clippy_lints/src/map_unit_fn.rs

   1 use crate::utils::paths;
   2 use crate::utils::{in_macro, iter_input_pats, match_type, method_chain_args, snippet, span_lint_and_then};
   3 use if_chain::if_chain;
   4 use rustc::hir;
   5 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   6 use rustc::ty;
   7 use rustc::{declare_tool_lint, lint_array};
   8 use rustc_errors::Applicability;
   9 use syntax::source_map::Span;
  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 impl LintPass for Pass {
  81     fn get_lints(&self) -> LintArray {
  82         lint_array!(OPTION_MAP_UNIT_FN, RESULT_MAP_UNIT_FN)
  83     }
  84 }
  85
  86 fn is_unit_type(ty: ty::Ty<'_>) -> bool {
  87     match ty.sty {
  88         ty::Tuple(slice) => slice.is_empty(),
  89         ty::Never => true,
  90         _ => false,
  91     }
  92 }
  93
  94 fn is_unit_function(cx: &LateContext<'_, '_>, expr: &hir::Expr) -> bool {
  95     let ty = cx.tables.expr_ty(expr);
  96
  97     if let ty::FnDef(id, _) = ty.sty {
  98         if let Some(fn_type) = cx.tcx.fn_sig(id).no_bound_vars() {
  99             return is_unit_type(fn_type.output());
 100         }
 101     }
 102     false
 103 }
 104
 105 fn is_unit_expression(cx: &LateContext<'_, '_>, expr: &hir::Expr) -> bool {
 106     is_unit_type(cx.tables.expr_ty(expr))
 107 }
 108
 109 /// The expression inside a closure may or may not have surrounding braces and
 110 /// semicolons, which causes problems when generating a suggestion. Given an
 111 /// expression that evaluates to '()' or '!', recursively remove useless braces
 112 /// and semi-colons until is suitable for including in the suggestion template
 113 fn reduce_unit_expression<'a>(cx: &LateContext<'_, '_>, expr: &'a hir::Expr) -> Option<Span> {
 114     if !is_unit_expression(cx, expr) {
 115         return None;
 116     }
 117
 118     match expr.node {
 119         hir::ExprKind::Call(_, _) | hir::ExprKind::MethodCall(_, _, _) => {
 120             // Calls can't be reduced any more
 121             Some(expr.span)
 122         },
 123         hir::ExprKind::Block(ref block, _) => {
 124             match (&block.stmts[..], block.expr.as_ref()) {
 125                 (&[], Some(inner_expr)) => {
 126                     // If block only contains an expression,
 127                     // reduce `{ X }` to `X`
 128                     reduce_unit_expression(cx, inner_expr)
 129                 },
 130                 (&[ref inner_stmt], None) => {
 131                     // If block only contains statements,
 132                     // reduce `{ X; }` to `X` or `X;`
 133                     match inner_stmt.node {
 134                         hir::StmtKind::Decl(ref d, _) => Some(d.span),
 135                         hir::StmtKind::Expr(ref e, _) => Some(e.span),
 136                         hir::StmtKind::Semi(_, _) => Some(inner_stmt.span),
 137                     }
 138                 },
 139                 _ => {
 140                     // For closures that contain multiple statements
 141                     // it's difficult to get a correct suggestion span
 142                     // for all cases (multi-line closures specifically)
 143                     //
 144                     // We do not attempt to build a suggestion for those right now.
 145                     None
 146                 },
 147             }
 148         },
 149         _ => None,
 150     }
 151 }
 152
 153 fn unit_closure<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, expr: &'a hir::Expr) -> Option<(&'tcx hir::Arg, &'a hir::Expr)> {
 154     if let hir::ExprKind::Closure(_, ref decl, inner_expr_id, _, _) = expr.node {
 155         let body = cx.tcx.hir().body(inner_expr_id);
 156         let body_expr = &body.value;
 157
 158         if_chain! {
 159             if decl.inputs.len() == 1;
 160             if is_unit_expression(cx, body_expr);
 161             if let Some(binding) = iter_input_pats(&decl, body).next();
 162             then {
 163                 return Some((binding, body_expr));
 164             }
 165         }
 166     }
 167     None
 168 }
 169
 170 /// Builds a name for the let binding variable (`var_arg`)
 171 ///
 172 /// `x.field` => `x_field`
 173 /// `y` => `_y`
 174 ///
 175 /// Anything else will return `_`.
