1 use crate::utils::{is_type_diagnostic_item, iter_input_pats, method_chain_args, snippet, span_lint_and_then};
2 use if_chain::if_chain;
3 use rustc_errors::Applicability;
5 use rustc_lint::{LateContext, LateLintPass};
6 use rustc_middle::ty::{self, Ty};
7 use rustc_session::{declare_lint_pass, declare_tool_lint};
8 use rustc_span::source_map::Span;
10 declare_clippy_lint! {
11 /// **What it does:** Checks for usage of `option.map(f)` where f is a function
12 /// or closure that returns the unit type.
14 /// **Why is this bad?** Readability, this can be written more clearly with
15 /// an if let statement
17 /// **Known problems:** None.
22 /// # fn do_stuff() -> Option<String> { Some(String::new()) }
23 /// # fn log_err_msg(foo: String) -> Option<String> { Some(foo) }
24 /// # fn format_msg(foo: String) -> String { String::new() }
25 /// let x: Option<String> = do_stuff();
26 /// x.map(log_err_msg);
27 /// # let x: Option<String> = do_stuff();
28 /// x.map(|msg| log_err_msg(format_msg(msg)));
31 /// The correct use would be:
34 /// # fn do_stuff() -> Option<String> { Some(String::new()) }
35 /// # fn log_err_msg(foo: String) -> Option<String> { Some(foo) }
36 /// # fn format_msg(foo: String) -> String { String::new() }
37 /// let x: Option<String> = do_stuff();
38 /// if let Some(msg) = x {
42 /// # let x: Option<String> = do_stuff();
43 /// if let Some(msg) = x {
44 /// log_err_msg(format_msg(msg));
47 pub OPTION_MAP_UNIT_FN,
49 "using `option.map(f)`, where `f` is a function or closure that returns `()`"
52 declare_clippy_lint! {
53 /// **What it does:** Checks for usage of `result.map(f)` where f is a function
54 /// or closure that returns the unit type.
56 /// **Why is this bad?** Readability, this can be written more clearly with
57 /// an if let statement
59 /// **Known problems:** None.
64 /// # fn do_stuff() -> Result<String, String> { Ok(String::new()) }
65 /// # fn log_err_msg(foo: String) -> Result<String, String> { Ok(foo) }
66 /// # fn format_msg(foo: String) -> String { String::new() }
67 /// let x: Result<String, String> = do_stuff();
68 /// x.map(log_err_msg);
69 /// # let x: Result<String, String> = do_stuff();
70 /// x.map(|msg| log_err_msg(format_msg(msg)));
73 /// The correct use would be:
76 /// # fn do_stuff() -> Result<String, String> { Ok(String::new()) }
77 /// # fn log_err_msg(foo: String) -> Result<String, String> { Ok(foo) }
78 /// # fn format_msg(foo: String) -> String { String::new() }
79 /// let x: Result<String, String> = do_stuff();
80 /// if let Ok(msg) = x {
83 /// # let x: Result<String, String> = do_stuff();
84 /// if let Ok(msg) = x {
85 /// log_err_msg(format_msg(msg));
88 pub RESULT_MAP_UNIT_FN,
90 "using `result.map(f)`, where `f` is a function or closure that returns `()`"
93 declare_lint_pass!(MapUnit => [OPTION_MAP_UNIT_FN, RESULT_MAP_UNIT_FN]);
95 fn is_unit_type(ty: Ty<'_>) -> bool {
97 ty::Tuple(slice) => slice.is_empty(),
103 fn is_unit_function(cx: &LateContext<'_, '_>, expr: &hir::Expr<'_>) -> bool {
104 let ty = cx.tables.expr_ty(expr);
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());
114 fn is_unit_expression(cx: &LateContext<'_, '_>, expr: &hir::Expr<'_>) -> bool {
115 is_unit_type(cx.tables.expr_ty(expr))
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<'a>(cx: &LateContext<'_, '_>, expr: &'a hir::Expr<'_>) -> Option<Span> {
123 if !is_unit_expression(cx, expr) {
128 hir::ExprKind::Call(_, _) | hir::ExprKind::MethodCall(_, _, _) => {
129 // Calls can't be reduced any more
132 hir::ExprKind::Block(ref 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)
139 (&[ref 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(ref local) => Some(local.span),
144 hir::StmtKind::Expr(ref e) => Some(e.span),
145 hir::StmtKind::Semi(..) => Some(inner_stmt.span),
146 hir::StmtKind::Item(..) => None,
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)
154 // We do not attempt to build a suggestion for those right now.
