Auto merge of #8127 - dswij:8090, r=xFrednet

Fix `enum_variants` FP on prefixes that are not camel-case

closes #8090

Fix FP on `enum_variants` when prefixes are only a substring of a camel-case word. Also adds some util helpers on `str_utils` to help parsing camel-case strings.

This changes how the lint behaves:

1. previously if the Prefix is only a length of 1, it's going to get ignored, i.e. these were previously ignored and now is warned
```rust
enum Foo {
    cFoo,
    cBar,
    cBaz,
}

enum Something {
    CCall,
    CCreate,
    CCryogenize,
}
```

2. non-ascii characters that doesn't have casing will not be split,
```rust
enum NonCaps {
    PrefixXXX,
    PrefixTea,
    PrefixCake,
}
```
will be considered as `PrefixXXX`, `Prefix`, `Prefix`, so this won't lint as opposed to fired previously.

changelog: [`enum_variant_names`] Fix FP when first prefix are only a substring of a camel-case word.

---

 (Edited by `@xFrednet` removed some non ascii characters)

clippy_utils/src/str_utils.rs

@@ -68,6 +68,20 @@ pub fn camel_case_until(s: &str) -> StrIndex {
 /// ```
 #[must_use]
 pub fn camel_case_start(s: &str) -> StrIndex {
+    camel_case_start_from_idx(s, 0)
+}
+
+/// Returns `StrIndex` of the last camel-case component of `s[idx..]`.
+///
+/// ```
+/// # use clippy_utils::str_utils::{camel_case_start_from_idx, StrIndex};
+/// assert_eq!(camel_case_start_from_idx("AbcDef", 0), StrIndex::new(0, 0));
+/// assert_eq!(camel_case_start_from_idx("AbcDef", 1), StrIndex::new(3, 3));
+/// assert_eq!(camel_case_start_from_idx("AbcDefGhi", 0), StrIndex::new(0, 0));
+/// assert_eq!(camel_case_start_from_idx("AbcDefGhi", 1), StrIndex::new(3, 3));
+/// assert_eq!(camel_case_start_from_idx("Abcdefg", 1), StrIndex::new(7, 7));
+/// ```
+pub fn camel_case_start_from_idx(s: &str, start_idx: usize) -> StrIndex {
     let char_count = s.chars().count();
     let range = 0..char_count;
     let mut iter = range.rev().zip(s.char_indices().rev());
@@ -78,9 +92,13 @@ pub fn camel_case_start(s: &str) -> StrIndex {
     } else {
         return StrIndex::new(char_count, s.len());
     }
+
     let mut down = true;
     let mut last_index = StrIndex::new(char_count, s.len());
     for (char_index, (byte_index, c)) in iter {
+        if byte_index < start_idx {
+            break;
+        }
         if down {
             if c.is_uppercase() {
                 down = false;
@@ -98,9 +116,55 @@ pub fn camel_case_start(s: &str) -> StrIndex {
             return last_index;
         }
     }
+
     last_index
 }
 
+/// Get the indexes of camel case components of a string `s`
+///
+/// ```
+/// # use clippy_utils::str_utils::{camel_case_indices, StrIndex};
+/// assert_eq!(
+///     camel_case_indices("AbcDef"),
+///     vec![StrIndex::new(0, 0), StrIndex::new(3, 3), StrIndex::new(6, 6)]
+/// );
+/// assert_eq!(
+///     camel_case_indices("abcDef"),
+///     vec![StrIndex::new(3, 3), StrIndex::new(6, 6)]
+/// );
+/// ```
+pub fn camel_case_indices(s: &str) -> Vec<StrIndex> {
+    let mut result = Vec::new();
+    let mut str_idx = camel_case_start(s);
+
+    while str_idx.byte_index < s.len() {
+        let next_idx = str_idx.byte_index + 1;
+        result.push(str_idx);
+        str_idx = camel_case_start_from_idx(s, next_idx);
+    }
+    result.push(str_idx);
+
+    result
+}
+
+/// Split camel case string into a vector of its components
+///
+/// ```
+/// # use clippy_utils::str_utils::{camel_case_split, StrIndex};
+/// assert_eq!(camel_case_split("AbcDef"), vec!["Abc", "Def"]);
+/// ```
+pub fn camel_case_split(s: &str) -> Vec<&str> {
+    let mut offsets = camel_case_indices(s)
+        .iter()
+        .map(|e| e.byte_index)
+        .collect::<Vec<usize>>();
+    if offsets[0] != 0 {
+        offsets.insert(0, 0);
+    }
+
+    offsets.windows(2).map(|w| &s[w[0]..w[1]]).collect()
+}
+
 /// Dealing with sting comparison can be complicated, this struct ensures that both the
 /// character and byte count are provided for correct indexing.
 #[derive(Debug, Default, PartialEq, Eq)]
@@ -231,4 +295,31 @@ mod test {
     fn until_caps() {
         assert_eq!(camel_case_until("ABCD"), StrIndex::new(0, 0));
     }
+
+    #[test]
+    fn camel_case_start_from_idx_full() {
+        assert_eq!(camel_case_start_from_idx("AbcDef", 0), StrIndex::new(0, 0));
+        assert_eq!(camel_case_start_from_idx("AbcDef", 1), StrIndex::new(3, 3));
+        assert_eq!(camel_case_start_from_idx("AbcDef", 4), StrIndex::new(6, 6));
+        assert_eq!(camel_case_start_from_idx("AbcDefGhi", 0), StrIndex::new(0, 0));
+        assert_eq!(camel_case_start_from_idx("AbcDefGhi", 1), StrIndex::new(3, 3));
+        assert_eq!(camel_case_start_from_idx("Abcdefg", 1), StrIndex::new(7, 7));
+    }
+
+    #[test]
+    fn camel_case_indices_full() {
+        assert_eq!(camel_case_indices("Abc\u{f6}\u{f6}DD"), vec![StrIndex::new(7, 9)]);
+    }
+
+    #[test]
+    fn camel_case_split_full() {
+        assert_eq!(camel_case_split("A"), vec!["A"]);
+        assert_eq!(camel_case_split("AbcDef"), vec!["Abc", "Def"]);
+        assert_eq!(camel_case_split("Abc"), vec!["Abc"]);
+        assert_eq!(camel_case_split("abcDef"), vec!["abc", "Def"]);
+        assert_eq!(
+            camel_case_split("\u{f6}\u{f6}AabABcd"),
+            vec!["\u{f6}\u{f6}", "Aab", "A", "Bcd"]
+        );
+    }
 }