 176 fn let_binding_name(cx: &LateContext<'_, '_>, var_arg: &hir::Expr) -> String {
 177     match &var_arg.node {
 178         hir::ExprKind::Field(_, _) => snippet(cx, var_arg.span, "_").replace(".", "_"),
 179         hir::ExprKind::Path(_) => format!("_{}", snippet(cx, var_arg.span, "")),
 180         _ => "_".to_string(),
 181     }
 182 }
 183
 184 fn suggestion_msg(function_type: &str, map_type: &str) -> String {
 185     format!(
 186         "called `map(f)` on an {0} value where `f` is a unit {1}",
 187         map_type, function_type
 188     )
 189 }
 190
 191 fn lint_map_unit_fn(cx: &LateContext<'_, '_>, stmt: &hir::Stmt, expr: &hir::Expr, map_args: &[hir::Expr]) {
 192     let var_arg = &map_args[0];
 193
 194     let (map_type, variant, lint) = if match_type(cx, cx.tables.expr_ty(var_arg), &paths::OPTION) {
 195         ("Option", "Some", OPTION_MAP_UNIT_FN)
 196     } else if match_type(cx, cx.tables.expr_ty(var_arg), &paths::RESULT) {
 197         ("Result", "Ok", RESULT_MAP_UNIT_FN)
 198     } else {
 199         return;
 200     };
 201     let fn_arg = &map_args[1];
 202
 203     if is_unit_function(cx, fn_arg) {
 204         let msg = suggestion_msg("function", map_type);
 205         let suggestion = format!(
 206             "if let {0}({1}) = {2} {{ {3}(...) }}",
 207             variant,
 208             let_binding_name(cx, var_arg),
 209             snippet(cx, var_arg.span, "_"),
 210             snippet(cx, fn_arg.span, "_")
 211         );
 212
 213         span_lint_and_then(cx, lint, expr.span, &msg, |db| {
 214             db.span_suggestion_with_applicability(stmt.span, "try this", suggestion, Applicability::Unspecified);
 215         });
 216     } else if let Some((binding, closure_expr)) = unit_closure(cx, fn_arg) {
 217         let msg = suggestion_msg("closure", map_type);
 218
 219         span_lint_and_then(cx, lint, expr.span, &msg, |db| {
 220             if let Some(reduced_expr_span) = reduce_unit_expression(cx, closure_expr) {
 221                 let suggestion = format!(
 222                     "if let {0}({1}) = {2} {{ {3} }}",
 223                     variant,
 224                     snippet(cx, binding.pat.span, "_"),
 225                     snippet(cx, var_arg.span, "_"),
 226                     snippet(cx, reduced_expr_span, "_")
 227                 );
 228                 db.span_suggestion_with_applicability(
 229                     stmt.span,
 230                     "try this",
 231                     suggestion,
 232                     Applicability::MachineApplicable, // snippet
 233                 );
 234             } else {
 235                 let suggestion = format!(
 236                     "if let {0}({1}) = {2} {{ ... }}",
 237                     variant,
 238                     snippet(cx, binding.pat.span, "_"),
 239                     snippet(cx, var_arg.span, "_")
 240                 );
 241                 db.span_suggestion_with_applicability(stmt.span, "try this", suggestion, Applicability::Unspecified);
 242             }
 243         });
 244     }
 245 }
 246
 247 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
 248     fn check_stmt(&mut self, cx: &LateContext<'_, '_>, stmt: &hir::Stmt) {
 249         if in_macro(stmt.span) {
 250             return;
 251         }
 252
 253         if let hir::StmtKind::Semi(ref expr, _) = stmt.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 }