1 use clippy_utils::diagnostics::span_lint_and_then;
2 use clippy_utils::source::snippet;
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;
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;
14 declare_clippy_lint! {
16 /// Checks for usage of `option.map(f)` where f is a function
17 /// or closure that returns the unit type `()`.
19 /// ### Why is this bad?
20 /// Readability, this can be written more clearly with
21 /// an if let statement
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)));
34 /// The correct use would be:
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 {
45 /// # let x: Option<String> = do_stuff();
46 /// if let Some(msg) = x {
47 /// log_err_msg(format_msg(msg));
50 pub OPTION_MAP_UNIT_FN,
52 "using `option.map(f)`, where `f` is a function or closure that returns `()`"
55 declare_clippy_lint! {
57 /// Checks for usage of `result.map(f)` where f is a function
58 /// or closure that returns the unit type `()`.
60 /// ### Why is this bad?
61 /// Readability, this can be written more clearly with
62 /// an if let statement
66 /// # fn do_stuff() -> Result<String, String> { Ok(String::new()) }
67 /// # fn log_err_msg(foo: String) -> Result<String, String> { Ok(foo) }
68 /// # fn format_msg(foo: String) -> String { String::new() }
69 /// let x: Result<String, String> = do_stuff();
70 /// x.map(log_err_msg);
71 /// # let x: Result<String, String> = do_stuff();
72 /// x.map(|msg| log_err_msg(format_msg(msg)));
75 /// The correct use would be:
78 /// # fn do_stuff() -> Result<String, String> { Ok(String::new()) }
79 /// # fn log_err_msg(foo: String) -> Result<String, String> { Ok(foo) }
80 /// # fn format_msg(foo: String) -> String { String::new() }
81 /// let x: Result<String, String> = do_stuff();
82 /// if let Ok(msg) = x {
85 /// # let x: Result<String, String> = do_stuff();
86 /// if let Ok(msg) = x {
87 /// log_err_msg(format_msg(msg));
90 pub RESULT_MAP_UNIT_FN,
92 "using `result.map(f)`, where `f` is a function or closure that returns `()`"
95 declare_lint_pass!(MapUnit => [OPTION_MAP_UNIT_FN, RESULT_MAP_UNIT_FN]);
97 fn is_unit_type(ty: Ty<'_>) -> bool {
99 ty::Tuple(slice) => slice.is_empty(),
105 fn is_unit_function(cx: &LateContext<'_>, expr: &hir::Expr<'_>) -> bool {
106 let ty = cx.typeck_results().expr_ty(expr);
108 if let ty::FnDef(id, _) = *ty.kind() {
109 if let Some(fn_type) = cx.tcx.fn_sig(id).no_bound_vars() {
110 return is_unit_type(fn_type.output());
116 fn is_unit_expression(cx: &LateContext<'_>, expr: &hir::Expr<'_>) -> bool {
117 is_unit_type(cx.typeck_results().expr_ty(expr))
120 /// The expression inside a closure may or may not have surrounding braces and
121 /// semicolons, which causes problems when generating a suggestion. Given an
122 /// expression that evaluates to '()' or '!', recursively remove useless braces
123 /// and semi-colons until is suitable for including in the suggestion template
124 fn reduce_unit_expression<'a>(cx: &LateContext<'_>, expr: &'a hir::Expr<'_>) -> Option<Span> {
125 if !is_unit_expression(cx, expr) {
130 hir::ExprKind::Call(_, _) | hir::ExprKind::MethodCall(_, _, _, _) => {
131 // Calls can't be reduced any more
134 hir::ExprKind::Block(block, _) => {
135 match (block.stmts, block.expr.as_ref()) {
136 (&[], Some(inner_expr)) => {
137 // If block only contains an expression,
138 // reduce `{ X }` to `X`
139 reduce_unit_expression(cx, inner_expr)
141 (&[ref inner_stmt], None) => {
142 // If block only contains statements,
143 // reduce `{ X; }` to `X` or `X;`
144 match inner_stmt.kind {
145 hir::StmtKind::Local(local) => Some(local.span),
146 hir::StmtKind::Expr(e) => Some(e.span),
147 hir::StmtKind::Semi(..) => Some(inner_stmt.span),
148 hir::StmtKind::Item(..) => None,
152 // For closures that contain multiple statements
153 // it's difficult to get a correct suggestion span
154 // for all cases (multi-line closures specifically)
156 // We do not attempt to build a suggestion for those right now.
165 fn unit_closure<'tcx>(
166 cx: &LateContext<'tcx>,
167 expr: &hir::Expr<'_>,
168 ) -> Option<(&'tcx hir::Param<'tcx>, &'tcx hir::Expr<'tcx>)> {
170 if let hir::ExprKind::Closure(_, decl, inner_expr_id, _, _) = expr.kind;
171 let body = cx.tcx.hir().body(inner_expr_id);
172 let body_expr = &body.value;
173 if decl.inputs.len() == 1;
174 if is_unit_expression(cx, body_expr);
175 if let Some(binding) = iter_input_pats(decl, body).next();
177 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.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)
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<'tcx> LateLintPass<'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(expr) = stmt.kind {
268 if let Some(arglists) = method_chain_args(expr, &["map"]) {
269 lint_map_unit_fn(cx, stmt, expr, arglists[0]);