Auto merge of #83855 - Dylan-DPC:rollup-oww62sh, r=Dylan-DPC

Rollup of 8 pull requests

Successful merges:

 - #73945 (Add an unstable --json=unused-externs flag to print unused externs)
 - #81619 (Implement `SourceIterator` and `InPlaceIterable` for `ResultShunt`)
 - #82726 (BTree: move blocks around in node.rs)
 - #83521 (2229: Fix diagnostic issue when using FakeReads in closures)
 - #83532 (Fix compiletest on FreeBSD)
 - #83793 (rustdoc: highlight macros more efficiently)
 - #83809 (Remove unneeded INITIAL_IDS const)
 - #83827 (cleanup leak after test to make miri happy)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

Author: bors

compiler/rustc_errors/src/emitter.rs

@@ -195,6 +195,9 @@ pub trait Emitter {
 
     fn emit_future_breakage_report(&mut self, _diags: Vec<(FutureBreakage, Diagnostic)>) {}
 
+    /// Emit list of unused externs
+    fn emit_unused_externs(&mut self, _lint_level: &str, _unused_externs: &[&str]) {}
+
     /// Checks if should show explanations about "rustc --explain"
     fn should_show_explain(&self) -> bool {
         true

compiler/rustc_errors/src/json.rs

@@ -159,6 +159,19 @@ impl Emitter for JsonEmitter {
         }
     }
 
+    fn emit_unused_externs(&mut self, lint_level: &str, unused_externs: &[&str]) {
+        let data = UnusedExterns { lint_level, unused_extern_names: unused_externs };
+        let result = if self.pretty {
+            writeln!(&mut self.dst, "{}", as_pretty_json(&data))
+        } else {
+            writeln!(&mut self.dst, "{}", as_json(&data))
+        }
+        .and_then(|_| self.dst.flush());
+        if let Err(e) = result {
+            panic!("failed to print unused externs: {:?}", e);
+        }
+    }
+
     fn source_map(&self) -> Option<&Lrc<SourceMap>> {
         Some(&self.sm)
     }
@@ -322,6 +335,18 @@ struct FutureIncompatReport {
     future_incompat_report: Vec<FutureBreakageItem>,
 }
 
+// NOTE: Keep this in sync with the equivalent structs in rustdoc's
+// doctest component (as well as cargo).
+// We could unify this struct the one in rustdoc but they have different
+// ownership semantics, so doing so would create wasteful allocations.
+#[derive(Encodable)]
+struct UnusedExterns<'a, 'b, 'c> {
+    /// The severity level of the unused dependencies lint
+    lint_level: &'a str,
+    /// List of unused externs by their names.
+    unused_extern_names: &'b [&'c str],
+}
+
 impl Diagnostic {
     fn from_errors_diagnostic(diag: &crate::Diagnostic, je: &JsonEmitter) -> Diagnostic {
         let sugg = diag.suggestions.iter().map(|sugg| Diagnostic {