Change help message to make some sense in broader context

Author: hkmatsumoto

compiler/rustc_parse/src/parser/stmt.rs

@@ -578,7 +578,7 @@ impl<'a> Parser<'a> {
                             // this only when parsing an index expression.
                             err.span_suggestion_verbose(
                                 self.token.span,
-                                "you might have meant to make a slice with range index",
+                                "you might have meant a range expression",
                                 "..",
                                 Applicability::MaybeIncorrect,
                             );

tests/ui/suggestions/range-index-instead-of-colon.rs

@@ -3,5 +3,5 @@
 fn main() {
     &[1, 2, 3][1:2];
     //~^ ERROR: expected one of
-    //~| HELP: you might have meant to make a slice with range index
+    //~| HELP: you might have meant a range expression
 }

tests/ui/suggestions/range-index-instead-of-colon.stderr

@@ -4,7 +4,7 @@ error: expected one of `.`, `?`, `]`, or an operator, found `:`
 LL |     &[1, 2, 3][1:2];
    |                 ^ expected one of `.`, `?`, `]`, or an operator
    |
-help: you might have meant to make a slice with range index
+help: you might have meant a range expression
    |
 LL |     &[1, 2, 3][1..2];
    |                 ~~