src/tools/clippy/clippy_lints/src/map_unit_fn.rs

   1 use clippy_utils::diagnostics::span_lint_and_then;
   2 use clippy_utils::source::{snippet, snippet_with_applicability, snippet_with_context};
   3 use clippy_utils::ty::is_type_diagnostic_item;
   4 use clippy_utils::{iter_input_pats, method_chain_args};
   5 use if_chain::if_chain;
   6 use rustc_errors::Applicability;
   7 use rustc_hir as hir;
   8 use rustc_lint::{LateContext, LateLintPass};
   9 use rustc_middle::ty::{self, Ty};
  10 use rustc_session::{declare_lint_pass, declare_tool_lint};
  11 use rustc_span::source_map::Span;
  12 use rustc_span::sym;
  13
  14 declare_clippy_lint! {
  15     /// ### What it does
  16     /// Checks for usage of `option.map(f)` where f is a function
  17     /// or closure that returns the unit type `()`.
  18     ///
  19     /// ### Why is this bad?
  20     /// Readability, this can be written more clearly with
  21     /// an if let statement
  22     ///
  23     /// ### Example
  24     /// ```rust
  25     /// # fn do_stuff() -> Option<String> { Some(String::new()) }
  26     /// # fn log_err_msg(foo: String) -> Option<String> { Some(foo) }
  27     /// # fn format_msg(foo: String) -> String { String::new() }
  28     /// let x: Option<String> = do_stuff();
  29     /// x.map(log_err_msg);
  30     /// # let x: Option<String> = do_stuff();
  31     /// x.map(|msg| log_err_msg(format_msg(msg)));
  32     /// ```
  33     ///
  34     /// The correct use would be:
  35     ///
  36     /// ```rust
  37     /// # fn do_stuff() -> Option<String> { Some(String::new()) }
  38     /// # fn log_err_msg(foo: String) -> Option<String> { Some(foo) }
  39     /// # fn format_msg(foo: String) -> String { String::new() }
  40     /// let x: Option<String> = do_stuff();
  41     /// if let Some(msg) = x {
  42     ///     log_err_msg(msg);
  43     /// }
  44     ///
  45     /// # let x: Option<String> = do_stuff();
  46     /// if let Some(msg) = x {
  47     ///     log_err_msg(format_msg(msg));
  48     /// }
  49     /// ```
  50     #[clippy::version = "pre 1.29.0"]
  51     pub OPTION_MAP_UNIT_FN,
  52     complexity,
  53     "using `option.map(f)`, where `f` is a function or closure that returns `()`"
  54 }
  55
  56 declare_clippy_lint! {
  57     /// ### What it does
  58     /// Checks for usage of `result.map(f)` where f is a function
  59     /// or closure that returns the unit type `()`.
  60     ///
  61     /// ### Why is this bad?
  62     /// Readability, this can be written more clearly with
  63     /// an if let statement
  64     ///
  65     /// ### Example
  66     /// ```rust
  67     /// # fn do_stuff() -> Result<String, String> { Ok(String::new()) }
  68     /// # fn log_err_msg(foo: String) -> Result<String, String> { Ok(foo) }
  69     /// # fn format_msg(foo: String) -> String { String::new() }
  70     /// let x: Result<String, String> = do_stuff();
  71     /// x.map(log_err_msg);
  72     /// # let x: Result<String, String> = do_stuff();
  73     /// x.map(|msg| log_err_msg(format_msg(msg)));
  74     /// ```
  75     ///
  76     /// The correct use would be:
  77     ///
  78     /// ```rust
  79     /// # fn do_stuff() -> Result<String, String> { Ok(String::new()) }
  80     /// # fn log_err_msg(foo: String) -> Result<String, String> { Ok(foo) }
  81     /// # fn format_msg(foo: String) -> String { String::new() }
  82     /// let x: Result<String, String> = do_stuff();
  83     /// if let Ok(msg) = x {
  84     ///     log_err_msg(msg);
  85     /// };
  86     /// # let x: Result<String, String> = do_stuff();
  87     /// if let Ok(msg) = x {
  88     ///     log_err_msg(format_msg(msg));
  89     /// };
  90     /// ```
  91     #[clippy::version = "pre 1.29.0"]
  92     pub RESULT_MAP_UNIT_FN,
