clippy_lints/src/map_unit_fn.rs

   1 use rustc::hir;
   2 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   3 use rustc::{declare_tool_lint, lint_array};
   4 use if_chain::if_chain;
   5 use rustc::ty;
   6 use rustc_errors::Applicability;
   7 use syntax::source_map::Span;
   8 use crate::utils::{in_macro, iter_input_pats, match_type, method_chain_args, snippet, span_lint_and_then};
   9 use crate::utils::paths;
  10
  11 #[derive(Clone)]
  12 pub struct Pass;
  13
  14 /// **What it does:** Checks for usage of `option.map(f)` where f is a function
  15 /// or closure that returns the unit type.
  16 ///
  17 /// **Why is this bad?** Readability, this can be written more clearly with
  18 /// an if let statement
  19 ///
  20 /// **Known problems:** None.
  21 ///
  22 /// **Example:**
  23 ///
  24 /// ```rust
  25 /// let x: Option<&str> = do_stuff();
  26 /// x.map(log_err_msg);
  27 /// x.map(|msg| log_err_msg(format_msg(msg)))
  28 /// ```
  29 ///
  30 /// The correct use would be:
  31 ///
  32 /// ```rust
  33 /// let x: Option<&str> = do_stuff();
  34 /// if let Some(msg) = x {
  35 ///     log_err_msg(msg)
  36 /// }
  37 /// if let Some(msg) = x {
  38 ///     log_err_msg(format_msg(msg))
  39 /// }
  40 /// ```
  41 declare_clippy_lint! {
  42     pub OPTION_MAP_UNIT_FN,
  43     complexity,
  44     "using `option.map(f)`, where f is a function or closure that returns ()"
  45 }
  46
  47 /// **What it does:** Checks for usage of `result.map(f)` where f is a function
  48 /// or closure that returns the unit type.
  49 ///
  50 /// **Why is this bad?** Readability, this can be written more clearly with
  51 /// an if let statement
  52 ///
  53 /// **Known problems:** None.
  54 ///
  55 /// **Example:**
  56 ///
  57 /// ```rust
  58 /// let x: Result<&str, &str> = do_stuff();
  59 /// x.map(log_err_msg);
  60 /// x.map(|msg| log_err_msg(format_msg(msg)))
  61 /// ```
  62 ///
  63 /// The correct use would be:
  64 ///
  65 /// ```rust
  66 /// let x: Result<&str, &str> = do_stuff();
  67 /// if let Ok(msg) = x {
  68 ///     log_err_msg(msg)
  69 /// }
  70 /// if let Ok(msg) = x {
  71 ///     log_err_msg(format_msg(msg))
  72 /// }
  73 /// ```
  74 declare_clippy_lint! {
  75     pub RESULT_MAP_UNIT_FN,
  76     complexity,
  77     "using `result.map(f)`, where f is a function or closure that returns ()"
  78 }
  79
  80
  81 impl LintPass for Pass {
  82     fn get_lints(&self) -> LintArray {
  83         lint_array!(OPTION_MAP_UNIT_FN, RESULT_MAP_UNIT_FN)
  84     }
  85 }
  86
  87 fn is_unit_type(ty: ty::Ty<'_>) -> bool {
  88     match ty.sty {
  89         ty::Tuple(slice) => slice.is_empty(),
  90         ty::Never => true,
  91         _ => false,
  92     }
  93 }
  94
  95 fn is_unit_function(cx: &LateContext<'_, '_>, expr: &hir::Expr) -> bool {
  96     let ty = cx.tables.expr_ty(expr);
  97
  98     if let ty::FnDef(id, _) = ty.sty {
  99         if let Some(fn_type) = cx.tcx.fn_sig(id).no_late_bound_regions() {
 100             return is_unit_type(fn_type.output());
 101         }
 102     }
 103     false
 104 }
 105
 106 fn is_unit_expression(cx: &LateContext<'_, '_>, expr: &hir::Expr) -> bool {
 107     is_unit_type(cx.tables.expr_ty(expr))
 108 }
 109
 110 /// The expression inside a closure may or may not have surrounding braces and
 111 /// semicolons, which causes problems when generating a suggestion. Given an
 112 /// expression that evaluates to '()' or '!', recursively remove useless braces
 113 /// and semi-colons until is suitable for including in the suggestion template
 114 fn reduce_unit_expression<'a>(cx: &LateContext<'_, '_>, expr: &'a hir::Expr) -> Option<Span> {
 115     if !is_unit_expression(cx, expr) {
 116         return None;
 117     }
 118
 119     match expr.node {
 120         hir::ExprKind::Call(_, _) |
 121         hir::ExprKind::MethodCall(_, _, _) => {
 122             // Calls can't be reduced any more
 123             Some(expr.span)
 124         },
 125         hir::ExprKind::Block(ref block, _) => {
 126             match (&block.stmts[..], block.expr.as_ref()) {
 127                 (&[], Some(inner_expr)) => {
 128                     // If block only contains an expression,
 129                     // reduce `{ X }` to `X`
 130                     reduce_unit_expression(cx, inner_expr)
 131                 },
 132                 (&[ref inner_stmt], None) => {
 133                     // If block only contains statements,
 134                     // reduce `{ X; }` to `X` or `X;`
 135                     match inner_stmt.node {
 136                         hir::StmtKind::Decl(ref d, _) => Some(d.span),
 137                         hir::StmtKind::Expr(ref e, _) => Some(e.span),
 138                         hir::StmtKind::Semi(_, _) => Some(inner_stmt.span),
 139                     }
 140                 },
 141                 _ => {
 142                     // For closures that contain multiple statements
 143                     // it's difficult to get a correct suggestion span
 144                     // for all cases (multi-line closures specifically)
 145                     //
 146                     // We do not attempt to build a suggestion for those right now.
