clippy_lints/src/strings.rs

   1 use rustc_errors::Applicability;
   2 use rustc_hir::{BinOpKind, Expr, ExprKind};
   3 use rustc_lint::{LateContext, LateLintPass, LintContext};
   4 use rustc_middle::lint::in_external_macro;
   5 use rustc_session::{declare_lint_pass, declare_tool_lint};
   6 use rustc_span::source_map::Spanned;
   7
   8 use if_chain::if_chain;
   9
  10 use crate::utils::SpanlessEq;
  11 use crate::utils::{get_parent_expr, is_allowed, is_type_diagnostic_item, span_lint, span_lint_and_sugg, walk_ptrs_ty};
  12
  13 declare_clippy_lint! {
  14     /// **What it does:** Checks for string appends of the form `x = x + y` (without
  15     /// `let`!).
  16     ///
  17     /// **Why is this bad?** It's not really bad, but some people think that the
  18     /// `.push_str(_)` method is more readable.
  19     ///
  20     /// **Known problems:** None.
  21     ///
  22     /// **Example:**
  23     ///
  24     /// ```rust
  25     /// let mut x = "Hello".to_owned();
  26     /// x = x + ", World";
  27     ///
  28     /// // More readable
  29     /// x += ", World";
  30     /// x.push_str(", World");
  31     /// ```
  32     pub STRING_ADD_ASSIGN,
  33     pedantic,
  34     "using `x = x + ..` where x is a `String` instead of `push_str()`"
  35 }
  36
  37 declare_clippy_lint! {
  38     /// **What it does:** Checks for all instances of `x + _` where `x` is of type
  39     /// `String`, but only if [`string_add_assign`](#string_add_assign) does *not*
  40     /// match.
  41     ///
  42     /// **Why is this bad?** It's not bad in and of itself. However, this particular
  43     /// `Add` implementation is asymmetric (the other operand need not be `String`,
  44     /// but `x` does), while addition as mathematically defined is symmetric, also
  45     /// the `String::push_str(_)` function is a perfectly good replacement.
  46     /// Therefore, some dislike it and wish not to have it in their code.
  47     ///
  48     /// That said, other people think that string addition, having a long tradition
  49     /// in other languages is actually fine, which is why we decided to make this
  50     /// particular lint `allow` by default.
  51     ///
  52     /// **Known problems:** None.
  53     ///
  54     /// **Example:**
  55     ///
  56     /// ```rust
  57     /// let x = "Hello".to_owned();
  58     /// x + ", World";
  59     /// ```
  60     pub STRING_ADD,
  61     restriction,
  62     "using `x + ..` where x is a `String` instead of `push_str()`"
  63 }
  64
  65 declare_clippy_lint! {
  66     /// **What it does:** Checks for the `as_bytes` method called on string literals
  67     /// that contain only ASCII characters.
  68     ///
  69     /// **Why is this bad?** Byte string literals (e.g., `b"foo"`) can be used
  70     /// instead. They are shorter but less discoverable than `as_bytes()`.
  71     ///
  72     /// **Known Problems:** None.
  73     ///
  74     /// **Example:**
  75     /// ```rust
  76     /// // Bad
  77     /// let bs = "a byte string".as_bytes();
  78     ///
  79     /// // Good
  80     /// let bs = b"a byte string";
  81     /// ```
  82     pub STRING_LIT_AS_BYTES,
  83     style,
  84     "calling `as_bytes` on a string literal instead of using a byte string literal"
  85 }
  86
  87 declare_lint_pass!(StringAdd => [STRING_ADD, STRING_ADD_ASSIGN]);
  88
  89 impl<'tcx> LateLintPass<'tcx> for StringAdd {
  90     fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
  91         if in_external_macro(cx.sess(), e.span) {
  92             return;
  93         }
  94
  95         if let ExprKind::Binary(
  96             Spanned {
  97                 node: BinOpKind::Add, ..
