From e65c060d78db59c5f21cb13f4ec322e407f0a642 Mon Sep 17 00:00:00 2001
From: Hirochika Matsumoto <hirochika.k.matsumoto@gmail.com>
Date: Wed, 3 May 2023 22:32:52 +0900
Subject: [PATCH] Detect Python-like slicing and suggest how to fix

Fix #108215
---
 compiler/rustc_ast/src/token.rs                |  5 +++++
 compiler/rustc_parse/src/parser/stmt.rs        | 16 ++++++++++++++++
 .../range-index-instead-of-colon.rs            |  8 ++++++++
 .../range-index-instead-of-colon.stderr        | 18 ++++++++++++++++++
 4 files changed, 47 insertions(+)
 create mode 100644 tests/ui/suggestions/range-index-instead-of-colon.rs
 create mode 100644 tests/ui/suggestions/range-index-instead-of-colon.stderr

diff --git a/compiler/rustc_ast/src/token.rs b/compiler/rustc_ast/src/token.rs
index 3189fcf7ff9..a7c6f8c5d8c 100644
--- a/compiler/rustc_ast/src/token.rs
+++ b/compiler/rustc_ast/src/token.rs
@@ -756,6 +756,11 @@ pub fn is_numeric_lit(&self) -> bool {
         )
     }
 
+    /// 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() {
diff --git a/compiler/rustc_parse/src/parser/stmt.rs b/compiler/rustc_parse/src/parser/stmt.rs
index 5316dde6096..94d200188ea 100644
--- a/compiler/rustc_parse/src/parser/stmt.rs
+++ b/compiler/rustc_parse/src/parser/stmt.rs
@@ -567,6 +567,22 @@ pub(crate) fn parse_block_tail(
                         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();
diff --git a/tests/ui/suggestions/range-index-instead-of-colon.rs b/tests/ui/suggestions/range-index-instead-of-colon.rs
new file mode 100644
index 00000000000..a5e0098544d
--- /dev/null
+++ b/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
+}
\ No newline at end of file
diff --git a/tests/ui/suggestions/range-index-instead-of-colon.stderr b/tests/ui/suggestions/range-index-instead-of-colon.stderr
new file mode 100644
index 00000000000..72e400fba0a
--- /dev/null
+++ b/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
+