cargo/util/context/
value.rs

1//! Deserialization of a [`Value<T>`] type which tracks where it was deserialized from.
2//!
3//! ## Rationale for `Value<T>`
4//!
5//! Often Cargo wants to report semantic error information or other sorts of
6//! error information about configuration keys but it also may wish to indicate
7//! as an error context where the key was defined as well (to help user
8//! debugging). The `Value<T>` type here can be used to deserialize a `T` value
9//! from configuration, but also record where it was deserialized from when it
10//! was read.
11//!
12//! Deserializing `Value<T>` is pretty special, and serde doesn't have built-in
13//! support for this operation. To implement this we extend serde's "data model"
14//! a bit. We configure deserialization of `Value<T>` to basically only work with
15//! our one deserializer using configuration.
16//!
17//! ## How `Value<T>` deserialization works
18//!
19//! `Value<T>` uses a custom protocol to inject source location information
20//! into serde's deserialization process:
21//!
22//! **Magic identifiers**: `Value<T>::deserialize` requests a struct with special
23//! [name](NAME) and [field names](FIELDS) that use invalid Rust syntax to avoid
24//! conflicts. This signals to Cargo's deserializer that location tracking is needed.
25//!
26//! **Custom deserializer response**: When Cargo's deserializer sees these magic
27//! identifiers, it switches to `ValueDeserializer` (from the [`de`] module)
28//! instead of normal struct deserialization.
29//!
30//! **Two-field protocol**: `ValueDeserializer` presents exactly two fields
31//! through map visiting:
32//! * The actual value (deserialized normally)
33//! * The definition context (encoded as a `(u32, String)` tuple acting as a
34//!   tagged union of [`Definition`] variants)
35//!
36//! This allows `Value<T>` to capture both the deserialized data and where it
37//! came from.
38//!
39//! **Note**: When modifying [`Definition`] variants, be sure to update both
40//! the `Definition::deserialize` implementation here and the
41//! `MapAccess::next_value_seed` implementation in `ValueDeserializer`.
42//!
43//! [`de`]: crate::util::context::de
44
45use serde::de;
46use std::cmp::Ordering;
47use std::fmt;
48use std::marker;
49use std::mem;
50use std::path::{Path, PathBuf};
51
52/// A type which can be deserialized as a configuration value which records
53/// where it was deserialized from.
54#[derive(Debug, PartialEq, Clone)]
55pub struct Value<T> {
56    /// The inner value that was deserialized.
57    pub val: T,
58    /// The location where `val` was defined in configuration (e.g. file it was
59    /// defined in, env var etc).
60    pub definition: Definition,
61}
62
63pub type OptValue<T> = Option<Value<T>>;
64
65// The names below are intended to be invalid Rust identifiers
66// to avoid conflicts with other valid structures.
67pub(crate) const VALUE_FIELD: &str = "$__cargo_private_value";
68pub(crate) const DEFINITION_FIELD: &str = "$__cargo_private_definition";
69pub(crate) const NAME: &str = "$__cargo_private_Value";
70pub(crate) static FIELDS: [&str; 2] = [VALUE_FIELD, DEFINITION_FIELD];
71
72/// Location where a config value is defined.
73#[derive(Clone, Debug, Eq)]
74pub enum Definition {
75    BuiltIn,
76    /// Defined in a `.cargo/config`, includes the path to the file.
77    Path(PathBuf),
78    /// Defined in an environment variable, includes the environment key.
79    Environment(String),
80    /// Passed in on the command line.
81    /// A path is attached when the config value is a path to a config file.
82    Cli(Option<PathBuf>),
83}
84
85impl PartialOrd for Definition {
86    fn partial_cmp(&self, other: &Definition) -> Option<Ordering> {
87        Some(self.cmp(other))
88    }
89}
90
91impl Ord for Definition {
92    fn cmp(&self, other: &Definition) -> Ordering {
93        if mem::discriminant(self) == mem::discriminant(other) {
94            Ordering::Equal
95        } else if self.is_higher_priority(other) {
96            Ordering::Greater
97        } else {
98            Ordering::Less
99        }
100    }
101}
102
103impl Definition {
104    /// Root directory where this is defined.
105    ///
106    /// If from a file, it is the directory above `.cargo/config.toml`.
107    /// CLI and env use the provided current working directory.
108    pub fn root<'a>(&'a self, cwd: &'a Path) -> &'a Path {
109        match self {
110            Definition::Path(p) | Definition::Cli(Some(p)) => p.parent().unwrap().parent().unwrap(),
111            Definition::Environment(_) | Definition::Cli(None) | Definition::BuiltIn => cwd,
112        }
113    }
114
115    /// Returns true if self is a higher priority to other.
116    ///
117    /// CLI is preferred over environment, which is preferred over files.
