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, LintContext, LintPass};
   9 use rustc::ty;
  10 use rustc::{declare_tool_lint, lint_array};
  11 use rustc_errors::Applicability;
  12 use syntax::ast::LitKind;
  13 use syntax::source_map::Span;
  14
  15 /// **What it does:** Checks for the use of `format!("string literal with no
  16 /// argument")` and `format!("{}", foo)` where `foo` is a string.
  17 ///
  18 /// **Why is this bad?** There is no point of doing that. `format!("foo")` can
  19 /// be replaced by `"foo".to_owned()` if you really need a `String`. The even
  20 /// worse `&format!("foo")` is often encountered in the wild. `format!("{}",
  21 /// foo)` can be replaced by `foo.clone()` if `foo: String` or `foo.to_owned()`
  22 /// if `foo: &str`.
  23 ///
  24 /// **Known problems:** None.
  25 ///
  26 /// **Examples:**
  27 /// ```rust
  28 /// format!("foo")
  29 /// format!("{}", foo)
  30 /// ```
  31 declare_clippy_lint! {
  32     pub USELESS_FORMAT,
  33     complexity,
  34     "useless use of `format!`"
  35 }
  36
  37 #[derive(Copy, Clone, Debug)]
  38 pub struct Pass;
  39
  40 impl LintPass for Pass {
  41     fn get_lints(&self) -> LintArray {
  42         lint_array![USELESS_FORMAT]
  43     }
  44
  45     fn name(&self) -> &'static str {
  46         "UselessFormat"
  47     }
  48 }
  49
  50 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
  51     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
  52         if let Some(span) = is_expn_of(expr.span, "format") {
  53             if in_macro(span) {
  54                 return;
  55             }
  56             match expr.node {
  57                 // `format!("{}", foo)` expansion
  58                 ExprKind::Call(ref fun, ref args) => {
  59                     if_chain! {
  60                         if let ExprKind::Path(ref qpath) = fun.node;
  61                         if let Some(fun_def_id) = opt_def_id(resolve_node(cx, qpath, fun.hir_id));
  62                         let new_v1 = match_def_path(cx.tcx, fun_def_id, &paths::FMT_ARGUMENTS_NEWV1);
  63                         let new_v1_fmt = match_def_path(
  64                             cx.tcx,
  65                             fun_def_id,
  66                             &paths::FMT_ARGUMENTS_NEWV1FORMATTED
  67                         );
  68                         if new_v1 || new_v1_fmt;
  69                         if check_single_piece(&args[0]);
  70                         if let Some(format_arg) = get_single_string_arg(cx, &args[1]);
  71                         if new_v1 || check_unformatted(&args[2]);
  72                         if let ExprKind::AddrOf(_, ref format_arg) = format_arg.node;
  73                         then {
  74                             let (message, sugg) = if_chain! {
  75                                 if let ExprKind::MethodCall(ref path, _, _) = format_arg.node;
  76                                 if path.ident.as_interned_str() == "to_string";
  77                                 then {
  78                                     ("`to_string()` is enough",
  79                                     snippet(cx, format_arg.span, "<arg>").to_string())
  80                                 } else {
  81                                     ("consider using .to_string()",
  82                                     format!("{}.to_string()", snippet(cx, format_arg.span, "<arg>")))
  83                                 }
  84                             };
  85
  86                             span_useless_format(cx, span, message, sugg);
  87                         }
  88                     }
  89                 },
  90                 // `format!("foo")` expansion contains `match () { () => [], }`
  91                 ExprKind::Match(ref matchee, _, _) => {
  92                     if let ExprKind::Tup(ref tup) = matchee.node {
  93                         if tup.is_empty() {
  94                             let sugg = format!("{}.to_string()", snippet(cx, expr.span, "<expr>").into_owned());
  95                             span_useless_format(cx, span, "consider using .to_string()", sugg);
  96                         }
  97                     }
  98                 },
  99                 _ => (),
 100             }
 101         }
 102     }
 103 }
 104
 105 fn span_useless_format<'a, 'tcx: 'a, T: LintContext<'tcx>>(cx: &'a T, span: Span, help: &str, mut sugg: String) {
 106     let to_replace = span.source_callsite();
 107
 108     // The callsite span contains the statement semicolon for some reason.
