Auto merge of #12604 - Eh2406:to_string, r=epage

Fewer temporary needless strings

### What does this PR try to resolve?

I noticed a few places where we were using `InternedString::to_string()` in places where we just needed a hash key or to display it. In these situations we could avoid a memory allocation by using the `InternedString` directly. I used the clippy `disallowed-methods` to look through all of the places who called `to_string` on an `InternedString`, and fixed all the places where it was a straightforward change to remove it. I don't think any of them matter in practice. But doing less work can't hurt.

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

This was an internal re-factor, and the tests still pass.

credential/cargo-credential/src/error.rs

@@ -42,9 +42,9 @@ pub enum Error {
 }
 
 impl From<String> for Error {
-    fn from(err: String) -> Self {
+    fn from(message: String) -> Self {
         Box::new(StringTypedError {
-            message: err.to_string(),
+            message,
             source: None,
         })
         .into()

src/cargo/core/compiler/custom_build.rs

@@ -281,7 +281,7 @@ fn build_work(cx: &mut Context<'_, '_>, unit: &Unit) -> CargoResult<Job> {
         .env("NUM_JOBS", &bcx.jobs().to_string())
         .env("TARGET", bcx.target_data.short_name(&unit.kind))
         .env("DEBUG", debug.to_string())
-        .env("OPT_LEVEL", &unit.profile.opt_level.to_string())
+        .env("OPT_LEVEL", &unit.profile.opt_level)
         .env(
             "PROFILE",
             match unit.profile.root {

src/cargo/core/compiler/mod.rs

@@ -252,7 +252,7 @@ fn rustc(cx: &mut Context<'_, '_>, unit: &Unit, exec: &Arc<dyn Executor>) -> Car
     let mut rustc = prepare_rustc(cx, unit)?;
     let build_plan = cx.bcx.build_config.build_plan;
 
-    let name = unit.pkg.name().to_string();
+    let name = unit.pkg.name();
     let buildkey = unit.buildkey();
 
     let outputs = cx.outputs(unit)?;
@@ -785,7 +785,7 @@ fn rustdoc(cx: &mut Context<'_, '_>, unit: &Unit) -> CargoResult<Work> {
     paths::create_dir_all(&doc_dir)?;
 
     let target_desc = unit.target.description_named();
-    let name = unit.pkg.name().to_string();
+    let name = unit.pkg.name();
     let build_script_outputs = Arc::clone(&cx.build_script_outputs);
     let package_id = unit.pkg.package_id();
     let manifest_path = PathBuf::from(unit.pkg.manifest_path());

src/cargo/core/package.rs

@@ -338,7 +338,7 @@ pub struct Downloads<'a, 'cfg> {
     /// Total bytes for all successfully downloaded packages.
     downloaded_bytes: u64,
     /// Size (in bytes) and package name of the largest downloaded package.
-    largest: (u64, String),
+    largest: (u64, InternedString),
     /// Time when downloading started.
     start: Instant,
     /// Indicates *all* downloads were successful.
@@ -459,7 +459,7 @@ impl<'cfg> PackageSet<'cfg> {
             ))),
             downloads_finished: 0,
             downloaded_bytes: 0,
-            largest: (0, String::new()),
+            largest: (0, InternedString::new("")),
             success: false,
             updated_at: Cell::new(Instant::now()),
             timeout,
@@ -891,7 +891,7 @@ impl<'a, 'cfg> Downloads<'a, 'cfg> {
         self.downloads_finished += 1;
         self.downloaded_bytes += dl.total.get();
         if dl.total.get() > self.largest.0 {
-            self.largest = (dl.total.get(), dl.id.name().to_string());
+            self.largest = (dl.total.get(), dl.id.name());
         }
 
         // We're about to synchronously extract the crate below. While we're

src/cargo/core/resolver/dep_cache.rs

@@ -531,10 +531,9 @@ impl RequirementError {
                             summary.package_id(),
                             feat
                         )),
-                        Some(p) => ActivateError::Conflict(
-                            p,
-                            ConflictReason::MissingFeatures(feat.to_string()),
-                        ),
+                        Some(p) => {
+                            ActivateError::Conflict(p, ConflictReason::MissingFeatures(feat))
+                        }
                     };
                 }
                 if deps.iter().any(|dep| dep.is_optional()) {
@@ -575,10 +574,9 @@ impl RequirementError {
                     )),
                     // This code path currently isn't used, since `foo/bar`
                     // and `dep:` syntax is not allowed in a dependency.
-                    Some(p) => ActivateError::Conflict(
-                        p,
-                        ConflictReason::MissingFeatures(dep_name.to_string()),
-                    ),
+                    Some(p) => {
+                        ActivateError::Conflict(p, ConflictReason::MissingFeatures(dep_name))
+                    }
                 }
             }
             RequirementError::Cycle(feat) => ActivateError::Fatal(anyhow::format_err!(

src/cargo/core/resolver/types.rs

@@ -285,7 +285,7 @@ pub enum ConflictReason {
     /// A dependency listed features that weren't actually available on the
     /// candidate. For example we tried to activate feature `foo` but the
     /// candidate we're activating didn't actually have the feature `foo`.
-    MissingFeatures(String),
+    MissingFeatures(InternedString),
 
     /// A dependency listed a feature that ended up being a required dependency.
     /// For example we tried to activate feature `foo` but the

src/cargo/util/toml/mod.rs

@@ -2326,7 +2326,7 @@ impl TomlManifest {
             let mut names_sources = BTreeMap::new();
             for dep in &deps {
                 let name = dep.name_in_toml();
-                let prev = names_sources.insert(name.to_string(), dep.source_id());
+                let prev = names_sources.insert(name, dep.source_id());
                 if prev.is_some() && prev != Some(dep.source_id()) {
                     bail!(
                         "Dependency '{}' has different source paths depending on the build \