clippy_lints/src/format.rs

   1 // Copyright 2014-2018 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution.
   3 //
   4 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   5 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   6 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   7 // option. This file may not be copied, modified, or distributed
   8 // except according to those terms.
   9
  10 use crate::rustc::hir::*;
  11 use crate::rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
  12 use crate::rustc::ty;
  13 use crate::rustc::{declare_tool_lint, lint_array};
  14 use crate::rustc_errors::Applicability;
  15 use crate::syntax::ast::LitKind;
  16 use crate::utils::paths;
  17 use crate::utils::{
  18     in_macro, is_expn_of, last_path_segment, match_def_path, match_type, opt_def_id, resolve_node, snippet,
  19     span_lint_and_then, walk_ptrs_ty,
  20 };
  21 use if_chain::if_chain;
  22
  23 /// **What it does:** Checks for the use of `format!("string literal with no
  24 /// argument")` and `format!("{}", foo)` where `foo` is a string.
  25 ///
  26 /// **Why is this bad?** There is no point of doing that. `format!("foo")` can
  27 /// be replaced by `"foo".to_owned()` if you really need a `String`. The even
  28 /// worse `&format!("foo")` is often encountered in the wild. `format!("{}",
  29 /// foo)` can be replaced by `foo.clone()` if `foo: String` or `foo.to_owned()`
  30 /// if `foo: &str`.
  31 ///
  32 /// **Known problems:** None.
  33 ///
  34 /// **Examples:**
  35 /// ```rust
  36 /// format!("foo")
  37 /// format!("{}", foo)
  38 /// ```
  39 declare_clippy_lint! {
  40     pub USELESS_FORMAT,
  41     complexity,
  42     "useless use of `format!`"
  43 }
  44
  45 #[derive(Copy, Clone, Debug)]
  46 pub struct Pass;
  47
  48 impl LintPass for Pass {
  49     fn get_lints(&self) -> LintArray {
  50         lint_array![USELESS_FORMAT]
  51     }
  52 }
  53
  54 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
  55     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
  56         if let Some(span) = is_expn_of(expr.span, "format") {
  57             if in_macro(span) {
  58                 return;
  59             }
  60             match expr.node {
  61                 // `format!("{}", foo)` expansion
  62                 ExprKind::Call(ref fun, ref args) => {
  63                     if_chain! {
  64                         if let ExprKind::Path(ref qpath) = fun.node;
  65                         if args.len() == 3;
  66                         if let Some(fun_def_id) = opt_def_id(resolve_node(cx, qpath, fun.hir_id));
  67                         if match_def_path(cx.tcx, fun_def_id, &paths::FMT_ARGUMENTS_NEWV1FORMATTED);
  68                         if check_single_piece(&args[0]);
  69                         if let Some(format_arg) = get_single_string_arg(cx, &args[1]);
  70                         if check_unformatted(&args[2]);
  71                         if let ExprKind::AddrOf(_, ref format_arg) = format_arg.node;
  72                         then {
  73                             let (message, sugg) = if_chain! {
  74                                 if let ExprKind::MethodCall(ref path, _, _) = format_arg.node;
  75                                 if path.ident.as_interned_str() == "to_string";
  76                                 then {
  77                                     ("`to_string()` is enough",
  78                                     snippet(cx, format_arg.span, "<arg>").to_string())
  79                                 } else {
  80                                     ("consider using .to_string()",
  81                                     format!("{}.to_string()", snippet(cx, format_arg.span, "<arg>")))
  82                                 }
  83                             };
  84
  85                             span_lint_and_then(cx, USELESS_FORMAT, span, "useless use of `format!`", |db| {
  86                                 db.span_suggestion_with_applicability(
  87                                     expr.span,
  88                                     message,
  89                                     sugg,
  90                                     Applicability::MachineApplicable,
  91                                 );
  92                             });
  93                         }
  94                     }
  95                 },
  96                 // `format!("foo")` expansion contains `match () { () => [], }`
