Suggest Semicolon in Incorrect Repeat Expressions

Author: veera-sivarajan

compiler/rustc_hir/src/hir.rs

@@ -1807,6 +1807,18 @@ impl Expr<'_> {
         }
     }
 
+    /// Check if expression is an integer literal that can be used
+    /// where `usize` is expected.
+    pub fn is_size_lit(&self) -> bool {
+        matches!(
+            self.kind,
+            ExprKind::Lit(Lit {
+                node: LitKind::Int(_, LitIntType::Unsuffixed | LitIntType::Unsigned(UintTy::Usize)),
+                ..
+            })
+        )
+    }
+
     /// If `Self.kind` is `ExprKind::DropTemps(expr)`, drill down until we get a non-`DropTemps`
     /// `Expr`. This is used in suggestions to ignore this `ExprKind` as it is semantically
     /// silent, only signaling the ownership system. By doing this, suggestions that check the

compiler/rustc_hir_typeck/messages.ftl

@@ -155,6 +155,8 @@ hir_typeck_remove_semi_for_coerce_ret = the `match` arms can conform to this ret
 hir_typeck_remove_semi_for_coerce_semi = the `match` is a statement because of this semicolon, consider removing it
 hir_typeck_remove_semi_for_coerce_suggestion = remove this semicolon
 
+hir_typeck_replace_comma_with_semicolon = replace comma with semicolon to create {$descr}
+
 hir_typeck_return_stmt_outside_of_fn_body =
     {$statement_kind} statement outside of function body
     .encl_body_label = the {$statement_kind} is part of this body...

compiler/rustc_hir_typeck/src/demand.rs

@@ -30,7 +30,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         if expr_ty == expected {
             return;
         }
-
         self.annotate_alternative_method_deref(err, expr, error);
         self.explain_self_literal(err, expr, expected, expr_ty);
 
@@ -39,6 +38,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             || self.suggest_missing_unwrap_expect(err, expr, expected, expr_ty)
             || self.suggest_remove_last_method_call(err, expr, expected)
             || self.suggest_associated_const(err, expr, expected)
+            || self.suggest_semicolon_in_repeat_expr(err, expr, expr_ty)
             || self.suggest_deref_ref_or_into(err, expr, expected, expr_ty, expected_ty_expr)
             || self.suggest_option_to_bool(err, expr, expr_ty, expected)
             || self.suggest_compatible_variants(err, expr, expected, expr_ty)

compiler/rustc_hir_typeck/src/errors.rs

@@ -724,3 +724,16 @@ pub(crate) struct PassToVariadicFunction<'tcx, 'a> {
     #[note(hir_typeck_teach_help)]
     pub(crate) teach: Option<()>,
 }
+
+#[derive(Subdiagnostic)]
+#[suggestion(
+    hir_typeck_replace_comma_with_semicolon,
+    applicability = "machine-applicable",
+    style = "verbose",
+    code = "; "
+)]
+pub struct ReplaceCommaWithSemicolon {
+    #[primary_span]
+    pub comma_span: Span,
+    pub descr: &'static str,
+}

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -1299,14 +1299,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 let span = expr.span.shrink_to_hi();
                 let subdiag = if self.type_is_copy_modulo_regions(self.param_env, ty) {
                     errors::OptionResultRefMismatch::Copied { span, def_path }
-                } else if let Some(clone_did) = self.tcx.lang_items().clone_trait()
-                    && rustc_trait_selection::traits::type_known_to_meet_bound_modulo_regions(
-                        self,
-                        self.param_env,
-                        ty,
-                        clone_did,
-                    )
-                {
+                } else if self.type_is_clone_modulo_regions(self.param_env, ty) {
                     errors::OptionResultRefMismatch::Cloned { span, def_path }
                 } else {
                     return false;
@@ -2159,6 +2152,79 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
     }
 
