src/tools/clippy/clippy_lints/src/format_push_string.rs

   1 use clippy_utils::diagnostics::span_lint_and_help;
   2 use clippy_utils::ty::is_type_diagnostic_item;
   3 use clippy_utils::{match_def_path, paths, peel_hir_expr_refs};
   4 use rustc_hir::{BinOpKind, Expr, ExprKind};
   5 use rustc_lint::{LateContext, LateLintPass};
   6 use rustc_session::{declare_lint_pass, declare_tool_lint};
   7 use rustc_span::sym;
   8
   9 declare_clippy_lint! {
  10     /// ### What it does
  11     /// Detects cases where the result of a `format!` call is
  12     /// appended to an existing `String`.
  13     ///
  14     /// ### Why is this bad?
  15     /// Introduces an extra, avoidable heap allocation.
  16     ///
  17     /// ### Example
  18     /// ```rust
  19     /// let mut s = String::new();
  20     /// s += &format!("0x{:X}", 1024);
  21     /// s.push_str(&format!("0x{:X}", 1024));
  22     /// ```
  23     /// Use instead:
  24     /// ```rust
  25     /// use std::fmt::Write as _; // import without risk of name clashing
  26     ///
  27     /// let mut s = String::new();
  28     /// let _ = write!(s, "0x{:X}", 1024);
  29     /// ```
  30     #[clippy::version = "1.61.0"]
  31     pub FORMAT_PUSH_STRING,
  32     perf,
  33     "`format!(..)` appended to existing `String`"
  34 }
  35 declare_lint_pass!(FormatPushString => [FORMAT_PUSH_STRING]);
  36
  37 fn is_string(cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
  38     is_type_diagnostic_item(cx, cx.typeck_results().expr_ty(e).peel_refs(), sym::String)
  39 }
  40 fn is_format(cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
  41     if let Some(macro_def_id) = e.span.ctxt().outer_expn_data().macro_def_id {
  42         cx.tcx.get_diagnostic_name(macro_def_id) == Some(sym::format_macro)
  43     } else {
  44         false
  45     }
  46 }
  47
  48 impl<'tcx> LateLintPass<'tcx> for FormatPushString {
  49     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  50         let arg = match expr.kind {
  51             ExprKind::MethodCall(_, [_, arg], _) => {
  52                 if let Some(fn_def_id) = cx.typeck_results().type_dependent_def_id(expr.hir_id) &&
  53                 match_def_path(cx, fn_def_id, &paths::PUSH_STR) {
  54                     arg
  55                 } else {
  56                     return;
  57                 }
  58             }
  59             ExprKind::AssignOp(op, left, arg)
  60             if op.node == BinOpKind::Add && is_string(cx, left) => {
  61                 arg
  62             },
  63             _ => return,
  64         };
  65         let (arg, _) = peel_hir_expr_refs(arg);
  66         if is_format(cx, arg) {
  67             span_lint_and_help(
  68                 cx,
  69                 FORMAT_PUSH_STRING,
  70                 expr.span,
  71                 "`format!(..)` appended to existing `String`",
  72                 None,
  73                 "consider using `write!` to avoid the extra allocation",
  74             );
  75         }
  76     }
  77 }