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| 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 | +} |
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