  93     complexity,
  94     "using `result.map(f)`, where `f` is a function or closure that returns `()`"
  95 }
  96
  97 declare_lint_pass!(MapUnit => [OPTION_MAP_UNIT_FN, RESULT_MAP_UNIT_FN]);
  98
  99 fn is_unit_type(ty: Ty<'_>) -> bool {
 100     ty.is_unit() || ty.is_never()
 101 }
 102
 103 fn is_unit_function(cx: &LateContext<'_>, expr: &hir::Expr<'_>) -> bool {
 104     let ty = cx.typeck_results().expr_ty(expr);
 105
 106     if let ty::FnDef(id, _) = *ty.kind() {
 107         if let Some(fn_type) = cx.tcx.fn_sig(id).no_bound_vars() {
 108             return is_unit_type(fn_type.output());
 109         }
 110     }
 111     false
 112 }
 113
 114 fn is_unit_expression(cx: &LateContext<'_>, expr: &hir::Expr<'_>) -> bool {
 115     is_unit_type(cx.typeck_results().expr_ty(expr))
 116 }
 117
 118 /// The expression inside a closure may or may not have surrounding braces and
 119 /// semicolons, which causes problems when generating a suggestion. Given an
 120 /// expression that evaluates to '()' or '!', recursively remove useless braces
 121 /// and semi-colons until is suitable for including in the suggestion template
 122 fn reduce_unit_expression(cx: &LateContext<'_>, expr: &hir::Expr<'_>) -> Option<Span> {
 123     if !is_unit_expression(cx, expr) {
 124         return None;
 125     }
 126
 127     match expr.kind {
 128         hir::ExprKind::Call(_, _) | hir::ExprKind::MethodCall(..) => {
 129             // Calls can't be reduced any more
 130             Some(expr.span)
 131         },
 132         hir::ExprKind::Block(block, _) => {
 133             match (block.stmts, block.expr.as_ref()) {
 134                 ([], Some(inner_expr)) => {
 135                     // If block only contains an expression,
 136                     // reduce `{ X }` to `X`
 137                     reduce_unit_expression(cx, inner_expr)
 138                 },
 139                 ([inner_stmt], None) => {
 140                     // If block only contains statements,
 141                     // reduce `{ X; }` to `X` or `X;`
 142                     match inner_stmt.kind {
 143                         hir::StmtKind::Local(local) => Some(local.span),
 144                         hir::StmtKind::Expr(e) => Some(e.span),
 145                         hir::StmtKind::Semi(..) => Some(inner_stmt.span),
 146                         hir::StmtKind::Item(..) => None,
 147                     }
 148                 },
 149                 _ => {
 150                     // For closures that contain multiple statements
 151                     // it's difficult to get a correct suggestion span
 152                     // for all cases (multi-line closures specifically)
 153                     //
 154                     // We do not attempt to build a suggestion for those right now.
 155                     None
 156                 },
 157             }
 158         },
 159         _ => None,
 160     }
 161 }
 162
 163 fn unit_closure<'tcx>(
 164     cx: &LateContext<'tcx>,
 165     expr: &hir::Expr<'_>,
 166 ) -> Option<(&'tcx hir::Param<'tcx>, &'tcx hir::Expr<'tcx>)> {
 167     if_chain! {
 168         if let hir::ExprKind::Closure(&hir::Closure { fn_decl, body, .. }) = expr.kind;
 169         let body = cx.tcx.hir().body(body);
 170         let body_expr = &body.value;
 171         if fn_decl.inputs.len() == 1;
 172         if is_unit_expression(cx, body_expr);
 173         if let Some(binding) = iter_input_pats(fn_decl, body).next();
 174         then {
 175             return Some((binding, body_expr));
 176         }
 177     }
 178     None
 179 }
 180
 181 /// Builds a name for the let binding variable (`var_arg`)
 182 ///
 183 /// `x.field` => `x_field`
 184 /// `y` => `_y`
 185 ///
 186 /// Anything else will return `a`.
