src/tools/clippy/clippy_lints/src/rc_clone_in_vec_init.rs

   1 use clippy_utils::diagnostics::span_lint_and_then;
   2 use clippy_utils::higher::VecArgs;
   3 use clippy_utils::last_path_segment;
   4 use clippy_utils::macros::root_macro_call_first_node;
   5 use clippy_utils::paths;
   6 use clippy_utils::source::{indent_of, snippet};
   7 use clippy_utils::ty::match_type;
   8 use rustc_errors::Applicability;
   9 use rustc_hir::{Expr, ExprKind, QPath, TyKind};
  10 use rustc_lint::{LateContext, LateLintPass};
  11 use rustc_session::{declare_lint_pass, declare_tool_lint};
  12 use rustc_span::{sym, Span, Symbol};
  13
  14 declare_clippy_lint! {
  15     /// ### What it does
  16     /// Checks for reference-counted pointers (`Arc`, `Rc`, `rc::Weak`, and `sync::Weak`)
  17     /// in `vec![elem; len]`
  18     ///
  19     /// ### Why is this bad?
  20     /// This will create `elem` once and clone it `len` times - doing so with `Arc`/`Rc`/`Weak`
  21     /// is a bit misleading, as it will create references to the same pointer, rather
  22     /// than different instances.
  23     ///
  24     /// ### Example
  25     /// ```rust
  26     /// let v = vec![std::sync::Arc::new("some data".to_string()); 100];
  27     /// // or
  28     /// let v = vec![std::rc::Rc::new("some data".to_string()); 100];
  29     /// ```
  30     /// Use instead:
  31     /// ```rust
  32     /// // Initialize each value separately:
  33     /// let mut data = Vec::with_capacity(100);
  34     /// for _ in 0..100 {
  35     ///     data.push(std::rc::Rc::new("some data".to_string()));
  36     /// }
  37     ///
  38     /// // Or if you want clones of the same reference,
  39     /// // Create the reference beforehand to clarify that
  40     /// // it should be cloned for each value
  41     /// let data = std::rc::Rc::new("some data".to_string());
  42     /// let v = vec![data; 100];
  43     /// ```
  44     #[clippy::version = "1.62.0"]
  45     pub RC_CLONE_IN_VEC_INIT,
  46     suspicious,
  47     "initializing reference-counted pointer in `vec![elem; len]`"
  48 }
  49 declare_lint_pass!(RcCloneInVecInit => [RC_CLONE_IN_VEC_INIT]);
  50
  51 impl LateLintPass<'_> for RcCloneInVecInit {
  52     fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
  53         let Some(macro_call) = root_macro_call_first_node(cx, expr) else { return; };
  54         let Some(VecArgs::Repeat(elem, len)) = VecArgs::hir(cx, expr) else { return; };
  55         let Some((symbol, func_span)) = ref_init(cx, elem) else { return; };
  56
  57         emit_lint(cx, symbol, macro_call.span, elem, len, func_span);
  58     }
  59 }
  60
  61 fn loop_init_suggestion(elem: &str, len: &str, indent: &str) -> String {
  62     format!(
  63         r#"{{
  64 {indent}    let mut v = Vec::with_capacity({len});
  65 {indent}    (0..{len}).for_each(|_| v.push({elem}));
  66 {indent}    v
  67 {indent}}}"#
  68     )
  69 }
  70
  71 fn extract_suggestion(elem: &str, len: &str, indent: &str) -> String {
  72     format!(
  73         "{{
  74 {indent}    let data = {elem};
  75 {indent}    vec![data; {len}]
  76 {indent}}}"
  77     )
  78 }
  79
  80 fn emit_lint(cx: &LateContext<'_>, symbol: Symbol, lint_span: Span, elem: &Expr<'_>, len: &Expr<'_>, func_span: Span) {
  81     let symbol_name = symbol.as_str();
  82
  83     span_lint_and_then(
  84         cx,
  85         RC_CLONE_IN_VEC_INIT,
  86         lint_span,
  87         "initializing a reference-counted pointer in `vec![elem; len]`",
  88         |diag| {
  89             let len_snippet = snippet(cx, len.span, "..");
  90             let elem_snippet = format!("{}(..)", snippet(cx, elem.span.with_hi(func_span.hi()), ".."));
  91             let indentation = " ".repeat(indent_of(cx, lint_span).unwrap_or(0));
  92             let loop_init_suggestion = loop_init_suggestion(&elem_snippet, len_snippet.as_ref(), &indentation);
  93             let extract_suggestion = extract_suggestion(&elem_snippet, len_snippet.as_ref(), &indentation);
  94
  95             diag.note(format!("each element will point to the same `{symbol_name}` instance"));
  96             diag.span_suggestion(
  97                 lint_span,
  98                 format!("consider initializing each `{symbol_name}` element individually"),
  99                 loop_init_suggestion,
 100                 Applicability::HasPlaceholders,
 101             );
 102             diag.span_suggestion(
 103                 lint_span,
 104                 format!(
 105                     "or if this is intentional, consider extracting the `{symbol_name}` initialization to a variable"
 106                 ),
 107                 extract_suggestion,
 108                 Applicability::HasPlaceholders,
 109             );
 110         },
 111     );
 112 }
 113
 114 /// Checks whether the given `expr` is a call to `Arc::new`, `Rc::new`, or evaluates to a `Weak`
 115 fn ref_init(cx: &LateContext<'_>, expr: &Expr<'_>) -> Option<(Symbol, Span)> {
 116     if_chain! {
 117         if let ExprKind::Call(func, _args) = expr.kind;
 118         if let ExprKind::Path(ref func_path @ QPath::TypeRelative(ty, _)) = func.kind;
 119         if let TyKind::Path(ref ty_path) = ty.kind;
 120         if let Some(def_id) = cx.qpath_res(ty_path, ty.hir_id).opt_def_id();
 121
 122         then {
 123             if last_path_segment(func_path).ident.name == sym::new
 124                 && let Some(symbol) = cx
 125                     .tcx
 126                     .get_diagnostic_name(def_id)
 127                     .filter(|symbol| symbol == &sym::Arc || symbol == &sym::Rc) {
 128                 return Some((symbol, func.span));
 129             }
 130
 131             let ty_path = cx.typeck_results().expr_ty(expr);
 132             if match_type(cx, ty_path, &paths::WEAK_RC) || match_type(cx, ty_path, &paths::WEAK_ARC) {
 133                 return Some((Symbol::intern("Weak"), func.span));
 134             }
 135         }
 136     }
 137
 138     None
 139 }