Rewrite source parser to use pulldown-cmark for comment parsing (#353)

Resolves #350

From a quick test of a few repos I have cloned, this fixes some false
positives around unused deps in doc comments (one found in `diesel`, one
found in `sqlx`)

Perf looks to be unchanged:

```
> hyperfine -i cargo-shear-old cargo-shear
Benchmark 1: cargo-shear-old
  Time (mean ± σ):     219.9 ms ±   4.7 ms    [User: 946.7 ms, System: 54.5 ms]
  Range (min … max):   212.2 ms … 230.4 ms    20 runs

Benchmark 2: cargo-shear
  Time (mean ± σ):     222.8 ms ±   6.7 ms    [User: 983.8 ms, System: 46.5 ms]
  Range (min … max):   213.3 ms … 243.1 ms    20 runs

Summary
  cargo-shear-old ran
    1.01 ± 0.04 times faster than cargo-shear
```

`pulldown-cmark` does have SIMD support, but only on x86.

Author: CathalMullan

Cargo.lock

@@ -23,6 +23,9 @@ walkdir = "2.5.0"
 cargo_metadata = "0.23.0"
 bpaf = { version = "0.9.19", features = ["derive", "batteries"] }
 ra_ap_syntax = "0.0.308"
+pulldown-cmark = { version = "0.13", default-features = false, features = [
+  "simd",
+] }
 rayon = "1.10.0"
 serde = { version = "1.0.228", features = ["derive"] }
 toml = { version = "0.9.8", features = ["serde"] }

Cargo.toml

@@ -9,12 +9,7 @@
 //! The analyzer walks through all source files in a package, collects import
 //! statements, and builds a set of used import names.
 
-use std::{
-    env,
-    ffi::OsString,
-    path::{Path, PathBuf},
-    process::Command,
-};
+use std::{env, ffi::OsString, path::PathBuf, process::Command};
 
 use anyhow::{Result, anyhow};
 use cargo_metadata::{Package, Target, TargetKind};
@@ -23,7 +18,7 @@ use rustc_hash::{FxHashMap, FxHashSet};
 use toml::Spanned;
 use walkdir::{DirEntry, WalkDir};
 
-use crate::{import_collector::collect_imports, manifest::Manifest};
+use crate::{manifest::Manifest, source_parser::ParsedSource};
 
 /// How a dependency is referenced in `[features]`.
 ///
@@ -164,14 +159,13 @@ impl DependencyAnalyzer {
             let target_kind = target.kind.first().ok_or_else(|| anyhow!("Target has no kind"))?;
             let rust_files = Self::get_target_rust_files(target);
 
-            let deps_vec: Vec<FxHashSet<String>> = rust_files
+            let imports: FxHashSet<String> = rust_files
                 .par_iter()
-                .map(|path| Self::process_rust_source(path))
-                .collect::<Result<Vec<_>>>()?;
-
-            let imports = deps_vec
+                .map(|path| ParsedSource::from_path(path.as_path()))
+                .collect::<Result<Vec<_>, _>>()?
                 .into_iter()
-                .fold(FxHashSet::default(), |a, b| a.union(&b).cloned().collect());
+                .flat_map(|parsed| parsed.imports)
+                .collect();
 
             Self::categorize_imports(categorized, target_kind, imports);
         }
@@ -236,8 +230,8 @@ impl DependencyAnalyzer {
                 ));
             }
 
-            let imports = collect_imports(&output_str);
-            Self::categorize_imports(categorized, target_kind, imports);
+            let parsed = ParsedSource::from_str(&output_str);
+            Self::categorize_imports(categorized, target_kind, parsed.imports);
         }
 
         Ok(())
@@ -287,12 +281,6 @@ impl DependencyAnalyzer {
         }
     }
 
-    /// Parse a Rust source file and collect all import names.
-    fn process_rust_source(path: &Path) -> Result<FxHashSet<String>> {
-        let source_text = std::fs::read_to_string(path)?;
-        Ok(collect_imports(&source_text))
-    }
-
     /// Collect import names for dependencies referenced in features.
     fn analyze_features(categorized: &mut CategorizedImports, manifest: &Manifest) {
         for (feature, values) in &manifest.features {