rust-clippy/clippy_lints/src/map_identity.rs

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use crate::utils::{
is_adjusted, is_type_diagnostic_item, match_path, match_trait_method, match_var, paths, remove_blocks,
span_lint_and_sugg,
};
use if_chain::if_chain;
use rustc_errors::Applicability;
use rustc_hir::{Body, Expr, ExprKind, Pat, PatKind, QPath, StmtKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::{declare_lint_pass, declare_tool_lint};
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use rustc_span::sym;
declare_clippy_lint! {
/// **What it does:** Checks for instances of `map(f)` where `f` is the identity function.
///
/// **Why is this bad?** It can be written more concisely without the call to `map`.
///
/// **Known problems:** None.
///
/// **Example:**
///
/// ```rust
/// let x = [1, 2, 3];
/// let y: Vec<_> = x.iter().map(|x| x).map(|x| 2*x).collect();
/// ```
/// Use instead:
/// ```rust
/// let x = [1, 2, 3];
/// let y: Vec<_> = x.iter().map(|x| 2*x).collect();
/// ```
pub MAP_IDENTITY,
complexity,
"using iterator.map(|x| x)"
}
declare_lint_pass!(MapIdentity => [MAP_IDENTITY]);
impl<'tcx> LateLintPass<'tcx> for MapIdentity {
fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
if expr.span.from_expansion() {
return;
}
if_chain! {
if let Some([caller, func]) = get_map_argument(cx, expr);
if is_expr_identity_function(cx, func);
then {
span_lint_and_sugg(
cx,
MAP_IDENTITY,
expr.span.trim_start(caller.span).unwrap(),
"unnecessary map of the identity function",
"remove the call to `map`",
String::new(),
Applicability::MachineApplicable
)
}
}
}
}
/// Returns the arguments passed into map() if the expression is a method call to
/// map(). Otherwise, returns None.
fn get_map_argument<'a>(cx: &LateContext<'_>, expr: &'a Expr<'a>) -> Option<&'a [Expr<'a>]> {
if_chain! {
if let ExprKind::MethodCall(ref method, _, ref args, _) = expr.kind;
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if args.len() == 2 && method.ident.name == sym::map;
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let caller_ty = cx.typeck_results().expr_ty(&args[0]);
if match_trait_method(cx, expr, &paths::ITERATOR)
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|| is_type_diagnostic_item(cx, caller_ty, sym::result_type)
|| is_type_diagnostic_item(cx, caller_ty, sym::option_type);
then {
Some(args)
} else {
None
}
}
}
/// Checks if an expression represents the identity function
/// Only examines closures and `std::convert::identity`
fn is_expr_identity_function(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
match expr.kind {
ExprKind::Closure(_, _, body_id, _, _) => is_body_identity_function(cx, cx.tcx.hir().body(body_id)),
ExprKind::Path(QPath::Resolved(_, ref path)) => match_path(path, &paths::STD_CONVERT_IDENTITY),
_ => false,
}
}
/// Checks if a function's body represents the identity function
/// Looks for bodies of the form `|x| x`, `|x| return x`, `|x| { return x }` or `|x| {
/// return x; }`
fn is_body_identity_function(cx: &LateContext<'_>, func: &Body<'_>) -> bool {
let params = func.params;
let body = remove_blocks(&func.value);
// if there's less/more than one parameter, then it is not the identity function
if params.len() != 1 {
return false;
}
match body.kind {
ExprKind::Path(QPath::Resolved(None, _)) => match_expr_param(cx, body, params[0].pat),
ExprKind::Ret(Some(ref ret_val)) => match_expr_param(cx, ret_val, params[0].pat),
ExprKind::Block(ref block, _) => {
if_chain! {
if block.stmts.len() == 1;
if let StmtKind::Semi(ref expr) | StmtKind::Expr(ref expr) = block.stmts[0].kind;
if let ExprKind::Ret(Some(ref ret_val)) = expr.kind;
then {
match_expr_param(cx, ret_val, params[0].pat)
} else {
false
}
}
},
_ => false,
}
}
/// Returns true iff an expression returns the same thing as a parameter's pattern
fn match_expr_param(cx: &LateContext<'_>, expr: &Expr<'_>, pat: &Pat<'_>) -> bool {
if let PatKind::Binding(_, _, ident, _) = pat.kind {
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match_var(expr, ident.name) && !(cx.typeck_results().hir_owner == expr.hir_id.owner && is_adjusted(cx, expr))
} else {
false
}
}