1 files changed, 77 insertions, 0 deletions
diff --git a/clippy_lints/src/format_push_string.rs b/clippy_lints/src/format_push_string.rs
new file mode 100644
index 00000000000..ee15ae9f59a
--- /dev/null
+++ b/clippy_lints/src/format_push_string.rs
@@ -0,0 +1,77 @@
+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.
+    ///
+    /// ### 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 = "1.61.0"]
+    pub FORMAT_PUSH_STRING,
+    perf,
+    "`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",
+            );
+        }
+    }
+}