diff options
Diffstat (limited to 'src/tools/clippy/clippy_lints/src/format_push_string.rs')
-rw-r--r-- | src/tools/clippy/clippy_lints/src/format_push_string.rs | 83 |
1 files changed, 83 insertions, 0 deletions
diff --git a/src/tools/clippy/clippy_lints/src/format_push_string.rs b/src/tools/clippy/clippy_lints/src/format_push_string.rs new file mode 100644 index 000000000..ebf5ab086 --- /dev/null +++ b/src/tools/clippy/clippy_lints/src/format_push_string.rs @@ -0,0 +1,83 @@ +use clippy_utils::diagnostics::span_lint_and_help; +use clippy_utils::ty::is_type_diagnostic_item; +use clippy_utils::{match_def_path, paths, peel_hir_expr_refs}; +use rustc_hir::{BinOpKind, Expr, ExprKind}; +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_diagnostic_item(cx, cx.typeck_results().expr_ty(e).peel_refs(), sym::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", + ); + } + } +} |