rust/clippy_lints/src/map_unit_fn.rs

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use crate::utils::{is_type_diagnostic_item, iter_input_pats, method_chain_args, snippet, span_lint_and_then};
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use if_chain::if_chain;
use rustc_errors::Applicability;
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use rustc_hir as hir;
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use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty::{self, Ty};
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use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::source_map::Span;
declare_clippy_lint! {
/// **What it does:** Checks for usage of `option.map(f)` where f is a function
/// or closure that returns the unit type.
///
/// **Why is this bad?** Readability, this can be written more clearly with
/// an if let statement
///
/// **Known problems:** None.
///
/// **Example:**
///
/// ```rust
/// # fn do_stuff() -> Option<String> { Some(String::new()) }
/// # fn log_err_msg(foo: String) -> Option<String> { Some(foo) }
/// # fn format_msg(foo: String) -> String { String::new() }
/// let x: Option<String> = do_stuff();
/// x.map(log_err_msg);
/// # let x: Option<String> = do_stuff();
/// x.map(|msg| log_err_msg(format_msg(msg)));
/// ```
///
/// The correct use would be:
///
/// ```rust
/// # fn do_stuff() -> Option<String> { Some(String::new()) }
/// # fn log_err_msg(foo: String) -> Option<String> { Some(foo) }
/// # fn format_msg(foo: String) -> String { String::new() }
/// let x: Option<String> = do_stuff();
/// if let Some(msg) = x {
/// log_err_msg(msg);
/// }
///
/// # let x: Option<String> = do_stuff();
/// if let Some(msg) = x {
/// log_err_msg(format_msg(msg));
/// }
/// ```
pub OPTION_MAP_UNIT_FN,
complexity,
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"using `option.map(f)`, where `f` is a function or closure that returns `()`"
}
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declare_clippy_lint! {
/// **What it does:** Checks for usage of `result.map(f)` where f is a function
/// or closure that returns the unit type.
///
/// **Why is this bad?** Readability, this can be written more clearly with
/// an if let statement
///
/// **Known problems:** None.
///
/// **Example:**
///
/// ```rust
/// # fn do_stuff() -> Result<String, String> { Ok(String::new()) }
/// # fn log_err_msg(foo: String) -> Result<String, String> { Ok(foo) }
/// # fn format_msg(foo: String) -> String { String::new() }
/// let x: Result<String, String> = do_stuff();
/// x.map(log_err_msg);
/// # let x: Result<String, String> = do_stuff();
/// x.map(|msg| log_err_msg(format_msg(msg)));
/// ```
///
/// The correct use would be:
///
/// ```rust
/// # fn do_stuff() -> Result<String, String> { Ok(String::new()) }
/// # fn log_err_msg(foo: String) -> Result<String, String> { Ok(foo) }
/// # fn format_msg(foo: String) -> String { String::new() }
/// let x: Result<String, String> = do_stuff();
/// if let Ok(msg) = x {
/// log_err_msg(msg);
/// };
/// # let x: Result<String, String> = do_stuff();
/// if let Ok(msg) = x {
/// log_err_msg(format_msg(msg));
/// };
/// ```
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pub RESULT_MAP_UNIT_FN,
complexity,
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"using `result.map(f)`, where `f` is a function or closure that returns `()`"
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}
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declare_lint_pass!(MapUnit => [OPTION_MAP_UNIT_FN, RESULT_MAP_UNIT_FN]);
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fn is_unit_type(ty: Ty<'_>) -> bool {
match ty.kind() {
ty::Tuple(slice) => slice.is_empty(),
ty::Never => true,
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_ => false,
}
}
fn is_unit_function(cx: &LateContext<'_>, expr: &hir::Expr<'_>) -> bool {
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let ty = cx.typeck_results().expr_ty(expr);
if let ty::FnDef(id, _) = *ty.kind() {
if let Some(fn_type) = cx.tcx.fn_sig(id).no_bound_vars() {
return is_unit_type(fn_type.output());
}
}
false
}
fn is_unit_expression(cx: &LateContext<'_>, expr: &hir::Expr<'_>) -> bool {
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is_unit_type(cx.typeck_results().expr_ty(expr))
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}
/// The expression inside a closure may or may not have surrounding braces and
/// semicolons, which causes problems when generating a suggestion. Given an
/// expression that evaluates to '()' or '!', recursively remove useless braces
/// and semi-colons until is suitable for including in the suggestion template
fn reduce_unit_expression<'a>(cx: &LateContext<'_>, expr: &'a hir::Expr<'_>) -> Option<Span> {
if !is_unit_expression(cx, expr) {
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return None;
}
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match expr.kind {
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hir::ExprKind::Call(_, _) | hir::ExprKind::MethodCall(_, _, _, _) => {
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// Calls can't be reduced any more
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Some(expr.span)
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},
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hir::ExprKind::Block(ref block, _) => {
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match (&block.stmts[..], block.expr.as_ref()) {
(&[], Some(inner_expr)) => {
// If block only contains an expression,
// reduce `{ X }` to `X`
reduce_unit_expression(cx, inner_expr)
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},
(&[ref inner_stmt], None) => {
// If block only contains statements,
// reduce `{ X; }` to `X` or `X;`
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match inner_stmt.kind {
hir::StmtKind::Local(ref local) => Some(local.span),
hir::StmtKind::Expr(ref e) => Some(e.span),
hir::StmtKind::Semi(..) => Some(inner_stmt.span),
hir::StmtKind::Item(..) => None,
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}
},
_ => {
// For closures that contain multiple statements
// it's difficult to get a correct suggestion span
// for all cases (multi-line closures specifically)
//
// We do not attempt to build a suggestion for those right now.