+    /// Suggest replacing comma with semicolon in incorrect repeat expressions
+    /// like `["_", 10]` or `vec![String::new(), 10]`.
+    pub(crate) fn suggest_semicolon_in_repeat_expr(
+        &self,
+        err: &mut Diag<'_>,
+        expr: &hir::Expr<'_>,
+        expr_ty: Ty<'tcx>,
+    ) -> bool {
+        // Check if `expr` is contained in array of two elements
+        if let hir::Node::Expr(array_expr) = self.tcx.parent_hir_node(expr.hir_id)
+            && let hir::ExprKind::Array(elements) = array_expr.kind
+            && let [first, second] = &elements[..]
+            && second.hir_id == expr.hir_id
+        {
+            // Span between the two elements of the array
+            let comma_span = first.span.between(second.span);
+
+            // Check if `expr` is a constant value of type `usize`.
+            // This can only detect const variable declarations and
+            // calls to const functions.
+
+            // Checking this here instead of rustc_hir::hir because
+            // this check needs access to `self.tcx` but rustc_hir
+            // has no access to `TyCtxt`.
+            let expr_is_const_usize = expr_ty.is_usize()
+                && match expr.kind {
+                    ExprKind::Path(QPath::Resolved(
+                        None,
+                        Path { res: Res::Def(DefKind::Const, _), .. },
+                    )) => true,
+                    ExprKind::Call(
+                        Expr {
+                            kind:
+                                ExprKind::Path(QPath::Resolved(
+                                    None,
+                                    Path { res: Res::Def(DefKind::Fn, fn_def_id), .. },
+                                )),
+                            ..
+                        },
+                        _,
+                    ) => self.tcx.is_const_fn(*fn_def_id),
+                    _ => false,
+                };
+
+            // `array_expr` is from a macro `vec!["a", 10]` if
+            // 1. array expression's span is imported from a macro
+            // 2. first element of array implements `Clone` trait
+            // 3. second element is an integer literal or is an expression of `usize` like type
+            if self.tcx.sess.source_map().is_imported(array_expr.span)
+                && self.type_is_clone_modulo_regions(self.param_env, self.check_expr(first))
+                && (second.is_size_lit() || expr_ty.is_usize_like())
+            {
+                err.subdiagnostic(errors::ReplaceCommaWithSemicolon {
+                    comma_span,
+                    descr: "a vector",
+                });
+                return true;
+            } else if self.type_is_copy_modulo_regions(self.param_env, self.check_expr(first))
+                && (second.is_size_lit() || expr_is_const_usize)
+            {
+                // `array_expr` is from an array `["a", 10]` if
+                // 1. first element of array implements `Copy` trait
+                // 2. second element is an integer literal or is a const value of type `usize`
+                err.subdiagnostic(errors::ReplaceCommaWithSemicolon {
+                    comma_span,
+                    descr: "an array",
+                });
+                return true;
+            }
+        }
+        false
+    }
+
     /// If the expected type is an enum (Issue #55250) with any variants whose
     /// sole field is of the found type, suggest such variants. (Issue #42764)
     pub(crate) fn suggest_compatible_variants(

compiler/rustc_middle/src/ty/sty.rs

@@ -988,6 +988,18 @@ impl<'tcx> Ty<'tcx> {
         }
     }
 
+    /// Check if type is an `usize`.
+    #[inline]
+    pub fn is_usize(self) -> bool {
+        matches!(self.kind(), Uint(UintTy::Usize))
+    }
+
+    /// Check if type is an `usize` or an integral type variable.
+    #[inline]
+    pub fn is_usize_like(self) -> bool {
+        matches!(self.kind(), Uint(UintTy::Usize) | Infer(IntVar(_)))
+    }
+
     #[inline]
     pub fn is_never(self) -> bool {
         matches!(self.kind(), Never)

compiler/rustc_trait_selection/src/infer.rs

@@ -26,6 +26,7 @@ impl<'tcx> InferCtxt<'tcx> {
         })
     }
 
+    /// Check if `ty` implements the `Copy` trait.
     fn type_is_copy_modulo_regions(&self, param_env: ty::ParamEnv<'tcx>, ty: Ty<'tcx>) -> bool {
         let ty = self.resolve_vars_if_possible(ty);
 
@@ -42,6 +43,14 @@ impl<'tcx> InferCtxt<'tcx> {
         traits::type_known_to_meet_bound_modulo_regions(self, param_env, ty, copy_def_id)
     }
 
+    /// Check if `ty` implements the `Clone` trait.
+    fn type_is_clone_modulo_regions(&self, param_env: ty::ParamEnv<'tcx>, ty: Ty<'tcx>) -> bool {
+        let ty = self.resolve_vars_if_possible(ty);
+        let clone_def_id = self.tcx.require_lang_item(LangItem::Clone, None);
+        traits::type_known_to_meet_bound_modulo_regions(self, param_env, ty, clone_def_id)
+    }
+
+    /// Check if `ty` implements the `Sized` trait.
     fn type_is_sized_modulo_regions(&self, param_env: ty::ParamEnv<'tcx>, ty: Ty<'tcx>) -> bool {
         let lang_item = self.tcx.require_lang_item(LangItem::Sized, None);
         traits::type_known_to_meet_bound_modulo_regions(self, param_env, ty, lang_item)