bevy_lint/lints/nursery/
zst_query.rs

1//! Checks for queries that query the data for a zero-sized type.
2//!
3//! # Motivation
4//!
5//! Zero-sized types (ZSTs) are types that have no size because they contain no runtime data. Any
6//! information they may hold is known at compile-time in the form of [constant generics], which do
7//! not need to be queried. As such, ZSTs are better used as query filters instead of query data.
8//!
9//! [constant generics]: https://doc.rust-lang.org/reference/items/generics.html#const-generics
10//!
11//! # Known Issues
12//!
13//! This lint raises false positives on queries like `Has<T>` and `AnyOf<T>` because they are ZSTs,
14//! even though they still retrieve data from the ECS. Please see [#279] for more information.
15//!
16//! [#279]: https://github.com/TheBevyFlock/bevy_cli/issues/279
17//!
18//! # Example
19//!
20//! ```
21//! # use bevy::prelude::*;
22//! #
23//! // This is a zero-sized type, sometimes known as a "marker component".
24//! #[derive(Component)]
25//! struct Player;
26//!
27//! fn move_player(mut query: Query<(&mut Transform, &Player)>) {
28//!     for (transform, _) in query.iter_mut() {
29//!         // ...
30//!     }
31//! }
32//! #
33//! # assert_eq!(std::mem::size_of::<Player>(), 0);
34//! ```
35//!
36//! Use instead:
37//!
38//! ```
39//! # use bevy::prelude::*;
40//! #
41//! #[derive(Component)]
42//! struct Player;
43//!
44//! fn move_player(mut query: Query<&mut Transform, With<Player>>) {
45//!     for transform in query.iter_mut() {
46//!         // ...
47//!     }
48//! }
49//! #
50//! # assert_eq!(std::mem::size_of::<Player>(), 0);
51//! ```
52
53use crate::{
54    declare_bevy_lint, declare_bevy_lint_pass,
55    utils::hir_parse::{detuple, generic_type_at},
56};
57use clippy_utils::{
58    diagnostics::span_lint_and_help,
59    ty::{is_normalizable, match_type, ty_from_hir_ty},
60};
61use rustc_abi::Size;
62use rustc_hir::AmbigArg;
63use rustc_lint::{LateContext, LateLintPass};
64use rustc_middle::ty::{
65    Ty,
66    layout::{LayoutOf, TyAndLayout},
67};
68
69declare_bevy_lint! {
70    pub ZST_QUERY,
71    // This will eventually be a `RESTRICTION` lint, but due to
72    // <https://github.com/TheBevyFlock/bevy_cli/issues/279> it is not yet ready for production.
73    super::NURSERY,
74    "queried a zero-sized type",
75}
76
77declare_bevy_lint_pass! {
78    pub ZstQuery => [ZST_QUERY.lint],
79}
80
81impl<'tcx> LateLintPass<'tcx> for ZstQuery {
82    fn check_ty(&mut self, cx: &LateContext<'tcx>, hir_ty: &'tcx rustc_hir::Ty<'tcx, AmbigArg>) {
83        if hir_ty.span.in_external_macro(cx.tcx.sess.source_map()) {
84            return;
85        }
86        let ty = ty_from_hir_ty(cx, hir_ty.as_unambig_ty());
87
88        let Some(query_kind) = QueryKind::try_from_ty(cx, ty) else {
89            return;
90        };
91
92        let Some(query_data_ty) = generic_type_at(cx, hir_ty.as_unambig_ty(), 2) else {
93            return;
94        };
95
96        for hir_ty in detuple(*query_data_ty) {
97            let ty = ty_from_hir_ty(cx, &hir_ty);
98
99            // We want to make sure we're evaluating `Foo` and not `&Foo`/`&mut Foo`
100            let peeled = ty.peel_refs();
101
102            if !is_zero_sized(cx, peeled).unwrap_or_default() {
103                continue;
104            }
105
106            // TODO: We can also special case `Option<&Foo>`/`Option<&mut Foo>` to
107            //       instead suggest `Has<Foo>`
108            span_lint_and_help(
109                cx,
110                ZST_QUERY.lint,
111                hir_ty.span,
112                ZST_QUERY.lint.desc,
113                None,
114                query_kind.help(peeled),
115            );
116        }
117    }
118}
119
120enum QueryKind {
121    Query,
122}
123
124impl QueryKind {
125    fn try_from_ty<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> Option<Self> {
126        if match_type(cx, ty, &crate::paths::QUERY) {
127            Some(Self::Query)
128        } else {
129            None
130        }
131    }
132
133    fn help(&self, ty: Ty<'_>) -> String {
134        // It should be noted that `With<Foo>` is not always the best filter to suggest.
135        // While it's most often going to be what users want, there's also `Added<Foo>`
136        // and `Changed<Foo>` which might be more appropriate in some cases
137        // (i.e. users are calling `foo.is_added()` or `foo.is_changed()` in the body of
138        // the system).
139        // In the future, we might want to span the usage site of `is_added()`/`is_changed()`
140        // and suggest to use `Added<Foo>`/`Changed<Foo>` instead.
141        match self {
142            Self::Query => format!(
143                // NOTE: This isn't actually true, please see #279 for more info and how this will
144                // be fixed!
145                "zero-sized types do not retrieve any data, consider using a filter instead: `With<{ty}>`"
146            ),
147        }
148    }
149}
150
151/// Checks if a type is zero-sized.
152///
153/// Returns:
154/// - `Some(true)` if the type is most likely a ZST
155/// - `Some(false)` if the type is most likely not a ZST
156/// - `None` if we cannot determine the size (e.g., type is not normalizable)
157fn is_zero_sized<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> Option<bool> {
158    // `cx.layout_of()` panics if the type is not normalizable.
159    if !is_normalizable(cx, cx.param_env, ty) {
160        return None;
161    }
162
163    // Note: we don't use `approx_ty_size` from `clippy_utils` here
164    // because it will return `0` as the default value if the type is not
165    // normalizable, which will put us at risk of emitting more false positives.
166    if let Ok(TyAndLayout { layout, .. }) = cx.layout_of(ty) {
167        Some(layout.size() == Size::ZERO)
168    } else {
169        None
170    }
171}
bevy_lint/lints/nursery/zst_query.rs

bevy_lint/lints/nursery/
zst_query.rs
