1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
|
use crate::lints::UnitBindingsDiag;
use crate::{LateLintPass, LintContext};
use rustc_hir as hir;
use rustc_middle::ty::Ty;
declare_lint! {
/// The `unit_bindings` lint detects cases where bindings are useless because they have
/// the unit type `()` as their inferred type. The lint is suppressed if the user explicitly
/// annotates the let binding with the unit type `()`, or if the let binding uses an underscore
/// wildcard pattern, i.e. `let _ = expr`, or if the binding is produced from macro expansions.
///
/// ### Example
///
/// ```rust,compile_fail
/// #![deny(unit_bindings)]
///
/// fn foo() {
/// println!("do work");
/// }
///
/// pub fn main() {
/// let x = foo(); // useless binding
/// }
/// ```
///
/// {{produces}}
///
/// ### Explanation
///
/// Creating a local binding with the unit type `()` does not do much and can be a sign of a
/// user error, such as in this example:
///
/// ```rust,no_run
/// fn main() {
/// let mut x = [1, 2, 3];
/// x[0] = 5;
/// let y = x.sort(); // useless binding as `sort` returns `()` and not the sorted array.
/// println!("{:?}", y); // prints "()"
/// }
/// ```
pub UNIT_BINDINGS,
Allow,
"binding is useless because it has the unit `()` type"
}
declare_lint_pass!(UnitBindings => [UNIT_BINDINGS]);
impl<'tcx> LateLintPass<'tcx> for UnitBindings {
fn check_local(&mut self, cx: &crate::LateContext<'tcx>, local: &'tcx hir::Local<'tcx>) {
// Suppress warning if user:
// - explicitly ascribes a type to the pattern
// - explicitly wrote `let pat = ();`
// - explicitly wrote `let () = init;`.
if !local.span.from_expansion()
&& let Some(tyck_results) = cx.maybe_typeck_results()
&& let Some(init) = local.init
&& let init_ty = tyck_results.expr_ty(init)
&& let local_ty = tyck_results.node_type(local.hir_id)
&& init_ty == Ty::new_unit(cx.tcx)
&& local_ty == Ty::new_unit(cx.tcx)
&& local.ty.is_none()
&& !matches!(init.kind, hir::ExprKind::Tup([]))
&& !matches!(local.pat.kind, hir::PatKind::Tuple([], ..))
{
cx.emit_spanned_lint(
UNIT_BINDINGS,
local.span,
UnitBindingsDiag { label: local.pat.span },
);
}
}
}
|