163 fn unit_closure<'a, 'tcx>(
164 cx: &LateContext<'a, 'tcx>,
165 expr: &'a hir::Expr<'a>,
166 ) -> Option<(&'tcx hir::Param<'tcx>, &'a hir::Expr<'a>)> {
167 if let hir::ExprKind::Closure(_, ref decl, inner_expr_id, _, _) = expr.kind {
168 let body = cx.tcx.hir().body(inner_expr_id);
169 let body_expr = &body.value;
172 if decl.inputs.len() == 1;
173 if is_unit_expression(cx, body_expr);
174 if let Some(binding) = iter_input_pats(&decl, body).next();
176 return Some((binding, body_expr));
183 /// Builds a name for the let binding variable (`var_arg`)
185 /// `x.field` => `x_field`
188 /// Anything else will return `a`.
189 fn let_binding_name(cx: &LateContext<'_, '_>, var_arg: &hir::Expr<'_>) -> String {
190 match &var_arg.kind {
191 hir::ExprKind::Field(_, _) => snippet(cx, var_arg.span, "_").replace(".", "_"),
192 hir::ExprKind::Path(_) => format!("_{}", snippet(cx, var_arg.span, "")),
193 _ => "a".to_string(),
198 fn suggestion_msg(function_type: &str, map_type: &str) -> String {
200 "called `map(f)` on an `{0}` value where `f` is a {1} that returns the unit type",
201 map_type, function_type
205 fn lint_map_unit_fn(cx: &LateContext<'_, '_>, stmt: &hir::Stmt<'_>, expr: &hir::Expr<'_>, map_args: &[hir::Expr<'_>]) {
206 let var_arg = &map_args[0];
208 let (map_type, variant, lint) = if is_type_diagnostic_item(cx, cx.tables.expr_ty(var_arg), sym!(option_type)) {
209 ("Option", "Some", OPTION_MAP_UNIT_FN)
210 } else if is_type_diagnostic_item(cx, cx.tables.expr_ty(var_arg), sym!(result_type)) {
211 ("Result", "Ok", RESULT_MAP_UNIT_FN)
215 let fn_arg = &map_args[1];
217 if is_unit_function(cx, fn_arg) {
218 let msg = suggestion_msg("function", map_type);
219 let suggestion = format!(
220 "if let {0}({binding}) = {1} {{ {2}({binding}) }}",
222 snippet(cx, var_arg.span, "_"),
223 snippet(cx, fn_arg.span, "_"),
224 binding = let_binding_name(cx, var_arg)
227 span_lint_and_then(cx, lint, expr.span, &msg, |diag| {
228 diag.span_suggestion(stmt.span, "try this", suggestion, Applicability::MachineApplicable);
230 } else if let Some((binding, closure_expr)) = unit_closure(cx, fn_arg) {
231 let msg = suggestion_msg("closure", map_type);
233 span_lint_and_then(cx, lint, expr.span, &msg, |diag| {
234 if let Some(reduced_expr_span) = reduce_unit_expression(cx, closure_expr) {
235 let suggestion = format!(
236 "if let {0}({1}) = {2} {{ {3} }}",
238 snippet(cx, binding.pat.span, "_"),
239 snippet(cx, var_arg.span, "_"),
240 snippet(cx, reduced_expr_span, "_")
242 diag.span_suggestion(
246 Applicability::MachineApplicable, // snippet
249 let suggestion = format!(
250 "if let {0}({1}) = {2} {{ ... }}",
252 snippet(cx, binding.pat.span, "_"),
253 snippet(cx, var_arg.span, "_"),
255 diag.span_suggestion(stmt.span, "try this", suggestion, Applicability::HasPlaceholders);
261 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MapUnit {
262 fn check_stmt(&mut self, cx: &LateContext<'_, '_>, stmt: &hir::Stmt<'_>) {
263 if stmt.span.from_expansion() {
267 if let hir::StmtKind::Semi(ref expr) = stmt.kind {
268 if let Some(arglists) = method_chain_args(expr, &["map"]) {
269 lint_map_unit_fn(cx, stmt, expr, arglists[0]);