Detect Python-like slicing and suggest how to fix

Fix #108215

compiler/rustc_ast/src/token.rs

@@ -756,6 +756,11 @@ impl Token {
         )
     }
 
+    /// Returns `true` if the token is the integer literal.
+    pub fn is_integer_lit(&self) -> bool {
+        matches!(self.kind, Literal(Lit { kind: LitKind::Integer, .. }))
+    }
+
     /// Returns `true` if the token is a non-raw identifier for which `pred` holds.
     pub fn is_non_raw_ident_where(&self, pred: impl FnOnce(Ident) -> bool) -> bool {
         match self.ident() {

compiler/rustc_parse/src/parser/stmt.rs

@@ -567,6 +567,22 @@ impl<'a> Parser<'a> {
                         snapshot.recover_diff_marker();
                     }
                     if self.token == token::Colon {
+                        // if a previous and next token of the current one is
+                        // integer literal (e.g. `1:42`), it's likely a range
+                        // expression for Pythonistas and we can suggest so.
+                        if self.prev_token.is_integer_lit()
+                            && self.look_ahead(1, |token| token.is_integer_lit())
+                        {
+                            // TODO(hkmatsumoto): Might be better to trigger
+                            // 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",
+                                "..",
+                                Applicability::MaybeIncorrect,
+                            );
+                        }
+
                         // if next token is following a colon, it's likely a path
                         // and we can suggest a path separator
                         self.bump();

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

@@ -0,0 +1,8 @@
+// edition:2021
+
+fn main() {
+    &[1, 2, 3][1:2];
+    //~^ ERROR: expected one of
+    //~| HELP: you might have meant to make a slice with range index
+    //~| HELP: maybe write a path separator here
+}

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

@@ -0,0 +1,18 @@
+error: expected one of `.`, `?`, `]`, or an operator, found `:`
+  --> $DIR/range-index-instead-of-colon.rs:4:17
+   |
+LL |     &[1, 2, 3][1:2];
+   |                 ^ expected one of `.`, `?`, `]`, or an operator
+   |
+   = note: type ascription syntax has been removed, see issue #101728 <https://github.com/rust-lang/rust/issues/101728>
+help: you might have meant to make a slice with range index
+   |
+LL |     &[1, 2, 3][1..2];
+   |                 ~~
+help: maybe write a path separator here
+   |
+LL |     &[1, 2, 3][1::2];
+   |                 ~~
+
+error: aborting due to previous error
+