None
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},
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}
},
_ => None,
}
}
fn unit_closure<'tcx>(
cx: &LateContext<'tcx>,
expr: &hir::Expr<'_>,
) -> Option<(&'tcx hir::Param<'tcx>, &'tcx hir::Expr<'tcx>)> {
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if let hir::ExprKind::Closure(_, ref decl, inner_expr_id, _, _) = expr.kind {
let body = cx.tcx.hir().body(inner_expr_id);
let body_expr = &body.value;
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if_chain! {
if decl.inputs.len() == 1;
if is_unit_expression(cx, body_expr);
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if let Some(binding) = iter_input_pats(&decl, body).next();
then {
return Some((binding, body_expr));
}
}
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}
None
}
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/// Builds a name for the let binding variable (`var_arg`)
///
/// `x.field` => `x_field`
/// `y` => `_y`
///
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/// Anything else will return `a`.
fn let_binding_name(cx: &LateContext<'_>, var_arg: &hir::Expr<'_>) -> String {
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match &var_arg.kind {
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hir::ExprKind::Field(_, _) => snippet(cx, var_arg.span, "_").replace(".", "_"),
hir::ExprKind::Path(_) => format!("_{}", snippet(cx, var_arg.span, "")),
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_ => "a".to_string(),
}
}
#[must_use]
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fn suggestion_msg(function_type: &str, map_type: &str) -> String {
format!(
"called `map(f)` on an `{0}` value where `f` is a {1} that returns the unit type",
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map_type, function_type
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)
}
fn lint_map_unit_fn(cx: &LateContext<'_>, stmt: &hir::Stmt<'_>, expr: &hir::Expr<'_>, map_args: &[hir::Expr<'_>]) {
let var_arg = &map_args[0];
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let (map_type, variant, lint) =
if is_type_diagnostic_item(cx, cx.typeck_results().expr_ty(var_arg), sym!(option_type)) {
("Option", "Some", OPTION_MAP_UNIT_FN)
} else if is_type_diagnostic_item(cx, cx.typeck_results().expr_ty(var_arg), sym!(result_type)) {
("Result", "Ok", RESULT_MAP_UNIT_FN)
} else {
return;
};
let fn_arg = &map_args[1];
if is_unit_function(cx, fn_arg) {
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let msg = suggestion_msg("function", map_type);
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let suggestion = format!(
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"if let {0}({binding}) = {1} {{ {2}({binding}) }}",
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variant,
snippet(cx, var_arg.span, "_"),
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snippet(cx, fn_arg.span, "_"),
binding = let_binding_name(cx, var_arg)
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);
span_lint_and_then(cx, lint, expr.span, &msg, |diag| {
diag.span_suggestion(stmt.span, "try this", suggestion, Applicability::MachineApplicable);
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});
} else if let Some((binding, closure_expr)) = unit_closure(cx, fn_arg) {
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let msg = suggestion_msg("closure", map_type);
span_lint_and_then(cx, lint, expr.span, &msg, |diag| {
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if let Some(reduced_expr_span) = reduce_unit_expression(cx, closure_expr) {
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let suggestion = format!(
"if let {0}({1}) = {2} {{ {3} }}",
variant,
snippet(cx, binding.pat.span, "_"),
snippet(cx, var_arg.span, "_"),
snippet(cx, reduced_expr_span, "_")
);
diag.span_suggestion(
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stmt.span,
"try this",
suggestion,
Applicability::MachineApplicable, // snippet
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);
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} else {
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let suggestion = format!(
"if let {0}({1}) = {2} {{ ... }}",
variant,
snippet(cx, binding.pat.span, "_"),
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snippet(cx, var_arg.span, "_"),
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);
diag.span_suggestion(stmt.span, "try this", suggestion, Applicability::HasPlaceholders);
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}
});
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}
}
impl<'tcx> LateLintPass<'tcx> for MapUnit {
fn check_stmt(&mut self, cx: &LateContext<'_>, stmt: &hir::Stmt<'_>) {
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if stmt.span.from_expansion() {
return;
}
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if let hir::StmtKind::Semi(ref expr) = stmt.kind {
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if let Some(arglists) = method_chain_args(expr, &["map"]) {
lint_map_unit_fn(cx, stmt, expr, arglists[0]);
}
}
}
}