tor_memquota/
if_enabled.rs

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1//! Helper type for disabling memory tracking when not wanted
2
3use crate::internal_prelude::*;
4
5/// Token indicating that memory quota tracking is enabled, at both compile and runtime
6///
7/// If support is compiled in this is a unit.
8///
9/// If the `memquota` cargo feature is not enabled, this type is uninhabited.
10/// Scattering values of this type around in relevant data structures
11/// and parameters lists
12/// allows the compiler to eliminate the unwanted code.
13#[derive(Clone, Copy, Debug, PartialEq)]
14pub struct EnabledToken {
15    /// Make non-exhaustive even within the crate
16    _hidden: (),
17
18    /// Uninhabited if the feature isn't enabled.
19    #[cfg(not(feature = "memquota"))]
20    _forbid: Void,
21}
22
23impl Eq for EnabledToken {}
24
25impl EnabledToken {
26    /// Obtain an `EnabledToken` (only available if tracking is compiled in)
27    #[allow(clippy::new_without_default)] // a conditional Default impl would be rather odd
28    #[cfg(feature = "memquota")]
29    pub const fn new() -> Self {
30        EnabledToken { _hidden: () }
31    }
32
33    /// Obtain an `EnabledToken` if memory-tracking is compiled in, or `None` otherwise
34    #[allow(clippy::unnecessary_wraps)] // Will be None if compiled out
35    #[allow(unreachable_code)]
36    pub const fn new_if_compiled_in() -> Option<Self> {
37        Some(EnabledToken {
38            _hidden: (),
39
40            #[cfg(not(feature = "memquota"))]
41            _forbid: return None,
42        })
43    }
44}
45
46/// Either `T`, if we're enabled, or nothing if we're no-op
47///
48/// Used for runtime control of whether the memory quota is enabled:
49/// we support explicitly creating a no-op tracker
50/// with [`MemoryQuotaTracker::new_noop`](crate::MemoryQuotaTracker::new_noop).
51///
52/// We use this rather than just `Option` because we also have data structures
53/// (trackers, `Account`s and so on)
54/// which have been torn down, or are "dummy" or "dangling",
55/// which are supposed to return errors rather than no-op successes.
56#[derive(Clone, Debug, Eq, PartialEq)]
57pub(crate) enum IfEnabled<T> {
58    /// We're enabled, and supposed to be tracking memory
59    ///
60    /// The 2nd member causes this variant to prove that tracking is enabled.
61    /// If tracking is disabled at compile time, this variant is uninhabited
62    /// and the whole `IfEnabled` becomes a unit.
63    Enabled(T, EnabledToken),
64
65    /// We're inenabled and everything should be a lightweight no-op
66    Noop,
67}
68
69use IfEnabled::*;
70
71impl<T> IfEnabled<T> {
72    /// Convert to `Option`: return `Some` if this is `Enabled`
73    pub(crate) fn into_enabled(self) -> Option<T> {
74        match self {
75            Enabled(y, _e) => Some(y),
76            Noop => None,
77        }
78    }
79
80    /// Take reference; analogous to `Option::as_ref`
81    pub(crate) fn as_ref(&self) -> IfEnabled<&T> {
82        match self {
83            Enabled(y, e) => Enabled(y, *e),
84            Noop => Noop,
85        }
86    }
87
88    /// Take reference and convert to `Option`
89    ///
90    /// Convenience helper equivalent to `.as_ref().into_enabled()`.
91    pub(crate) fn as_enabled(&self) -> Option<&T> {
92        self.as_ref().into_enabled()
93    }
94
95    /// Return the contents of the `Enabled`, or declare it a [`Bug`]
96    #[track_caller]
97    pub(crate) fn enabled_or_bug(self) -> Result<T, Bug> {
98        match self {
99            Enabled(y, _e) => Ok(y),
100            Noop => Err(internal!("IfEnabled unexpectedly Noop")),
101        }
102    }
103}
tor_memquota/if_enabled.rs

tor_memquota/
if_enabled.rs
