Auto merge of #38066 - bluss:string-slice-error, r=sfackler

Use more specific panic message for &str slicing errors

Separate out of bounds errors from character boundary errors, and print
more details for character boundary errors.

It reports the first error it finds in:

1. begin out of bounds
2. end out of bounds
3. begin <= end violated
3. begin not char boundary
5. end not char boundary.

Example:

    &"abcαβγ"[..4]

    thread 'str::test_slice_fail_boundary_1' panicked at 'byte index 4 is not
    a char boundary; it is inside 'α' (bytes 3..5) of `abcαβγ`'

Fixes #38052

1 files changed, 2 insertions, 2 deletions

diff --git a/src/doc/book/strings.md b/src/doc/book/strings.md
index 6af15d87683..a2146b669e3 100644
--- a/src/doc/book/strings.md
+++ b/src/doc/book/strings.md
@@ -163,8 +163,8 @@ let hachi = &dog[0..2];
 with this error:
 
 ```text
-thread 'main' panicked at 'index 0 and/or 2 in `忠犬ハチ公` do not lie on
-character boundary'
+thread 'main' panicked at 'byte index 2 is not a char boundary; it is inside '忠'
+(bytes 0..3) of `忠犬ハチ公`'
 ```
 
 ## Concatenation