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 let Some(fun_def_id) = opt_def_id(resolve_node(cx, qpath, fun.hir_id));
  57                         let new_v1 = match_def_path(cx.tcx, fun_def_id, &paths::FMT_ARGUMENTS_NEWV1);
  58                         let new_v1_fmt = match_def_path(
  59                             cx.tcx,
  60                             fun_def_id,
  61                             &paths::FMT_ARGUMENTS_NEWV1FORMATTED
  62                         );
  63                         if new_v1 || new_v1_fmt;
  64                         if check_single_piece(&args[0]);
  65                         if let Some(format_arg) = get_single_string_arg(cx, &args[1]);
  66                         if new_v1 || check_unformatted(&args[2]);
  67                         if let ExprKind::AddrOf(_, ref format_arg) = format_arg.node;
  68                         then {
  69                             let (message, sugg) = if_chain! {
  70                                 if let ExprKind::MethodCall(ref path, _, _) = format_arg.node;
  71                                 if path.ident.as_interned_str() == "to_string";
  72                                 then {
  73                                     ("`to_string()` is enough",
  74                                     snippet(cx, format_arg.span, "<arg>").to_string())
  75                                 } else {
  76                                     ("consider using .to_string()",
  77                                     format!("{}.to_string()", snippet(cx, format_arg.span, "<arg>")))
  78                                 }
  79                             };
  80
  81                             span_lint_and_then(cx, USELESS_FORMAT, span, "useless use of `format!`", |db| {
  82                                 db.span_suggestion_with_applicability(
  83                                     expr.span,
  84                                     message,
  85                                     sugg,
  86                                     Applicability::MachineApplicable,
  87                                 );
  88                             });
  89                         }
  90                     }
  91                 },
  92                 // `format!("foo")` expansion contains `match () { () => [], }`
  93                 ExprKind::Match(ref matchee, _, _) => {
  94                     if let ExprKind::Tup(ref tup) = matchee.node {
  95                         if tup.is_empty() {
  96                             let sugg = format!("{}.to_string()", snippet(cx, expr.span, "<expr>").into_owned());
  97                             span_lint_and_then(cx, USELESS_FORMAT, span, "useless use of `format!`", |db| {
  98                                 db.span_suggestion_with_applicability(
  99                                     span,
 100                                     "consider using .to_string()",
 101                                     sugg,
 102                                     Applicability::MachineApplicable, // snippet
 103                                 );
 104                             });
 105                         }
 106                     }
 107                 },
 108                 _ => (),
 109             }
 110         }
 111     }
 112 }
 113
 114 /// Checks if the expressions matches `&[""]`
 115 fn check_single_piece(expr: &Expr) -> bool {
 116     if_chain! {
 117         if let ExprKind::AddrOf(_, ref expr) = expr.node; // &[""]
 118         if let ExprKind::Array(ref exprs) = expr.node; // [""]
 119         if exprs.len() == 1;
 120         if let ExprKind::Lit(ref lit) = exprs[0].node;
 121         if let LitKind::Str(ref lit, _) = lit.node;
 122         then {
 123             return lit.as_str().is_empty();
 124         }
 125     }
 126
 127     false
 128 }
 129
 130 /// Checks if the expressions matches
 131 /// ```rust,ignore
 132 /// &match (&"arg",) {
 133 /// (__arg0,) => [::std::fmt::ArgumentV1::new(__arg0,
 134 /// ::std::fmt::Display::fmt)],
 135 /// }
 136 /// ```
 137 /// and that the type of `__arg0` is `&str` or `String`,
 138 /// then returns the span of first element of the matched tuple.
 139 fn get_single_string_arg<'a>(cx: &LateContext<'_, '_>, expr: &'a Expr) -> Option<&'a Expr> {
 140     if_chain! {
 141         if let ExprKind::AddrOf(_, ref expr) = expr.node;
 142         if let ExprKind::Match(ref match_expr, ref arms, _) = expr.node;
 143         if arms.len() == 1;
 144         if arms[0].pats.len() == 1;
 145         if let PatKind::Tuple(ref pat, None) = arms[0].pats[0].node;
 146         if pat.len() == 1;
 147         if let ExprKind::Array(ref exprs) = arms[0].body.node;
 148         if exprs.len() == 1;
 149         if let ExprKind::Call(_, ref args) = exprs[0].node;
 150         if args.len() == 2;
 151         if let ExprKind::Path(ref qpath) = args[1].node;
 152         if let Some(fun_def_id) = opt_def_id(resolve_node(cx, qpath, args[1].hir_id));
 153         if match_def_path(cx.tcx, fun_def_id, &paths::DISPLAY_FMT_METHOD);
 154         then {
 155             let ty = walk_ptrs_ty(cx.tables.pat_ty(&pat[0]));
 156             if ty.sty == ty::Str || match_type(cx, ty, &paths::STRING) {
 157                 if let ExprKind::Tup(ref values) = match_expr.node {
 158                     return Some(&values[0]);
 159                 }
 160             }
 161         }
 162     }
 163
 164     None
 165 }
 166
 167 /// Checks if the expression matches
 168 /// ```rust,ignore
 169 /// &[_ {
 170 ///    format: _ {
 171 ///         width: _::Implied,
 172 ///         ...
 173 ///    },
 174 ///    ...,
 175 /// }]
 176 /// ```
 177 fn check_unformatted(expr: &Expr) -> bool {
 178     if_chain! {
 179         if let ExprKind::AddrOf(_, ref expr) = expr.node;
 180         if let ExprKind::Array(ref exprs) = expr.node;
 181         if exprs.len() == 1;
 182         if let ExprKind::Struct(_, ref fields, _) = exprs[0].node;
 183         if let Some(format_field) = fields.iter().find(|f| f.ident.name == "format");
 184         if let ExprKind::Struct(_, ref fields, _) = format_field.expr.node;
 185         if let Some(width_field) = fields.iter().find(|f| f.ident.name == "width");
 186         if let ExprKind::Path(ref width_qpath) = width_field.expr.node;
 187         if last_path_segment(width_qpath).ident.name == "Implied";
 188         if let Some(precision_field) = fields.iter().find(|f| f.ident.name == "precision");
 189         if let ExprKind::Path(ref precision_path) = precision_field.expr.node;
 190         if last_path_segment(precision_path).ident.name == "Implied";
 191         then {
 192             return true;
 193         }
 194     }
 195
 196     false
 197 }