  97                 ExprKind::Match(ref matchee, _, _) => {
  98                     if let ExprKind::Tup(ref tup) = matchee.node {
  99                         if tup.is_empty() {
 100                             let sugg = format!("{}.to_string()", snippet(cx, expr.span, "<expr>").into_owned());
 101                             span_lint_and_then(cx, USELESS_FORMAT, span, "useless use of `format!`", |db| {
 102                                 db.span_suggestion_with_applicability(
 103                                     span,
 104                                     "consider using .to_string()",
 105                                     sugg,
 106                                     Applicability::MachineApplicable, // snippet
 107                                 );
 108                             });
 109                         }
 110                     }
 111                 },
 112                 _ => (),
 113             }
 114         }
 115     }
 116 }
 117
 118 /// Checks if the expressions matches `&[""]`
 119 fn check_single_piece(expr: &Expr) -> bool {
 120     if_chain! {
 121         if let ExprKind::AddrOf(_, ref expr) = expr.node; // &[""]
 122         if let ExprKind::Array(ref exprs) = expr.node; // [""]
 123         if exprs.len() == 1;
 124         if let ExprKind::Lit(ref lit) = exprs[0].node;
 125         if let LitKind::Str(ref lit, _) = lit.node;
 126         then {
 127             return lit.as_str().is_empty();
 128         }
 129     }
 130
 131     false
 132 }
 133
 134 /// Checks if the expressions matches
 135 /// ```rust,ignore
 136 /// &match (&"arg",) {
 137 /// (__arg0,) => [::std::fmt::ArgumentV1::new(__arg0,
 138 /// ::std::fmt::Display::fmt)],
 139 /// }
 140 /// ```
 141 /// and that the type of `__arg0` is `&str` or `String`,
 142 /// then returns the span of first element of the matched tuple.
 143 fn get_single_string_arg<'a>(cx: &LateContext<'_, '_>, expr: &'a Expr) -> Option<&'a Expr> {
 144     if_chain! {
 145         if let ExprKind::AddrOf(_, ref expr) = expr.node;
 146         if let ExprKind::Match(ref match_expr, ref arms, _) = expr.node;
 147         if arms.len() == 1;
 148         if arms[0].pats.len() == 1;
 149         if let PatKind::Tuple(ref pat, None) = arms[0].pats[0].node;
 150         if pat.len() == 1;
 151         if let ExprKind::Array(ref exprs) = arms[0].body.node;
 152         if exprs.len() == 1;
 153         if let ExprKind::Call(_, ref args) = exprs[0].node;
 154         if args.len() == 2;
 155         if let ExprKind::Path(ref qpath) = args[1].node;
 156         if let Some(fun_def_id) = opt_def_id(resolve_node(cx, qpath, args[1].hir_id));
 157         if match_def_path(cx.tcx, fun_def_id, &paths::DISPLAY_FMT_METHOD);
 158         then {
 159             let ty = walk_ptrs_ty(cx.tables.pat_ty(&pat[0]));
 160             if ty.sty == ty::Str || match_type(cx, ty, &paths::STRING) {
 161                 if let ExprKind::Tup(ref values) = match_expr.node {
 162                     return Some(&values[0]);
 163                 }
 164             }
 165         }
 166     }
 167
 168     None
 169 }
 170
 171 /// Checks if the expression matches
 172 /// ```rust,ignore
 173 /// &[_ {
 174 ///    format: _ {
 175 ///         width: _::Implied,
 176 ///         ...
 177 ///    },
 178 ///    ...,
 179 /// }]
 180 /// ```
 181 fn check_unformatted(expr: &Expr) -> bool {
 182     if_chain! {
 183         if let ExprKind::AddrOf(_, ref expr) = expr.node;
 184         if let ExprKind::Array(ref exprs) = expr.node;
 185         if exprs.len() == 1;
 186         if let ExprKind::Struct(_, ref fields, _) = exprs[0].node;
 187         if let Some(format_field) = fields.iter().find(|f| f.ident.name == "format");
 188         if let ExprKind::Struct(_, ref fields, _) = format_field.expr.node;
 189         if let Some(width_field) = fields.iter().find(|f| f.ident.name == "width");
 190         if let ExprKind::Path(ref width_qpath) = width_field.expr.node;
 191         if last_path_segment(width_qpath).ident.name == "Implied";
 192         if let Some(precision_field) = fields.iter().find(|f| f.ident.name == "precision");
 193         if let ExprKind::Path(ref precision_path) = precision_field.expr.node;
 194         if last_path_segment(precision_path).ident.name == "Implied";
 195         then {
 196             return true;
 197         }
 198     }
 199
 200     false
 201 }