uv-resolver: document some of our intermediate data structures

There are some key invariants that I had to re-learn by reading the
code. This hopefully makes those invariants easier to discover by future
me (and others).

crates/uv-resolver/src/resolver/mod.rs

@@ -1854,17 +1854,30 @@ enum Response {
     },
 }
 
-/// An enum used by [`DependencyProvider`] that holds information about package dependencies.
-/// For each [Package] there is a set of versions allowed as a dependency.
+/// Information about the dependencies for a particular package.
+///
+/// This effectively distills the dependency metadata of a package down into
+/// its pubgrub specific constituent parts: each dependency package has a range
+/// of possible versions.
 #[derive(Clone)]
 enum Dependencies {
     /// Package dependencies are not available.
     Unavailable(UnavailableVersion),
     /// Container for all available package versions.
+    ///
+    /// Note that in universal mode, it is possible and allowed for multiple
+    /// `PubGrubPackage` values in this list to have the same package name.
+    /// These conflicts are resolved via `Dependencies::fork`.
     Available(Vec<(PubGrubPackage, Range<Version>)>),
 }
 
 impl Dependencies {
+    /// Turn this flat list of dependencies into a potential set of forked
+    /// groups of dependencies.
+    ///
+    /// A fork *only* occurs when there are multiple dependencies with the same
+    /// name *and* those dependency specifications have corresponding marker
+    /// expressions that are completely disjoint with one another.
     fn fork(self) -> ForkedDependencies {
         use std::collections::hash_map::Entry;
 
@@ -1991,11 +2004,19 @@ impl Dependencies {
     }
 }
 
+/// Information about the (possibly forked) dependencies for a particular
+/// package.
+///
+/// This is like `Dependencies` but with an extra variant that only occurs when
+/// a `Dependencies` list has multiple dependency specifications with the same
+/// name and non-overlapping marker expressions (i.e., a fork occurs).
 #[derive(Debug)]
 enum ForkedDependencies {
     /// Package dependencies are not available.
     Unavailable(UnavailableVersion),
     /// No forking occurred.
+    ///
+    /// This is the same as `Dependencies::Available`.
     Unforked(Vec<(PubGrubPackage, Range<Version>)>),
     /// Forked containers for all available package versions.
     ///
@@ -2005,14 +2026,48 @@ enum ForkedDependencies {
     Forked(Vec<Fork>),
 }
 
+/// A single fork in a list of dependencies.
+///
+/// A fork corresponds to the full list of dependencies for a package,
+/// but with any conflicting dependency specifications omitted. For
+/// example, if we have `a<2 ; sys_platform == 'foo'` and `a>=2 ;
+/// sys_platform == 'bar'`, then because the dependency specifications
+/// have the same name and because the marker expressions are disjoint,
+/// a fork occurs. One fork will contain `a<2` but not `a>=2`, while
+/// the other fork will contain `a>=2` but not `a<2`.
 #[derive(Clone, Debug)]
 struct Fork {
+    /// The list of dependencies for this fork, guaranteed to be conflict
+    /// free. (i.e., There are no two packages with the same name with
+    /// non-overlapping marker expressions.)
     dependencies: Vec<(PubGrubPackage, Range<Version>)>,
 }
 
+/// Intermediate state that represents a *possible* grouping of forks
+/// for one package name.
+///
+/// This accumulates state while examining a `Dependencies` list. In
+/// particular, it accumulates conflicting dependency specifications and marker
+/// expressions. As soon as a fork can be ruled out, this state is switched to
+/// `NoForkPossible`. If, at the end of visiting all `Dependencies`, we still
+/// have `PossibleForks::PossiblyForking`, then a fork exists if and only if
+/// one of its groups has length bigger than `1`.
+///
+/// One common way for a fork to be known to be impossible is if there exists
+/// conflicting dependency specifications where at least one is unconditional.
+/// For example, `a<2` and `a>=2 ; sys_platform == 'foo'`. In this case, `a<2`
+/// has a marker expression that is always true and thus never disjoint with
+/// any other marker expression. Therefore, there can be no fork for `a`.
+///
+/// Note that we use indices into a `Dependencies` list to represent packages.
+/// This avoids excessive cloning.
 #[derive(Debug)]
 enum PossibleForks<'a> {
+    /// A group of dependencies (all with the same package name) where it is
+    /// known that no forks exist.
     NoForkPossible(Vec<usize>),
+    /// A group of groups dependencies (all with the same package name) where
+    /// it is possible for each group to correspond to a fork.
     PossiblyForking(PossibleForkGroups<'a>),
 }
 
@@ -2064,8 +2119,11 @@ impl<'a> PossibleForks<'a> {
     }
 }
 
+/// A list of groups of dependencies (all with the same package name), where
+/// each group may correspond to a fork.
 #[derive(Debug)]
 struct PossibleForkGroups<'a> {
+    /// The list of forks.
     forks: Vec<PossibleFork<'a>>,
 }
 
@@ -2083,6 +2141,20 @@ impl<'a> PossibleForkGroups<'a> {
     }
 }
 
+/// Intermediate state representing a single possible fork.
+///
+/// The key invariant here is that, beyond a singleton fork, for all packages
+/// in this fork, its marker expression must be overlapping with at least one
+/// other package's marker expression. That is, when considering whether a
+/// dependency specification with a conflicting package name provokes a fork
+/// or not, one must look at the existing possible groups of forks. If any of
+/// those groups have a package with an overlapping marker expression, then
+/// the conflicting package name cannot possibly introduce a new fork. But if
+/// there is no existing fork with an overlapping marker expression, then the
+/// conflict provokes a new fork.
+///
+/// As with other intermediate data types, we use indices into a list of
+/// `Dependencies` to represent packages to avoid excessive cloning.
 #[derive(Debug)]
 struct PossibleFork<'a> {
     packages: Vec<(usize, &'a MarkerTree)>,