[guppy] handle platform dependencies + update feature graph construction

This is a complete overhaul of the way platform-specific dependencies are
handled. Based on my experimentation and reading the Cargo source code, I
believe that this is now correct.

Doing so also required feature graph construction to be updated, so the
feature graph construction is ready for the new feature resolver, and is now
platform-sensitive as well. The APIs for the feature graph are still to
come, but making the data model be full-fidelity allows for both the current
and new feature resolvers to be implemented.

Now that target logic is internalized in guppy, cargo-guppy no longer needs
to do its own evaluation.

For more about the new feature resolver, see:

* https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#features
* rust-lang/cargo#7914
* rust-lang/cargo#7915
* rust-lang/cargo#7916

cargo-guppy/Cargo.toml

@@ -13,4 +13,3 @@ itertools = "0.9.0"
 serde = "1.0.40"
 serde_json = "1.0.40"
 structopt = "0.3.0"
-target-spec = { version = "0.2.0", path = "../target-spec" }

cargo-guppy/src/lib.rs

@@ -3,9 +3,10 @@
 
 use anyhow;
 use clap::arg_enum;
+use guppy::graph::EnabledStatus;
 use guppy::{
     graph::{DependencyLink, DotWrite, PackageDotVisitor, PackageGraph, PackageMetadata},
-    MetadataCommand, PackageId,
+    MetadataCommand, PackageId, Platform, TargetFeatures,
 };
 use itertools;
 use std::cmp;
@@ -15,7 +16,6 @@ use std::fs;
 use std::io::Write;
 use std::iter;
 use structopt::StructOpt;
-use target_spec;
 
 mod diff;
 
@@ -305,6 +305,13 @@ fn narrow_graph(pkg_graph: &mut PackageGraph, options: &FilterOptions) {
         }
     }
 
+    let platform = if let Some(ref target) = options.target {
+        // Accept all features for the target filtering below.
+        Some(Platform::new(target, TargetFeatures::All).unwrap())
+    } else {
+        None
+    };
+
     pkg_graph.retain_edges(|_, DependencyLink { from, to, edge }| {
         // filter by the kind of dependency (--kind)
         // NOTE: We always retain all workspace deps in the graph, otherwise
@@ -316,12 +323,11 @@ fn narrow_graph(pkg_graph: &mut PackageGraph, options: &FilterOptions) {
         };
 
         // filter out irrelevant dependencies for a specific target (--target)
-        let include_target = if let Some(ref target) = options.target {
+        let include_target = if let Some(platform) = &platform {
             edge.normal()
-                .and_then(|meta| meta.target())
-                .and_then(|edge_target| {
-                    let res = target_spec::eval(edge_target, target).unwrap_or(true);
-                    Some(res)
+                .map(|meta| {
+                    // Include this dependency if it's optional or mandatory.
+                    meta.enabled_on(platform).unwrap() != EnabledStatus::Never
                 })
                 .unwrap_or(true)
         } else {

guppy/Cargo.toml

@@ -39,6 +39,7 @@ proptest-derive = { version = "0.1.2", optional = true }
 semver = "0.9.0"
 serde = { version = "1.0.99", features = ["derive"] }
 serde_json = "1.0.40"
+target-spec = { version = "0.2.0", path = "../target-spec" }
 
 [dev-dependencies]
 assert_matches = "1.3.0"

guppy/src/errors.rs

@@ -25,6 +25,16 @@ pub enum Error {
     UnknownPackageId(PackageId),
     /// A feature ID was unknown to this `FeatureGraph`.
     UnknownFeatureId(PackageId, Option<String>),
+    /// The platform `guppy` is running on is unknown.
+    UnknownCurrentPlatform,
+    /// An error occurred while evaluating a target specification against the given platform.
+    #[non_exhaustive]
+    TargetEvalError {
+        /// The given platform.
+        platform: &'static str,
+        /// The error that occurred while evaluating the target specification.
+        err: Box<dyn error::Error + Sync + Send + 'static>,
+    },
     /// An internal error occurred within this `PackageGraph`.
     PackageGraphInternalError(String),
 }
@@ -46,6 +56,12 @@ impl fmt::Display for Error {
                 Some(feature) => write!(f, "Unknown feature ID: '{}' '{}'", package_id, feature),
                 None => write!(f, "Unknown feature ID: '{}' (base)", package_id),
             },
+            UnknownCurrentPlatform => write!(f, "Unknown current platform"),
+            TargetEvalError { platform, err } => write!(
+                f,
+                "Error while evaluating target specifications against platform '{}': {}",
+                platform, err
+            ),
             PackageGraphInternalError(msg) => write!(f, "Internal error in package graph: {}", msg),
         }
     }
@@ -59,6 +75,8 @@ impl error::Error for Error {
             PackageGraphConstructError(_) => None,
             UnknownPackageId(_) => None,
             UnknownFeatureId(_, _) => None,
+            UnknownCurrentPlatform => None,
+            TargetEvalError { err, .. } => Some(err.as_ref()),
             PackageGraphInternalError(_) => None,
         }
     }