seq: improve error handling for invalid -f values

Improve the error message produced by `seq` when given invalid format
specifiers for the `-f` option. Before this commit:

    $ seq -f "%" 1
    seq: %: invalid conversion specification
    $ seq -f "%g%" 1
    seq: %: invalid conversion specification

After this commit:

    $ seq -f "%" 1
    seq: format '%' ends in %
    $ seq -f "%g%" 1
    seq: format '%g%' has too many % directives

This matches the behavior of GNU `seq`.

src/uucore/src/lib/features/format/mod.rs

@@ -22,7 +22,7 @@
 //!  3. Parse both `printf` specifiers and escape sequences (for e.g. `printf`)
 //!
 //! This module aims to combine all three use cases. An iterator parsing each
-//! of these cases is provided by [`parse_escape_only`], [`parse_spec_only`]
+//! of these cases is provided by [`parse_spec_only`], [`parse_escape_only`]
 //! and [`parse_spec_and_escape`], respectively.
 //!
 //! There is a special [`Format`] type, which can be used to parse a format
@@ -46,6 +46,8 @@ use std::{
     ops::ControlFlow,
 };
 
+use os_display::Quotable;
+
 use crate::error::UError;
 
 pub use self::{
@@ -63,6 +65,8 @@ pub enum FormatError {
     NeedAtLeastOneSpec(Vec<u8>),
     WrongSpecType,
     InvalidPrecision(String),
+    /// The format specifier ends with a %, as in `%f%`.
+    EndsWithPercent(Vec<u8>),
 }
 
 impl Error for FormatError {}
@@ -92,6 +96,9 @@ impl Display for FormatError {
                 "format '{}' has no % directive",
                 String::from_utf8_lossy(s)
             ),
+            Self::EndsWithPercent(s) => {
+                write!(f, "format {} ends in %", String::from_utf8_lossy(s).quote())
+            }
             Self::InvalidPrecision(precision) => write!(f, "invalid precision: '{precision}'"),
             // TODO: Error message below needs some work
             Self::WrongSpecType => write!(f, "wrong % directive type was given"),
@@ -190,6 +197,7 @@ pub fn parse_spec_only(
     let mut current = fmt;
     std::iter::from_fn(move || match current {
         [] => None,
+        [b'%'] => Some(Err(FormatError::EndsWithPercent(fmt.to_vec()))),
         [b'%', b'%', rest @ ..] => {
             current = rest;
             Some(Ok(FormatItem::Char(b'%')))
@@ -323,11 +331,14 @@ impl<F: Formatter> Format<F> {
 
         let mut suffix = Vec::new();
         for item in &mut iter {
-            match item? {
+            match item {
-                FormatItem::Spec(_) => {
+                // If the `format_string` is of the form `%f%f` or
+                // `%f%`, then return an error.
+                Ok(FormatItem::Spec(_)) | Err(FormatError::EndsWithPercent(_)) => {
                     return Err(FormatError::TooManySpecs(format_string.as_ref().to_vec()));
                 }
-                FormatItem::Char(c) => suffix.push(c),
+                Ok(FormatItem::Char(c)) => suffix.push(c),
+                Err(e) => return Err(e),
             }
         }
 

tests/by-util/test_seq.rs

@@ -787,6 +787,16 @@ fn test_invalid_format() {
         .fails()
         .no_stdout()
         .stderr_contains("format '%g%g' has too many % directives");
+    new_ucmd!()
+        .args(&["-f", "%g%", "1"])
+        .fails()
+        .no_stdout()
+        .stderr_contains("format '%g%' has too many % directives");
+    new_ucmd!()
+        .args(&["-f", "%", "1"])
+        .fails()
+        .no_stdout()
+        .stderr_contains("format '%' ends in %");
 }
 
 #[test]