rust/clippy_lints/src/format_push_string.rs

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use clippy_utils::diagnostics::span_lint_and_help;
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use clippy_utils::ty::is_type_lang_item;
use clippy_utils::{higher, match_def_path, paths};
use rustc_hir::{BinOpKind, Expr, ExprKind, LangItem, MatchSource};
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::declare_lint_pass;
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
/// ```no_run
/// let mut s = String::new();
/// s += &format!("0x{:X}", 1024);
/// s.push_str(&format!("0x{:X}", 1024));
/// ```
/// Use instead:
/// ```no_run
/// 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 {
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is_type_lang_item(cx, cx.typeck_results().expr_ty(e).peel_refs(), LangItem::String)
}
fn is_format(cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
let e = e.peel_blocks().peel_borrows();
if e.span.from_expansion()
&& 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 if let Some(higher::If { then, r#else, .. }) = higher::If::hir(e) {
is_format(cx, then) || r#else.is_some_and(|e| is_format(cx, e))
} else {
match higher::IfLetOrMatch::parse(cx, e) {
Some(higher::IfLetOrMatch::Match(_, arms, MatchSource::Normal)) => {
arms.iter().any(|arm| is_format(cx, arm.body))
},
Some(higher::IfLetOrMatch::IfLet(_, _, then, r#else, _)) => {
is_format(cx, then) || r#else.is_some_and(|e| is_format(cx, e))
},
_ => 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,
};
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",
);
}
}
}