 187 fn let_binding_name(cx: &LateContext<'_>, var_arg: &hir::Expr<'_>) -> String {
 188     match &var_arg.kind {
 189         hir::ExprKind::Field(_, _) => snippet(cx, var_arg.span, "_").replace('.', "_"),
 190         hir::ExprKind::Path(_) => format!("_{}", snippet(cx, var_arg.span, "")),
 191         _ => "a".to_string(),
 192     }
 193 }
 194
 195 #[must_use]
 196 fn suggestion_msg(function_type: &str, map_type: &str) -> String {
 197     format!("called `map(f)` on an `{map_type}` value where `f` is a {function_type} that returns the unit type `()`")
 198 }
 199
 200 fn lint_map_unit_fn(
 201     cx: &LateContext<'_>,
 202     stmt: &hir::Stmt<'_>,
 203     expr: &hir::Expr<'_>,
 204     map_args: (&hir::Expr<'_>, &[hir::Expr<'_>]),
 205 ) {
 206     let var_arg = &map_args.0;
 207
 208     let (map_type, variant, lint) = if is_type_diagnostic_item(cx, cx.typeck_results().expr_ty(var_arg), sym::Option) {
 209         ("Option", "Some", OPTION_MAP_UNIT_FN)
 210     } else if is_type_diagnostic_item(cx, cx.typeck_results().expr_ty(var_arg), sym::Result) {
 211         ("Result", "Ok", RESULT_MAP_UNIT_FN)
 212     } else {
 213         return;
 214     };
 215     let fn_arg = &map_args.1[0];
 216
 217     if is_unit_function(cx, fn_arg) {
 218         let mut applicability = Applicability::MachineApplicable;
 219         let msg = suggestion_msg("function", map_type);
 220         let suggestion = format!(
 221             "if let {0}({binding}) = {1} {{ {2}({binding}) }}",
 222             variant,
 223             snippet_with_applicability(cx, var_arg.span, "_", &mut applicability),
 224             snippet_with_applicability(cx, fn_arg.span, "_", &mut applicability),
 225             binding = let_binding_name(cx, var_arg)
 226         );
 227
 228         span_lint_and_then(cx, lint, expr.span, &msg, |diag| {
 229             diag.span_suggestion(stmt.span, "try this", suggestion, applicability);
 230         });
 231     } else if let Some((binding, closure_expr)) = unit_closure(cx, fn_arg) {
 232         let msg = suggestion_msg("closure", map_type);
 233
 234         span_lint_and_then(cx, lint, expr.span, &msg, |diag| {
 235             if let Some(reduced_expr_span) = reduce_unit_expression(cx, closure_expr) {
 236                 let mut applicability = Applicability::MachineApplicable;
 237                 let suggestion = format!(
 238                     "if let {0}({1}) = {2} {{ {3} }}",
 239                     variant,
 240                     snippet_with_applicability(cx, binding.pat.span, "_", &mut applicability),
 241                     snippet_with_applicability(cx, var_arg.span, "_", &mut applicability),
 242                     snippet_with_context(cx, reduced_expr_span, var_arg.span.ctxt(), "_", &mut applicability).0,
 243                 );
 244                 diag.span_suggestion(stmt.span, "try this", suggestion, applicability);
 245             } else {
 246                 let suggestion = format!(
 247                     "if let {0}({1}) = {2} {{ ... }}",
 248                     variant,
 249                     snippet(cx, binding.pat.span, "_"),
 250                     snippet(cx, var_arg.span, "_"),
 251                 );
 252                 diag.span_suggestion(stmt.span, "try this", suggestion, Applicability::HasPlaceholders);
 253             }
 254         });
 255     }
 256 }
 257
 258 impl<'tcx> LateLintPass<'tcx> for MapUnit {
 259     fn check_stmt(&mut self, cx: &LateContext<'_>, stmt: &hir::Stmt<'_>) {
 260         if stmt.span.from_expansion() {
 261             return;
 262         }
 263
 264         if let hir::StmtKind::Semi(expr) = stmt.kind {
 265             if let Some(arglists) = method_chain_args(expr, &["map"]) {
 266                 lint_map_unit_fn(cx, stmt, expr, arglists[0]);
 267             }
 268         }
 269     }
 270 }