 147                     None
 148                 }
 149             }
 150         },
 151         _ => None,
 152     }
 153 }
 154
 155 fn unit_closure<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, expr: &'a hir::Expr) -> Option<(&'tcx hir::Arg, &'a hir::Expr)> {
 156     if let hir::ExprKind::Closure(_, ref decl, inner_expr_id, _, _) = expr.node {
 157         let body = cx.tcx.hir.body(inner_expr_id);
 158         let body_expr = &body.value;
 159
 160         if_chain! {
 161             if decl.inputs.len() == 1;
 162             if is_unit_expression(cx, body_expr);
 163             if let Some(binding) = iter_input_pats(&decl, body).next();
 164             then {
 165                 return Some((binding, body_expr));
 166             }
 167         }
 168     }
 169     None
 170 }
 171
 172 /// Builds a name for the let binding variable (var_arg)
 173 ///
 174 /// `x.field` => `x_field`
 175 /// `y` => `_y`
 176 ///
 177 /// Anything else will return `_`.
 178 fn let_binding_name(cx: &LateContext<'_, '_>, var_arg: &hir::Expr) -> String {
 179     match &var_arg.node {
 180         hir::ExprKind::Field(_, _) => snippet(cx, var_arg.span, "_").replace(".", "_"),
 181         hir::ExprKind::Path(_) => format!("_{}", snippet(cx, var_arg.span, "")),
 182         _ => "_".to_string()
 183     }
 184 }
 185
 186 fn suggestion_msg(function_type: &str, map_type: &str) -> String {
 187     format!(
 188         "called `map(f)` on an {0} value where `f` is a unit {1}",
 189         map_type,
 190         function_type
 191     )
 192 }
 193
 194 fn lint_map_unit_fn(cx: &LateContext<'_, '_>, stmt: &hir::Stmt, expr: &hir::Expr, map_args: &[hir::Expr]) {
 195     let var_arg = &map_args[0];
 196     let fn_arg = &map_args[1];
 197
 198     let (map_type, variant, lint) =
 199         if match_type(cx, cx.tables.expr_ty(var_arg), &paths::OPTION) {
 200             ("Option", "Some", OPTION_MAP_UNIT_FN)
 201         } else if match_type(cx, cx.tables.expr_ty(var_arg), &paths::RESULT) {
 202             ("Result", "Ok", RESULT_MAP_UNIT_FN)
 203         } else {
 204             return
 205         };
 206
 207     if is_unit_function(cx, fn_arg) {
 208         let msg = suggestion_msg("function", map_type);
 209         let suggestion = format!("if let {0}({1}) = {2} {{ {3}(...) }}",
 210                                  variant,
 211                                  let_binding_name(cx, var_arg),
 212                                  snippet(cx, var_arg.span, "_"),
 213                                  snippet(cx, fn_arg.span, "_"));
 214
 215         span_lint_and_then(cx, lint, expr.span, &msg, |db| {
 216             db.span_suggestion_with_applicability(stmt.span,
 217                                                   "try this",
 218                                                   suggestion,
 219                                                   Applicability::Unspecified);
 220         });
 221     } else if let Some((binding, closure_expr)) = unit_closure(cx, fn_arg) {
 222         let msg = suggestion_msg("closure", map_type);
 223
 224         span_lint_and_then(cx, lint, expr.span, &msg, |db| {
 225             if let Some(reduced_expr_span) = reduce_unit_expression(cx, closure_expr) {
 226                 let suggestion = format!("if let {0}({1}) = {2} {{ {3} }}",
 227                                          variant,
 228                                          snippet(cx, binding.pat.span, "_"),
 229                                          snippet(cx, var_arg.span, "_"),
 230                                          snippet(cx, reduced_expr_span, "_"));
 231                 db.span_suggestion(stmt.span, "try this", suggestion);
 232             } else {
 233                 let suggestion = format!("if let {0}({1}) = {2} {{ ... }}",
 234                                          variant,
 235                                          snippet(cx, binding.pat.span, "_"),
 236                                          snippet(cx, var_arg.span, "_"));
 237                 db.span_suggestion_with_applicability(stmt.span,
 238                                                       "try this",
 239                                                       suggestion,
 240                                                       Applicability::Unspecified);
 241             }
 242         });
 243     }
 244 }
 245
 246 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
 247     fn check_stmt(&mut self, cx: &LateContext<'_, '_>, stmt: &hir::Stmt) {
 248         if in_macro(stmt.span) {
 249             return;
 250         }
 251
 252         if let hir::StmtKind::Semi(ref expr, _) = stmt.node {
 253             if let hir::ExprKind::MethodCall(_, _, _) = expr.node {
 254                 if let Some(arglists) = method_chain_args(expr, &["map"]) {
 255                     lint_map_unit_fn(cx, stmt, expr, arglists[0]);
 256                 }
 257             }
 258         }
 259     }
 260 }