clippy_lints/src/format.rs

   1 use crate::utils::paths;
   2 use crate::utils::{
   3     in_macro, is_expn_of, last_path_segment, match_def_path, match_type, opt_def_id, resolve_node, snippet,
   4     span_lint_and_then, walk_ptrs_ty,
   5 };
   6 use if_chain::if_chain;
   7 use rustc::hir::*;
   8 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   9 use rustc::ty;
  10 use rustc::{declare_tool_lint, lint_array};
  11 use rustc_errors::Applicability;
  12 use syntax::ast::LitKind;
  13
  14 /// **What it does:** Checks for the use of `format!("string literal with no
  15 /// argument")` and `format!("{}", foo)` where `foo` is a string.
  16 ///
  17 /// **Why is this bad?** There is no point of doing that. `format!("foo")` can
  18 /// be replaced by `"foo".to_owned()` if you really need a `String`. The even
  19 /// worse `&format!("foo")` is often encountered in the wild. `format!("{}",
  20 /// foo)` can be replaced by `foo.clone()` if `foo: String` or `foo.to_owned()`
  21 /// if `foo: &str`.
  22 ///
  23 /// **Known problems:** None.
  24 ///
  25 /// **Examples:**
  26 /// ```rust
  27 /// format!("foo")
  28 /// format!("{}", foo)
  29 /// ```
  30 declare_clippy_lint! {
  31     pub USELESS_FORMAT,
  32     complexity,
  33     "useless use of `format!`"
  34 }
  35
  36 #[derive(Copy, Clone, Debug)]
  37 pub struct Pass;
  38
  39 impl LintPass for Pass {
  40     fn get_lints(&self) -> LintArray {
  41         lint_array![USELESS_FORMAT]
  42     }
  43 }
  44
  45 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
  46     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
  47         if let Some(span) = is_expn_of(expr.span, "format") {
  48             if in_macro(span) {
  49                 return;
  50             }
  51             match expr.node {
  52                 // `format!("{}", foo)` expansion
  53                 ExprKind::Call(ref fun, ref args) => {
  54                     if_chain! {
  55                         if let ExprKind::Path(ref qpath) = fun.node;
  56                         if args.len() == 3;
  57                         if let Some(fun_def_id) = opt_def_id(resolve_node(cx, qpath, fun.hir_id));
  58                         if match_def_path(cx.tcx, fun_def_id, &paths::FMT_ARGUMENTS_NEWV1FORMATTED);
  59                         if check_single_piece(&args[0]);
  60                         if let Some(format_arg) = get_single_string_arg(cx, &args[1]);
  61                         if check_unformatted(&args[2]);
  62                         if let ExprKind::AddrOf(_, ref format_arg) = format_arg.node;
  63                         then {
  64                             let (message, sugg) = if_chain! {
  65                                 if let ExprKind::MethodCall(ref path, _, _) = format_arg.node;
  66                                 if path.ident.as_interned_str() == "to_string";
  67                                 then {
  68                                     ("`to_string()` is enough",
  69                                     snippet(cx, format_arg.span, "<arg>").to_string())
  70                                 } else {
  71                                     ("consider using .to_string()",
  72                                     format!("{}.to_string()", snippet(cx, format_arg.span, "<arg>")))
  73                                 }
  74                             };
  75
  76                             span_lint_and_then(cx, USELESS_FORMAT, span, "useless use of `format!`", |db| {
  77                                 db.span_suggestion_with_applicability(
  78                                     expr.span,
  79                                     message,
  80                                     sugg,
  81                                     Applicability::MachineApplicable,
  82                                 );
  83                             });
  84                         }
  85                     }
  86                 },
  87                 // `format!("foo")` expansion contains `match () { () => [], }`
  88                 ExprKind::Match(ref matchee, _, _) => {
  89                     if let ExprKind::Tup(ref tup) = matchee.node {
  90                         if tup.is_empty() {
