clippy_lints/src/useless_conversion.rs

   1 use crate::utils::{
   2     match_def_path, match_trait_method, paths, same_tys, snippet, snippet_with_macro_callsite, span_lint_and_sugg,
   3 };
   4 use rustc_errors::Applicability;
   5 use rustc_hir::{Expr, ExprKind, HirId, MatchSource};
   6 use rustc_lint::{LateContext, LateLintPass};
   7 use rustc_session::{declare_tool_lint, impl_lint_pass};
   8
   9 declare_clippy_lint! {
  10     /// **What it does:** Checks for `Into`/`From`/`IntoIter` calls that useless converts
  11     /// to the same type as caller.
  12     ///
  13     /// **Why is this bad?** Redundant code.
  14     ///
  15     /// **Known problems:** None.
  16     ///
  17     /// **Example:**
  18     ///
  19     /// ```rust
  20     /// // Bad
  21     /// // format!() returns a `String`
  22     /// let s: String = format!("hello").into();
  23     ///
  24     /// // Good
  25     /// let s: String = format!("hello");
  26     /// ```
  27     pub USELESS_CONVERSION,
  28     complexity,
  29     "calls to `Into`/`From`/`IntoIter` that performs useless conversions to the same type"
  30 }
  31
  32 #[derive(Default)]
  33 pub struct UselessConversion {
  34     try_desugar_arm: Vec<HirId>,
  35 }
  36
  37 impl_lint_pass!(UselessConversion => [USELESS_CONVERSION]);
  38
  39 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for UselessConversion {
  40     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr<'_>) {
  41         if e.span.from_expansion() {
  42             return;
  43         }
  44
  45         if Some(&e.hir_id) == self.try_desugar_arm.last() {
  46             return;
  47         }
  48
  49         match e.kind {
  50             ExprKind::Match(_, ref arms, MatchSource::TryDesugar) => {
  51                 let e = match arms[0].body.kind {
  52                     ExprKind::Ret(Some(ref e)) | ExprKind::Break(_, Some(ref e)) => e,
  53                     _ => return,
  54                 };
  55                 if let ExprKind::Call(_, ref args) = e.kind {
  56                     self.try_desugar_arm.push(args[0].hir_id);
  57                 }
  58             },
  59
  60             ExprKind::MethodCall(ref name, .., ref args) => {
  61                 if match_trait_method(cx, e, &paths::INTO) && &*name.ident.as_str() == "into" {
  62                     let a = cx.tables.expr_ty(e);
  63                     let b = cx.tables.expr_ty(&args[0]);
  64                     if same_tys(cx, a, b) {
  65                         let sugg = snippet_with_macro_callsite(cx, args[0].span, "<expr>").to_string();
  66
  67                         span_lint_and_sugg(
  68                             cx,
  69                             USELESS_CONVERSION,
  70                             e.span,
  71                             "useless conversion",
  72                             "consider removing `.into()`",
  73                             sugg,
  74                             Applicability::MachineApplicable, // snippet
  75                         );
  76                     }
  77                 }
  78                 if match_trait_method(cx, e, &paths::INTO_ITERATOR) && &*name.ident.as_str() == "into_iter" {
  79                     let a = cx.tables.expr_ty(e);
  80                     let b = cx.tables.expr_ty(&args[0]);
  81                     if same_tys(cx, a, b) {
  82                         let sugg = snippet(cx, args[0].span, "<expr>").into_owned();
  83                         span_lint_and_sugg(
  84                             cx,
  85                             USELESS_CONVERSION,
  86                             e.span,
  87                             "useless conversion",
  88                             "consider removing `.into_iter()`",
  89                             sugg,
  90                             Applicability::MachineApplicable, // snippet
  91                         );
  92                     }
  93                 }
  94             },
  95
  96             ExprKind::Call(ref path, ref args) => {
  97                 if let ExprKind::Path(ref qpath) = path.kind {
  98                     if let Some(def_id) = cx.tables.qpath_res(qpath, path.hir_id).opt_def_id() {
  99                         if match_def_path(cx, def_id, &paths::FROM_FROM) {
 100                             let a = cx.tables.expr_ty(e);
 101                             let b = cx.tables.expr_ty(&args[0]);
 102                             if same_tys(cx, a, b) {
 103                                 let sugg = snippet(cx, args[0].span.source_callsite(), "<expr>").into_owned();
 104                                 let sugg_msg =
 105                                     format!("consider removing `{}()`", snippet(cx, path.span, "From::from"));
 106                                 span_lint_and_sugg(
 107                                     cx,
 108                                     USELESS_CONVERSION,
 109                                     e.span,
 110                                     "useless conversion",
 111                                     &sugg_msg,
 112                                     sugg,
 113                                     Applicability::MachineApplicable, // snippet
 114                                 );
 115                             }
 116                         }
 117                     }
 118                 }
 119             },
 120
 121             _ => {},
 122         }
 123     }
 124
 125     fn check_expr_post(&mut self, _: &LateContext<'a, 'tcx>, e: &'tcx Expr<'_>) {
 126         if Some(&e.hir_id) == self.try_desugar_arm.last() {
 127             self.try_desugar_arm.pop();
 128         }
 129     }
 130 }