Cleanup: group code for single_char_add_str lint in one file (#15750)

A lot of code was duplicated instead of being shared.

changelog: none

Author: Alexendoo

clippy_lints/src/methods/mod.rs

@@ -103,8 +103,6 @@ mod search_is_some;
 mod seek_from_current;
 mod seek_to_start_instead_of_rewind;
 mod single_char_add_str;
-mod single_char_insert_string;
-mod single_char_push_string;
 mod skip_while_next;
 mod sliced_string_as_bytes;
 mod stable_sort_primitive;

clippy_lints/src/methods/single_char_add_str.rs

@@ -1,14 +1,80 @@
-use crate::methods::{single_char_insert_string, single_char_push_string};
-use rustc_hir as hir;
+use super::SINGLE_CHAR_ADD_STR;
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::source::{snippet_with_applicability, str_literal_to_char_literal};
+use rustc_ast::BorrowKind;
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind};
 use rustc_lint::LateContext;
+use rustc_middle::ty;
 use rustc_span::sym;
 
-pub(super) fn check(cx: &LateContext<'_>, expr: &hir::Expr<'_>, receiver: &hir::Expr<'_>, args: &[hir::Expr<'_>]) {
+pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, receiver: &Expr<'_>, args: &[Expr<'_>]) {
     if let Some(fn_def_id) = cx.typeck_results().type_dependent_def_id(expr.hir_id) {
-        match cx.tcx.get_diagnostic_name(fn_def_id) {
-            Some(sym::string_push_str) => single_char_push_string::check(cx, expr, receiver, args),
-            Some(sym::string_insert_str) => single_char_insert_string::check(cx, expr, receiver, args),
-            _ => {},
+        let mut applicability = Applicability::MachineApplicable;
+        let (short_name, arg, extra) = match cx.tcx.get_diagnostic_name(fn_def_id) {
+            Some(sym::string_insert_str) => (
+                "insert",
+                &args[1],
+                Some(|applicability| {
+                    format!(
+                        "{}, ",
+                        snippet_with_applicability(cx, args[0].span, "..", applicability)
+                    )
+                }),
+            ),
+            Some(sym::string_push_str) => ("push", &args[0], None),
+            _ => return,
+        };
+
+        if let Some(extension_string) = str_literal_to_char_literal(cx, arg, &mut applicability, false) {
+            let base_string_snippet =
+                snippet_with_applicability(cx, receiver.span.source_callsite(), "_", &mut applicability);
+            span_lint_and_sugg(
+                cx,
+                SINGLE_CHAR_ADD_STR,
+                expr.span,
+                format!("calling `{short_name}_str()` using a single-character string literal"),
+                format!("consider using `{short_name}` with a character literal"),
+                format!(
+                    "{base_string_snippet}.{short_name}({}{extension_string})",
+                    extra.map_or(String::new(), |f| f(&mut applicability))
+                ),
+                applicability,
+            );
+        } else if let ExprKind::AddrOf(BorrowKind::Ref, _, inner) = arg.kind
+            && let ExprKind::MethodCall(path_segment, method_arg, [], _) = inner.kind
+            && path_segment.ident.name == sym::to_string
+            && (is_ref_char(cx, method_arg) || is_char(cx, method_arg))
+        {
+            let base_string_snippet =
+                snippet_with_applicability(cx, receiver.span.source_callsite(), "_", &mut applicability);
+            let extension_string = match (
+                snippet_with_applicability(cx, method_arg.span.source_callsite(), "_", &mut applicability),
+                is_ref_char(cx, method_arg),
+            ) {
+                (snippet, false) => snippet,
+                (snippet, true) => format!("*{snippet}").into(),
+            };
+            span_lint_and_sugg(
+                cx,
+                SINGLE_CHAR_ADD_STR,
+                expr.span,
+                format!("calling `{short_name}_str()` using a single-character converted to string"),
+                format!("consider using `{short_name}` without `to_string()`"),
+                format!(
+                    "{base_string_snippet}.{short_name}({}{extension_string})",
+                    extra.map_or(String::new(), |f| f(&mut applicability))
+                ),
+                applicability,
+            );
         }
     }
 }
+
+fn is_ref_char(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
+    matches!(cx.typeck_results().expr_ty(expr).kind(), ty::Ref(_, ty, _) if ty.is_char())
+}
+
+fn is_char(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
+    cx.typeck_results().expr_ty(expr).is_char()
+}