  91                             let sugg = format!("{}.to_string()", snippet(cx, expr.span, "<expr>").into_owned());
  92                             span_lint_and_then(cx, USELESS_FORMAT, span, "useless use of `format!`", |db| {
  93                                 db.span_suggestion_with_applicability(
  94                                     span,
  95                                     "consider using .to_string()",
  96                                     sugg,
  97                                     Applicability::MachineApplicable, // snippet
  98                                 );
  99                             });
 100                         }
 101                     }
 102                 },
 103                 _ => (),
 104             }
 105         }
 106     }
 107 }
 108
 109 /// Checks if the expressions matches `&[""]`
 110 fn check_single_piece(expr: &Expr) -> bool {
 111     if_chain! {
 112         if let ExprKind::AddrOf(_, ref expr) = expr.node; // &[""]
 113         if let ExprKind::Array(ref exprs) = expr.node; // [""]
 114         if exprs.len() == 1;
 115         if let ExprKind::Lit(ref lit) = exprs[0].node;
 116         if let LitKind::Str(ref lit, _) = lit.node;
 117         then {
 118             return lit.as_str().is_empty();
 119         }
 120     }
 121
 122     false
 123 }
 124
 125 /// Checks if the expressions matches
 126 /// ```rust,ignore
 127 /// &match (&"arg",) {
 128 /// (__arg0,) => [::std::fmt::ArgumentV1::new(__arg0,
 129 /// ::std::fmt::Display::fmt)],
 130 /// }
 131 /// ```
 132 /// and that the type of `__arg0` is `&str` or `String`,
 133 /// then returns the span of first element of the matched tuple.
 134 fn get_single_string_arg<'a>(cx: &LateContext<'_, '_>, expr: &'a Expr) -> Option<&'a Expr> {
 135     if_chain! {
 136         if let ExprKind::AddrOf(_, ref expr) = expr.node;
 137         if let ExprKind::Match(ref match_expr, ref arms, _) = expr.node;
 138         if arms.len() == 1;
 139         if arms[0].pats.len() == 1;
 140         if let PatKind::Tuple(ref pat, None) = arms[0].pats[0].node;
 141         if pat.len() == 1;
 142         if let ExprKind::Array(ref exprs) = arms[0].body.node;
 143         if exprs.len() == 1;
 144         if let ExprKind::Call(_, ref args) = exprs[0].node;
 145         if args.len() == 2;
 146         if let ExprKind::Path(ref qpath) = args[1].node;
 147         if let Some(fun_def_id) = opt_def_id(resolve_node(cx, qpath, args[1].hir_id));
 148         if match_def_path(cx.tcx, fun_def_id, &paths::DISPLAY_FMT_METHOD);
 149         then {
 150             let ty = walk_ptrs_ty(cx.tables.pat_ty(&pat[0]));
 151             if ty.sty == ty::Str || match_type(cx, ty, &paths::STRING) {
 152                 if let ExprKind::Tup(ref values) = match_expr.node {
 153                     return Some(&values[0]);
 154                 }
 155             }
 156         }
 157     }
 158
 159     None
 160 }
 161
 162 /// Checks if the expression matches
 163 /// ```rust,ignore
 164 /// &[_ {
 165 ///    format: _ {
 166 ///         width: _::Implied,
 167 ///         ...
 168 ///    },
 169 ///    ...,
 170 /// }]
 171 /// ```
 172 fn check_unformatted(expr: &Expr) -> bool {
 173     if_chain! {
 174         if let ExprKind::AddrOf(_, ref expr) = expr.node;
 175         if let ExprKind::Array(ref exprs) = expr.node;
 176         if exprs.len() == 1;
 177         if let ExprKind::Struct(_, ref fields, _) = exprs[0].node;
 178         if let Some(format_field) = fields.iter().find(|f| f.ident.name == "format");
 179         if let ExprKind::Struct(_, ref fields, _) = format_field.expr.node;
 180         if let Some(width_field) = fields.iter().find(|f| f.ident.name == "width");
 181         if let ExprKind::Path(ref width_qpath) = width_field.expr.node;
 182         if last_path_segment(width_qpath).ident.name == "Implied";
 183         if let Some(precision_field) = fields.iter().find(|f| f.ident.name == "precision");
 184         if let ExprKind::Path(ref precision_path) = precision_field.expr.node;
 185         if last_path_segment(precision_path).ident.name == "Implied";
 186         then {
 187             return true;
 188         }
 189     }
 190
 191     false
 192 }