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use clippy_utils::diagnostics::span_lint_and_help;
use clippy_utils::ty::is_type_lang_item;
use clippy_utils::{match_def_path, paths, peel_hir_expr_refs};
use rustc_hir::{BinOpKind, Expr, ExprKind, LangItem};
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::sym;
declare_clippy_lint! {
/// ### What it does
/// Detects cases where the result of a `format!` call is
/// appended to an existing `String`.
///
/// ### Why is this bad?
/// Introduces an extra, avoidable heap allocation.
///
/// ### Known problems
/// `format!` returns a `String` but `write!` returns a `Result`.
/// Thus you are forced to ignore the `Err` variant to achieve the same API.
///
/// While using `write!` in the suggested way should never fail, this isn't necessarily clear to the programmer.
///
/// ### Example
/// ```rust
/// let mut s = String::new();
/// s += &format!("0x{:X}", 1024);
/// s.push_str(&format!("0x{:X}", 1024));
/// ```
/// Use instead:
/// ```rust
/// use std::fmt::Write as _; // import without risk of name clashing
///
/// let mut s = String::new();
/// let _ = write!(s, "0x{:X}", 1024);
/// ```
#[clippy::version = "1.62.0"]
pub FORMAT_PUSH_STRING,
restriction,
"`format!(..)` appended to existing `String`"
}
declare_lint_pass!(FormatPushString => [FORMAT_PUSH_STRING]);
fn is_string(cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
is_type_lang_item(cx, cx.typeck_results().expr_ty(e).peel_refs(), LangItem::String)
}
fn is_format(cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
if let Some(macro_def_id) = e.span.ctxt().outer_expn_data().macro_def_id {
cx.tcx.get_diagnostic_name(macro_def_id) == Some(sym::format_macro)
} else {
false
}
}
impl<'tcx> LateLintPass<'tcx> for FormatPushString {
fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
let arg = match expr.kind {
ExprKind::MethodCall(_, _, [arg], _) => {
if let Some(fn_def_id) = cx.typeck_results().type_dependent_def_id(expr.hir_id) &&
match_def_path(cx, fn_def_id, &paths::PUSH_STR) {
arg
} else {
return;
}
}
ExprKind::AssignOp(op, left, arg)
if op.node == BinOpKind::Add && is_string(cx, left) => {
arg
},
_ => return,
};
let (arg, _) = peel_hir_expr_refs(arg);
if is_format(cx, arg) {
span_lint_and_help(
cx,
FORMAT_PUSH_STRING,
expr.span,
"`format!(..)` appended to existing `String`",
None,
"consider using `write!` to avoid the extra allocation",
);
}
}
}
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