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//! Deserialization of a `Value<T>` type which tracks where it was deserialized
//! from.
//!
//! Often Cargo wants to report semantic error information or other sorts of
//! error information about configuration keys but it also may wish to indicate
//! as an error context where the key was defined as well (to help user
//! debugging). The `Value<T>` type here can be used to deserialize a `T` value
//! from configuration, but also record where it was deserialized from when it
//! was read.
use crate::util::config::Config;
use serde::de;
use std::cmp::Ordering;
use std::fmt;
use std::marker;
use std::mem;
use std::path::{Path, PathBuf};
/// A type which can be deserialized as a configuration value which records
/// where it was deserialized from.
#[derive(Debug, PartialEq, Clone)]
pub struct Value<T> {
/// The inner value that was deserialized.
pub val: T,
/// The location where `val` was defined in configuration (e.g. file it was
/// defined in, env var etc).
pub definition: Definition,
}
pub type OptValue<T> = Option<Value<T>>;
// Deserializing `Value<T>` is pretty special, and serde doesn't have built-in
// support for this operation. To implement this we extend serde's "data model"
// a bit. We configure deserialization of `Value<T>` to basically only work with
// our one deserializer using configuration.
//
// We define that `Value<T>` deserialization asks the deserializer for a very
// special struct name and struct field names. In doing so the deserializer will
// recognize this and synthesize a magical value for the `definition` field when
// we deserialize it. This protocol is how we're able to have a channel of
// information flowing from the configuration deserializer into the
// deserialization implementation here.
//
// You'll want to also check out the implementation of `ValueDeserializer` in
// `de.rs`. Also note that the names below are intended to be invalid Rust
// identifiers to avoid how they might conflict with other valid structures.
// Finally the `definition` field is transmitted as a tuple of i32/string, which
// is effectively a tagged union of `Definition` itself.
pub(crate) const VALUE_FIELD: &str = "$__cargo_private_value";
pub(crate) const DEFINITION_FIELD: &str = "$__cargo_private_definition";
pub(crate) const NAME: &str = "$__cargo_private_Value";
pub(crate) static FIELDS: [&str; 2] = [VALUE_FIELD, DEFINITION_FIELD];
/// Location where a config value is defined.
#[derive(Clone, Debug, Eq)]
pub enum Definition {
/// Defined in a `.cargo/config`, includes the path to the file.
Path(PathBuf),
/// Defined in an environment variable, includes the environment key.
Environment(String),
/// Passed in on the command line.
/// A path is attached when the config value is a path to a config file.
Cli(Option<PathBuf>),
}
impl PartialOrd for Definition {
fn partial_cmp(&self, other: &Definition) -> Option<Ordering> {
Some(self.cmp(other))
}
}
impl Ord for Definition {
fn cmp(&self, other: &Definition) -> Ordering {
if mem::discriminant(self) == mem::discriminant(other) {
Ordering::Equal
} else if self.is_higher_priority(other) {
Ordering::Greater
} else {
Ordering::Less
}
}
}
impl Definition {
/// Root directory where this is defined.
///
/// If from a file, it is the directory above `.cargo/config`.
/// CLI and env are the current working directory.
pub fn root<'a>(&'a self, config: &'a Config) -> &'a Path {
match self {
Definition::Path(p) | Definition::Cli(Some(p)) => p.parent().unwrap().parent().unwrap(),
Definition::Environment(_) | Definition::Cli(None) => config.cwd(),
}
}
/// Returns true if self is a higher priority to other.
///
/// CLI is preferred over environment, which is preferred over files.
pub fn is_higher_priority(&self, other: &Definition) -> bool {
matches!(
(self, other),
(Definition::Cli(_), Definition::Environment(_))
| (Definition::Cli(_), Definition::Path(_))
| (Definition::Environment(_), Definition::Path(_))
)
}
}
impl PartialEq for Definition {
fn eq(&self, other: &Definition) -> bool {
// configuration values are equivalent no matter where they're defined,
// but they need to be defined in the same location. For example if
// they're defined in the environment that's different than being
// defined in a file due to path interpretations.
mem::discriminant(self) == mem::discriminant(other)
}
}
impl fmt::Display for Definition {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match self {
Definition::Path(p) | Definition::Cli(Some(p)) => p.display().fmt(f),
Definition::Environment(key) => write!(f, "environment variable `{}`", key),
Definition::Cli(None) => write!(f, "--config cli option"),
}
}
}
impl<'de, T> de::Deserialize<'de> for Value<T>
where
T: de::Deserialize<'de>,
{
fn deserialize<D>(deserializer: D) -> Result<Value<T>, D::Error>
where
D: de::Deserializer<'de>,
{
struct ValueVisitor<T> {
_marker: marker::PhantomData<T>,
}
impl<'de, T> de::Visitor<'de> for ValueVisitor<T>
where
T: de::Deserialize<'de>,
{
type Value = Value<T>;
fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
formatter.write_str("a value")
}
fn visit_map<V>(self, mut visitor: V) -> Result<Value<T>, V::Error>
where
V: de::MapAccess<'de>,
{
let value = visitor.next_key::<ValueKey>()?;
if value.is_none() {
return Err(de::Error::custom("value not found"));
}
let val: T = visitor.next_value()?;
let definition = visitor.next_key::<DefinitionKey>()?;
if definition.is_none() {
return Err(de::Error::custom("definition not found"));
}
let definition: Definition = visitor.next_value()?;
Ok(Value { val, definition })
}
}
deserializer.deserialize_struct(
NAME,
&FIELDS,
ValueVisitor {
_marker: marker::PhantomData,
},
)
}
}
struct FieldVisitor {
expected: &'static str,
}
impl<'de> de::Visitor<'de> for FieldVisitor {
type Value = ();
fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
formatter.write_str("a valid value field")
}
fn visit_str<E>(self, s: &str) -> Result<(), E>
where
E: de::Error,
{
if s == self.expected {
Ok(())
} else {
Err(de::Error::custom("expected field with custom name"))
}
}
}
struct ValueKey;
impl<'de> de::Deserialize<'de> for ValueKey {
fn deserialize<D>(deserializer: D) -> Result<ValueKey, D::Error>
where
D: de::Deserializer<'de>,
{
deserializer.deserialize_identifier(FieldVisitor {
expected: VALUE_FIELD,
})?;
Ok(ValueKey)
}
}
struct DefinitionKey;
impl<'de> de::Deserialize<'de> for DefinitionKey {
fn deserialize<D>(deserializer: D) -> Result<DefinitionKey, D::Error>
where
D: de::Deserializer<'de>,
{
deserializer.deserialize_identifier(FieldVisitor {
expected: DEFINITION_FIELD,
})?;
Ok(DefinitionKey)
}
}
impl<'de> de::Deserialize<'de> for Definition {
fn deserialize<D>(deserializer: D) -> Result<Definition, D::Error>
where
D: de::Deserializer<'de>,
{
let (discr, value) = <(u32, String)>::deserialize(deserializer)?;
match discr {
0 => Ok(Definition::Path(value.into())),
1 => Ok(Definition::Environment(value)),
2 => {
let path = (value.len() > 0).then_some(value.into());
Ok(Definition::Cli(path))
}
_ => panic!("unexpected discriminant {discr} value {value}"),
}
}
}