  98             },
  99             ref left,
 100             _,
 101         ) = e.kind
 102         {
 103             if is_string(cx, left) {
 104                 if !is_allowed(cx, STRING_ADD_ASSIGN, e.hir_id) {
 105                     let parent = get_parent_expr(cx, e);
 106                     if let Some(p) = parent {
 107                         if let ExprKind::Assign(ref target, _, _) = p.kind {
 108                             // avoid duplicate matches
 109                             if SpanlessEq::new(cx).eq_expr(target, left) {
 110                                 return;
 111                             }
 112                         }
 113                     }
 114                 }
 115                 span_lint(
 116                     cx,
 117                     STRING_ADD,
 118                     e.span,
 119                     "you added something to a string. Consider using `String::push_str()` instead",
 120                 );
 121             }
 122         } else if let ExprKind::Assign(ref target, ref src, _) = e.kind {
 123             if is_string(cx, target) && is_add(cx, src, target) {
 124                 span_lint(
 125                     cx,
 126                     STRING_ADD_ASSIGN,
 127                     e.span,
 128                     "you assigned the result of adding something to this string. Consider using \
 129                      `String::push_str()` instead",
 130                 );
 131             }
 132         }
 133     }
 134 }
 135
 136 fn is_string(cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
 137     is_type_diagnostic_item(cx, walk_ptrs_ty(cx.typeck_results().expr_ty(e)), sym!(string_type))
 138 }
 139
 140 fn is_add(cx: &LateContext<'_>, src: &Expr<'_>, target: &Expr<'_>) -> bool {
 141     match src.kind {
 142         ExprKind::Binary(
 143             Spanned {
 144                 node: BinOpKind::Add, ..
 145             },
 146             ref left,
 147             _,
 148         ) => SpanlessEq::new(cx).eq_expr(target, left),
 149         ExprKind::Block(ref block, _) => {
 150             block.stmts.is_empty() && block.expr.as_ref().map_or(false, |expr| is_add(cx, expr, target))
 151         },
 152         _ => false,
 153     }
 154 }
 155
 156 // Max length a b"foo" string can take
 157 const MAX_LENGTH_BYTE_STRING_LIT: usize = 32;
 158
 159 declare_lint_pass!(StringLitAsBytes => [STRING_LIT_AS_BYTES]);
 160
 161 impl<'tcx> LateLintPass<'tcx> for StringLitAsBytes {
 162     fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
 163         use crate::utils::{snippet, snippet_with_applicability};
 164         use rustc_ast::ast::LitKind;
 165
 166         if_chain! {
 167             if let ExprKind::MethodCall(path, _, args, _) = &e.kind;
 168             if path.ident.name == sym!(as_bytes);
 169             if let ExprKind::Lit(lit) = &args[0].kind;
 170             if let LitKind::Str(lit_content, _) = &lit.node;
 171             then {
 172                 let callsite = snippet(cx, args[0].span.source_callsite(), r#""foo""#);
 173                 let mut applicability = Applicability::MachineApplicable;
 174                 if callsite.starts_with("include_str!") {
 175                     span_lint_and_sugg(
 176                         cx,
 177                         STRING_LIT_AS_BYTES,
 178                         e.span,
 179                         "calling `as_bytes()` on `include_str!(..)`",
 180                         "consider using `include_bytes!(..)` instead",
 181                         snippet_with_applicability(cx, args[0].span, r#""foo""#, &mut applicability).replacen(
 182                             "include_str",
 183                             "include_bytes",
 184                             1,
 185                         ),
 186                         applicability,
 187                     );
 188                 } else if lit_content.as_str().is_ascii()
 189                     && lit_content.as_str().len() <= MAX_LENGTH_BYTE_STRING_LIT
 190                     && !args[0].span.from_expansion()
 191                 {
 192                     span_lint_and_sugg(
 193                         cx,
 194                         STRING_LIT_AS_BYTES,
 195                         e.span,
 196                         "calling `as_bytes()` on a string literal",
 197                         "consider using a byte string literal instead",
 198                         format!(
 199                             "b{}",
 200                             snippet_with_applicability(cx, args[0].span, r#""foo""#, &mut applicability)
 201                         ),
 202                         applicability,
 203                     );
 204                 }
 205             }
 206         }
 207     }
 208 }