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;
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 #[clippy::version = "pre 1.29.0"]
51 pub OPTION_MAP_UNIT_FN,
53 "using `option.map(f)`, where `f` is a function or closure that returns `()`"
56 declare_clippy_lint! {
58 /// Checks for usage of `result.map(f)` where f is a function
59 /// or closure that returns the unit type `()`.
61 /// ### Why is this bad?
62 /// Readability, this can be written more clearly with
63 /// an if let statement
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)));
76 /// The correct use would be:
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 {
86 /// # let x: Result<String, String> = do_stuff();
87 /// if let Ok(msg) = x {
88 /// log_err_msg(format_msg(msg));
91 #[clippy::version = "pre 1.29.0"]
92 pub RESULT_MAP_UNIT_FN,
94 "using `result.map(f)`, where `f` is a function or closure that returns `()`"
97 declare_lint_pass!(MapUnit => [OPTION_MAP_UNIT_FN, RESULT_MAP_UNIT_FN]);
99 fn is_unit_type(ty: Ty<'_>) -> bool {
100 ty.is_unit() || ty.is_never()
103 fn is_unit_function(cx: &LateContext<'_>, expr: &hir::Expr<'_>) -> bool {
104 let ty = cx.typeck_results().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.typeck_results().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(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(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,
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<'tcx>(
164 cx: &LateContext<'tcx>,
165 expr: &hir::Expr<'_>,
166 ) -> Option<(&'tcx hir::Param<'tcx>, &'tcx hir::Expr<'tcx>)> {
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();
175 return Some((binding, body_expr));
181 /// Builds a name for the let binding variable (`var_arg`)
183 /// `x.field` => `x_field`
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(),
196 fn suggestion_msg(function_type: &str, map_type: &str) -> String {
198 "called `map(f)` on an `{0}` value where `f` is a {1} that returns the unit type `()`",
199 map_type, function_type
203 fn lint_map_unit_fn(cx: &LateContext<'_>, stmt: &hir::Stmt<'_>, expr: &hir::Expr<'_>, map_args: &[hir::Expr<'_>]) {
204 let var_arg = &map_args[0];
206 let (map_type, variant, lint) = if is_type_diagnostic_item(cx, cx.typeck_results().expr_ty(var_arg), sym::Option) {
207 ("Option", "Some", OPTION_MAP_UNIT_FN)
208 } else if is_type_diagnostic_item(cx, cx.typeck_results().expr_ty(var_arg), sym::Result) {
209 ("Result", "Ok", RESULT_MAP_UNIT_FN)
213 let fn_arg = &map_args[1];
215 if is_unit_function(cx, fn_arg) {
216 let mut applicability = Applicability::MachineApplicable;
217 let msg = suggestion_msg("function", map_type);
218 let suggestion = format!(
219 "if let {0}({binding}) = {1} {{ {2}({binding}) }}",
221 snippet_with_applicability(cx, var_arg.span, "_", &mut applicability),
222 snippet_with_applicability(cx, fn_arg.span, "_", &mut applicability),
223 binding = let_binding_name(cx, var_arg)
226 span_lint_and_then(cx, lint, expr.span, &msg, |diag| {
227 diag.span_suggestion(stmt.span, "try this", suggestion, applicability);
229 } else if let Some((binding, closure_expr)) = unit_closure(cx, fn_arg) {
230 let msg = suggestion_msg("closure", map_type);
232 span_lint_and_then(cx, lint, expr.span, &msg, |diag| {
233 if let Some(reduced_expr_span) = reduce_unit_expression(cx, closure_expr) {
234 let mut applicability = Applicability::MachineApplicable;
235 let suggestion = format!(
236 "if let {0}({1}) = {2} {{ {3} }}",
238 snippet_with_applicability(cx, binding.pat.span, "_", &mut applicability),
239 snippet_with_applicability(cx, var_arg.span, "_", &mut applicability),
240 snippet_with_context(cx, reduced_expr_span, var_arg.span.ctxt(), "_", &mut applicability).0,
242 diag.span_suggestion(stmt.span, "try this", suggestion, applicability);
244 let suggestion = format!(
245 "if let {0}({1}) = {2} {{ ... }}",
247 snippet(cx, binding.pat.span, "_"),
248 snippet(cx, var_arg.span, "_"),
250 diag.span_suggestion(stmt.span, "try this", suggestion, Applicability::HasPlaceholders);
256 impl<'tcx> LateLintPass<'tcx> for MapUnit {
257 fn check_stmt(&mut self, cx: &LateContext<'_>, stmt: &hir::Stmt<'_>) {
258 if stmt.span.from_expansion() {
262 if let hir::StmtKind::Semi(expr) = stmt.kind {
263 if let Some(arglists) = method_chain_args(expr, &["map"]) {
264 lint_map_unit_fn(cx, stmt, expr, arglists[0]);