Make use of Clippy repo read-only

driver/build/extra_symbols.rs

@@ -1,16 +1,15 @@
-use anyhow::{Context, Result, anyhow, bail};
+use anyhow::{Context, Result, anyhow};
 use dylint_internal::{
-    clippy_utils::{Rev, Revs, clippy_repository, parse_as_nightly},
+    clippy_utils::{Rev, Revs, clippy_repository, file_at_revision, parse_as_nightly},
     env,
     rustup::parse_toolchain,
 };
 use git2::Oid;
 use proc_macro2::TokenTree;
 use std::{
-    fs::{OpenOptions, read_to_string, write},
+    fs::{OpenOptions, write},
     io::Write,
-    path::PathBuf,
-    process::Command,
+    path::{Path, PathBuf},
 };
 use syn::{Item, ItemMacro, Macro, parse_file};
 
@@ -87,22 +86,7 @@ fn rev_from_channel(channel: &str) -> Result<Rev> {
 
 fn clippy_utils_sym_rs(rev: Oid) -> Result<String> {
     let repository = clippy_repository(false)?;
-    let Some(workdir) = repository.workdir() else {
-        bail!("Repository has no working directory");
-    };
-
-    let mut command = Command::new("git");
-    command
-        .current_dir(workdir)
-        .args(["checkout", &rev.to_string()]);
-    let status = command
-        .status()
-        .with_context(|| format!("command failed: {command:?}"))?;
-    assert!(status.success());
-
-    let sym_path = workdir.join("clippy_utils/src/sym.rs");
-    read_to_string(&sym_path)
-        .with_context(|| format!("`read_to_string` failed for `{}`", sym_path.display()))
+    file_at_revision(&repository, rev, Path::new("clippy_utils/src/sym.rs"))
 }
 
 fn quoted_symbols(sym_rs: &str) -> Result<Vec<String>> {

internal/src/clippy_utils/mod.rs

@@ -7,7 +7,7 @@ use std::{
 use toml_edit::{DocumentMut, Item, Value};
 
 mod repository;
-pub use repository::{clippy_repository, parse_as_nightly};
+pub use repository::{clippy_repository, file_at_revision, parse_as_nightly};
 
 mod revs_no_preinstall;
 pub use revs_no_preinstall::{Rev, Revs};
@@ -28,7 +28,11 @@ pub fn clippy_utils_package_version(path: &Path) -> Result<Version> {
             cargo_toml.to_string_lossy(),
         )
     })?;
-    let table = toml::from_str::<toml::Table>(&contents)?;
+    package_version_from_contents(&contents)
+}
+
+fn package_version_from_contents(contents: &str) -> Result<Version> {
+    let table = toml::from_str::<toml::Table>(contents)?;
     let version = table
         .get("package")
         .and_then(toml::Value::as_table)
@@ -76,7 +80,11 @@ pub fn toolchain_channel(path: &Path) -> Result<String> {
     let Some((_, contents)) = read_rust_toolchain_file(path)? else {
         bail!("Could not find rust-toolchain file at `{}`", path.display());
     };
-    let table = toml::from_str::<toml::Table>(&contents)?;
+    toolchain_channel_from_contents(&contents)
+}
+
+fn toolchain_channel_from_contents(contents: &str) -> Result<String> {
+    let table = toml::from_str::<toml::Table>(contents)?;
     table
         .get("toolchain")
         .and_then(toml::Value::as_table)

internal/src/clippy_utils/repository.rs

@@ -1,6 +1,9 @@
-use crate::{clone, git2::Repository};
+use crate::{
+    clone,
+    git2::{Oid, Repository},
+};
 use anyhow::{Context, Result};
-use std::{cell::RefCell, rc::Rc};
+use std::{cell::RefCell, path::Path, rc::Rc};
 use tempfile::{TempDir, tempdir};
 
 const RUST_CLIPPY_URL: &str = "https://github.com/rust-lang/rust-clippy";
@@ -26,6 +29,18 @@ pub fn clippy_repository(quiet: bool) -> Result<Rc<Repository>> {
     })
 }
 
+/// Reads a UTF-8 file from a commit without changing HEAD or the working tree.
+pub fn file_at_revision(repository: &Repository, rev: Oid, path: &Path) -> Result<String> {
+    (|| -> Result<String> {
+        let commit = repository.find_commit(rev)?;
+        let tree = commit.tree()?;
+        let entry = tree.get_path(path)?;
+        let blob = repository.find_blob(entry.id())?;
+        Ok(std::str::from_utf8(blob.content())?.to_owned())
+    })()
+    .with_context(|| format!("Could not read `{}` at revision `{rev}`", path.display()))
+}
+
 pub fn parse_as_nightly(channel: &str) -> Option<[u32; 3]> {
     channel.strip_prefix("nightly-").and_then(parse_date)
 }