clippy_lints/src/methods/unnecessary_join.rs

   1 use clippy_utils::{diagnostics::span_lint_and_sugg, ty::is_type_lang_item};
   2 use rustc_ast::ast::LitKind;
   3 use rustc_errors::Applicability;
   4 use rustc_hir::{Expr, ExprKind, LangItem};
   5 use rustc_lint::LateContext;
   6 use rustc_middle::ty::{Ref, Slice};
   7 use rustc_span::Span;
   8
   9 use super::UNNECESSARY_JOIN;
  10
  11 pub(super) fn check<'tcx>(
  12     cx: &LateContext<'tcx>,
  13     expr: &'tcx Expr<'tcx>,
  14     join_self_arg: &'tcx Expr<'tcx>,
  15     join_arg: &'tcx Expr<'tcx>,
  16     span: Span,
  17 ) {
  18     let applicability = Applicability::MachineApplicable;
  19     let collect_output_adjusted_type = cx.typeck_results().expr_ty_adjusted(join_self_arg);
  20     if_chain! {
  21         // the turbofish for collect is ::<Vec<String>>
  22         if let Ref(_, ref_type, _) = collect_output_adjusted_type.kind();
  23         if let Slice(slice) = ref_type.kind();
  24         if is_type_lang_item(cx, *slice, LangItem::String);
  25         // the argument for join is ""
  26         if let ExprKind::Lit(spanned) = &join_arg.kind;
  27         if let LitKind::Str(symbol, _) = spanned.node;
  28         if symbol.is_empty();
  29         then {
  30             span_lint_and_sugg(
  31                 cx,
  32                 UNNECESSARY_JOIN,
  33                 span.with_hi(expr.span.hi()),
  34                 r#"called `.collect<Vec<String>>().join("")` on an iterator"#,
  35                 "try using",
  36                 "collect::<String>()".to_owned(),
  37                 applicability,
  38             );
  39         }
  40     }
  41 }