Auto merge of #11445 - willcrichton:refactor-target-generator, r=weihanglo

Refactor generate_targets into separate module

### What does this PR try to resolve?

The `generate_targets` function is fairly complicated with an absurd number of parameters. This PR refactors the function into a `TargetGenerator` struct that represents the state of the generator, and reduces the amount of parameter-passing between the relevant functions. Additionally, the `generate_targets` function has been refactored into two smaller functions `create_proposals` and `proposals_to_units`. The docscrape-specific functionality from #11430 has been pulled out into a separate function, as promised.

### How should we test and review this PR?

This PR does not change any functionality, so no new tests are added. It should be reviewed for code style.

r? `@weihanglo`

Author: bors

src/cargo/ops/cargo_compile/compile_filter.rs

@@ -28,11 +28,11 @@ pub enum FilterRule {
 /// Filter to apply to the root package to select which Cargo targets will be built.
 /// (examples, bins, benches, tests, ...)
 ///
-/// The actual filter process happens inside [`generate_targets`].
+/// The actual filter process happens inside [`generate_units`].
 ///
 /// Not to be confused with [`Packages`], which opts in packages to be built.
 ///
-/// [`generate_targets`]: super::generate_targets
+/// [`generate_units`]: super::UnitGenerator::generate_units
 /// [`Packages`]: crate::ops::Packages
 #[derive(Debug)]
 pub enum CompileFilter {
@@ -176,7 +176,7 @@ impl CompileFilter {
     /// may include additional example targets to ensure they can be compiled.
     ///
     /// Note that the actual behavior is subject to `filter_default_targets`
-    /// and `generate_targets` though.
+    /// and `generate_units` though.
     pub fn all_test_targets() -> Self {
         Self::Only {
             all_targets: false,
@@ -234,7 +234,7 @@ impl CompileFilter {
     }
 
     /// Selects targets for "cargo run". for logic to select targets for other
-    /// subcommands, see `generate_targets` and `filter_default_targets`.
+    /// subcommands, see `generate_units` and `filter_default_targets`.
     pub fn target_run(&self, target: &Target) -> bool {
         match *self {
             CompileFilter::Default { .. } => true,