move single_char_add_str to its own module

Author: TaKO8Ki

clippy_lints/src/methods/single_char_add_str.rs

@@ -0,0 +1,15 @@
+use crate::methods::{single_char_insert_string, single_char_push_string};
+use crate::utils::match_def_path;
+use crate::utils::paths;
+use rustc_hir as hir;
+use rustc_lint::LateContext;
+
+pub(super) fn check(cx: &LateContext<'_>, expr: &hir::Expr<'_>, args: &[hir::Expr<'_>]) {
+    if let Some(fn_def_id) = cx.typeck_results().type_dependent_def_id(expr.hir_id) {
+        if match_def_path(cx, fn_def_id, &paths::PUSH_STR) {
+            single_char_push_string::check(cx, expr, args);
+        } else if match_def_path(cx, fn_def_id, &paths::INSERT_STR) {
+            single_char_insert_string::check(cx, expr, args);
+        }
+    }
+}