118    pub fn is_higher_priority(&self, other: &Definition) -> bool {
119        matches!(
120            (self, other),
121            (Definition::Cli(_), Definition::Environment(_))
122                | (Definition::Cli(_), Definition::Path(_))
123                | (Definition::Cli(_), Definition::BuiltIn)
124                | (Definition::Environment(_), Definition::Path(_))
125                | (Definition::Environment(_), Definition::BuiltIn)
126                | (Definition::Path(_), Definition::BuiltIn)
127        )
128    }
129}
130
131impl PartialEq for Definition {
132    fn eq(&self, other: &Definition) -> bool {
133        // configuration values are equivalent no matter where they're defined,
134        // but they need to be defined in the same location. For example if
135        // they're defined in the environment that's different than being
136        // defined in a file due to path interpretations.
137        mem::discriminant(self) == mem::discriminant(other)
138    }
139}
140
141impl fmt::Display for Definition {
142    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
143        match self {
144            Definition::Path(p) | Definition::Cli(Some(p)) => p.display().fmt(f),
145            Definition::Environment(key) => write!(f, "environment variable `{}`", key),
146            Definition::Cli(None) => write!(f, "--config cli option"),
147            Definition::BuiltIn => write!(f, "default"),
148        }
149    }
150}
151
152impl<T> From<T> for Value<T> {
153    fn from(val: T) -> Self {
154        Self {
155            val,
156            definition: Definition::BuiltIn,
157        }
158    }
159}
160
161impl<'de, T> de::Deserialize<'de> for Value<T>
162where
163    T: de::Deserialize<'de>,
164{
165    fn deserialize<D>(deserializer: D) -> Result<Value<T>, D::Error>
166    where
167        D: de::Deserializer<'de>,
168    {
169        struct ValueVisitor<T> {
170            _marker: marker::PhantomData<T>,
171        }
172
173        impl<'de, T> de::Visitor<'de> for ValueVisitor<T>
174        where
175            T: de::Deserialize<'de>,
176        {
177            type Value = Value<T>;
178
179            fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
180                formatter.write_str("a value")
181            }
182
183            fn visit_map<V>(self, mut visitor: V) -> Result<Value<T>, V::Error>
184            where
185                V: de::MapAccess<'de>,
186            {
187                let value = visitor.next_key::<ValueKey>()?;
188                if value.is_none() {
189                    return Err(de::Error::custom("value not found"));
190                }
191                let val: T = visitor.next_value()?;
192
193                let definition = visitor.next_key::<DefinitionKey>()?;
194                if definition.is_none() {
195                    return Err(de::Error::custom("definition not found"));
196                }
197                let definition: Definition = visitor.next_value()?;
198                Ok(Value { val, definition })
199            }
200        }
201
202        deserializer.deserialize_struct(
203            NAME,
204            &FIELDS,
205            ValueVisitor {
206                _marker: marker::PhantomData,
207            },
208        )
209    }
210}
211
212struct FieldVisitor {
213    expected: &'static str,
214}
215
216impl<'de> de::Visitor<'de> for FieldVisitor {
217    type Value = ();
218
219    fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
220        formatter.write_str("a valid value field")
221    }
222
223    fn visit_str<E>(self, s: &str) -> Result<(), E>
224    where
225        E: de::Error,
226    {
227        if s == self.expected {
228            Ok(())
229        } else {
230            Err(de::Error::custom("expected field with custom name"))
231        }
232    }
233}
234
235struct ValueKey;
236
237impl<'de> de::Deserialize<'de> for ValueKey {
238    fn deserialize<D>(deserializer: D) -> Result<ValueKey, D::Error>
239    where
240        D: de::Deserializer<'de>,
241    {
242        deserializer.deserialize_identifier(FieldVisitor {
243            expected: VALUE_FIELD,
244        })?;
245        Ok(ValueKey)
246    }
247}
248
249struct DefinitionKey;
250
251impl<'de> de::Deserialize<'de> for DefinitionKey {
252    fn deserialize<D>(deserializer: D) -> Result<DefinitionKey, D::Error>
253    where
254        D: de::Deserializer<'de>,
255    {
256        deserializer.deserialize_identifier(FieldVisitor {
257            expected: DEFINITION_FIELD,
258        })?;
259        Ok(DefinitionKey)
260    }
261}
262
263impl<'de> de::Deserialize<'de> for Definition {
264    fn deserialize<D>(deserializer: D) -> Result<Definition, D::Error>
265    where
266        D: de::Deserializer<'de>,
267    {
268        let (discr, value) = <(u32, String)>::deserialize(deserializer)?;
269        match discr {
270            0 => Ok(Definition::BuiltIn),
271            1 => Ok(Definition::Path(value.into())),
272            2 => Ok(Definition::Environment(value)),
273            3 => {
274                let path = (value.len() > 0).then_some(value.into());
275                Ok(Definition::Cli(path))
276            }
277            _ => panic!("unexpected discriminant {discr} value {value}"),
278        }
279    }
280}
cargo/util/context/value.rs

cargo/util/context/
value.rs
