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