@@ -302,7 +302,10 @@ impl core::fmt::Display for DeserError<'_> {
 
         let mut span_start = self.span.start();
         let mut span_end = self.span.end();
+        trace!("span range = {span_start}..{span_end}");
+
         use alloc::borrow::Cow;
+        use log::trace;
         let mut input_str: Cow<'_, str> = Cow::Borrowed(orig_input_str);
 
         // --- Context-sensitive truncation logic ---

tests/sequence.rs

@@ -30,6 +30,39 @@ fn test_simplest_value_singleton_list_positional() {
     assert_eq!(args_single.strings, vec!["joe", "le", "rigolo"]);
 }
 
+#[test]
+fn test_noargs_single_positional() {
+    #[derive(Facet, Debug, PartialEq)]
+    struct Args {
+        #[facet(positional)]
+        strings: String,
+    }
+    let err = facet_args::from_slice::<Args>(&[]).unwrap_err();
+    insta::assert_snapshot!(err);
+}
+
+#[test]
+fn test_noargs_vec_positional_default() {
+    #[derive(Facet, Debug, PartialEq)]
+    struct Args {
+        #[facet(positional, default)]
+        strings: Vec<String>,
+    }
+    let args = facet_args::from_slice::<Args>(&[]).unwrap();
+    assert!(args.strings.is_empty());
+}
+
+#[test]
+fn test_noargs_vec_positional_no_default() {
+    #[derive(Facet, Debug, PartialEq)]
+    struct Args {
+        #[facet(positional)]
+        strings: Vec<String>,
+    }
+    let err = facet_args::from_slice::<Args>(&[]).unwrap_err();
+    insta::assert_snapshot!(err);
+}
+
 #[test]
 #[ignore]
 fn test_value_singleton_list() {

tests/snapshots/sequence__noargs_single_positional.snap

@@ -0,0 +1,11 @@
+---
+source: tests/sequence.rs
+expression: err
+---
+Error: 
+   ╭─[ <unknown>:1:1 ]
+   │
+ 1 │ 
+   │ │ 
+   │ ╰─ Field 'Args::strings' was not initialized
+───╯

tests/snapshots/sequence__noargs_vec_positional.snap

@@ -0,0 +1,11 @@
+---
+source: tests/sequence.rs
+expression: err
+---
+Error: 
+   ╭─[ <unknown>:1:1 ]
+   │
+ 1 │ 
+   │ │ 
+   │ ╰─ Field 'Args::strings' was not initialized
+───╯

tests/snapshots/sequence__noargs_vec_positional_no_default.snap

@@ -0,0 +1,11 @@
+---
+source: tests/sequence.rs
+expression: err
+---
+Error: 
+   ╭─[ <unknown>:1:1 ]
+   │
+ 1 │ 
+   │ │ 
+   │ ╰─ Field 'Args::strings' was not initialized
+───╯

tests/snapshots/sequence__seq_empty.snap

@@ -0,0 +1,11 @@
+---
+source: tests/sequence.rs
+expression: err
+---
+Error: 
+   ╭─[ <unknown>:1:1 ]
+   │
+ 1 │ 
+   │ │ 
+   │ ╰─ Field 'Args::strings' was not initialized
+───╯