literal representation: simplification

Simplify calculation in grouping. Add test case to ensure `count()`
can't be zero in that branch.

Author: Michael Wright

clippy_lints/src/literal_representation.rs

@@ -268,7 +268,7 @@ impl<'a> NumericLiteral<'a> {
         let first_group_size;
 
         if partial_group_first {
-            first_group_size = (digits.clone().count() + group_size - 1) % group_size + 1;
+            first_group_size = (digits.clone().count() - 1) % group_size + 1;
             if pad {
                 for _ in 0..group_size - first_group_size {
                     output.push('0');

tests/ui/inconsistent_digit_grouping.fixed

@@ -18,4 +18,7 @@ fn main() {
     let _ = 0x0100_0000;
     let _ = 0x1000_0000;
     let _ = 0x0001_0000_0000_u64;
+
+    // Test suggestion when fraction has no digits
+    let _: f32 = 123_456.;
 }

tests/ui/inconsistent_digit_grouping.rs

@@ -18,4 +18,7 @@ fn main() {
     let _ = 0x1000000;
     let _ = 0x10000000;
     let _ = 0x100000000_u64;
+
+    // Test suggestion when fraction has no digits
+    let _: f32 = 1_23_456.;
 }

tests/ui/inconsistent_digit_grouping.stderr

@@ -56,5 +56,11 @@ error: long literal lacking separators
 LL |     let _ = 0x100000000_u64;
    |             ^^^^^^^^^^^^^^^ help: consider: `0x0001_0000_0000_u64`
 
-error: aborting due to 9 previous errors
+error: digits grouped inconsistently by underscores
+  --> $DIR/inconsistent_digit_grouping.rs:23:18
+   |
+LL |     let _: f32 = 1_23_456.;
+   |                  ^^^^^^^^^ help: consider: `123_456.`
+
+error: aborting due to 10 previous errors