 109     let snippet = snippet(cx, to_replace, "..");
 110     if snippet.ends_with(';') {
 111         sugg.push(';');
 112     }
 113
 114     span_lint_and_then(cx, USELESS_FORMAT, span, "useless use of `format!`", |db| {
 115         db.span_suggestion(
 116             to_replace,
 117             help,
 118             sugg,
 119             Applicability::MachineApplicable, // snippet
 120         );
 121     });
 122 }
 123
 124 /// Checks if the expressions matches `&[""]`
 125 fn check_single_piece(expr: &Expr) -> bool {
 126     if_chain! {
 127         if let ExprKind::AddrOf(_, ref expr) = expr.node; // &[""]
 128         if let ExprKind::Array(ref exprs) = expr.node; // [""]
 129         if exprs.len() == 1;
 130         if let ExprKind::Lit(ref lit) = exprs[0].node;
 131         if let LitKind::Str(ref lit, _) = lit.node;
 132         then {
 133             return lit.as_str().is_empty();
 134         }
 135     }
 136
 137     false
 138 }
 139
 140 /// Checks if the expressions matches
 141 /// ```rust,ignore
 142 /// &match (&"arg",) {
 143 /// (__arg0,) => [::std::fmt::ArgumentV1::new(__arg0,
 144 /// ::std::fmt::Display::fmt)],
 145 /// }
 146 /// ```
 147 /// and that the type of `__arg0` is `&str` or `String`,
 148 /// then returns the span of first element of the matched tuple.
 149 fn get_single_string_arg<'a>(cx: &LateContext<'_, '_>, expr: &'a Expr) -> Option<&'a Expr> {
 150     if_chain! {
 151         if let ExprKind::AddrOf(_, ref expr) = expr.node;
 152         if let ExprKind::Match(ref match_expr, ref arms, _) = expr.node;
 153         if arms.len() == 1;
 154         if arms[0].pats.len() == 1;
 155         if let PatKind::Tuple(ref pat, None) = arms[0].pats[0].node;
 156         if pat.len() == 1;
 157         if let ExprKind::Array(ref exprs) = arms[0].body.node;
 158         if exprs.len() == 1;
 159         if let ExprKind::Call(_, ref args) = exprs[0].node;
 160         if args.len() == 2;
 161         if let ExprKind::Path(ref qpath) = args[1].node;
 162         if let Some(fun_def_id) = opt_def_id(resolve_node(cx, qpath, args[1].hir_id));
 163         if match_def_path(cx.tcx, fun_def_id, &paths::DISPLAY_FMT_METHOD);
 164         then {
 165             let ty = walk_ptrs_ty(cx.tables.pat_ty(&pat[0]));
 166             if ty.sty == ty::Str || match_type(cx, ty, &paths::STRING) {
 167                 if let ExprKind::Tup(ref values) = match_expr.node {
 168                     return Some(&values[0]);
 169                 }
 170             }
 171         }
 172     }
 173
 174     None
 175 }
 176
 177 /// Checks if the expression matches
 178 /// ```rust,ignore
 179 /// &[_ {
 180 ///    format: _ {
 181 ///         width: _::Implied,
 182 ///         ...
 183 ///    },
 184 ///    ...,
 185 /// }]
 186 /// ```
 187 fn check_unformatted(expr: &Expr) -> bool {
 188     if_chain! {
 189         if let ExprKind::AddrOf(_, ref expr) = expr.node;
 190         if let ExprKind::Array(ref exprs) = expr.node;
 191         if exprs.len() == 1;
 192         if let ExprKind::Struct(_, ref fields, _) = exprs[0].node;
 193         if let Some(format_field) = fields.iter().find(|f| f.ident.name == "format");
 194         if let ExprKind::Struct(_, ref fields, _) = format_field.expr.node;
 195         if let Some(width_field) = fields.iter().find(|f| f.ident.name == "width");
 196         if let ExprKind::Path(ref width_qpath) = width_field.expr.node;
 197         if last_path_segment(width_qpath).ident.name == "Implied";
 198         if let Some(precision_field) = fields.iter().find(|f| f.ident.name == "precision");
 199         if let ExprKind::Path(ref precision_path) = precision_field.expr.node;
 200         if last_path_segment(precision_path).ident.name == "Implied";
 201         then {
 202             return true;
 203         }
 204     }
 205
 206     false
 207 }