Example of getting diagnostics

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diff --git a/src/doc/rustc-dev-guide/src/SUMMARY.md b/src/doc/rustc-dev-guide/src/SUMMARY.md
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--- a/src/doc/rustc-dev-guide/src/SUMMARY.md
+++ b/src/doc/rustc-dev-guide/src/SUMMARY.md
@@ -50,6 +50,7 @@
     - [The Rustc Driver and Interface](./rustc-driver.md)
         - [Rustdoc](./rustdoc.md)
         - [Ex: Type checking through `rustc_interface`](./rustc-driver-interacting-with-the-ast.md)
+        - [Ex: Getting diagnostics through `rustc_interface`](./rustc-driver-getting-diagnostics.md)
     - [Syntax and the AST](./syntax-intro.md)
         - [Lexing and Parsing](./the-parser.md)
         - [`#[test]` Implementation](./test-implementation.md)
diff --git a/src/doc/rustc-dev-guide/src/rustc-driver-getting-diagnostics.md b/src/doc/rustc-dev-guide/src/rustc-driver-getting-diagnostics.md
new file mode 100644
index 00000000000..d4b39555539
--- /dev/null
+++ b/src/doc/rustc-dev-guide/src/rustc-driver-getting-diagnostics.md
@@ -0,0 +1,101 @@
+# Example: Getting diagnostic through `rustc_interface`
+
+`rustc_interface` allows you to intercept diagnostics that would otherwise be printed to stderr.
+
+## Getting diagnostics
+
+NOTE: For the example to compile, you will need to first run the following:
+
+    rustup component add rustc-dev
+
+To get diagnostics from the compiler, configure `rustc_interface::Config` to output diagnostic to a buffer, and run `TyCtxt.analysis`:
+
+```rust
+#![feature(rustc_private)]
+
+extern crate rustc;
+extern crate rustc_error_codes;
+extern crate rustc_errors;
+extern crate rustc_hash;
+extern crate rustc_hir;
+extern crate rustc_interface;
+extern crate rustc_session;
+extern crate rustc_span;
+
+use rustc_errors::registry;
+use rustc_session::config;
+use rustc_span::source_map;
+use std::io;
+use std::path;
+use std::process;
+use std::str;
+use std::sync;
+
+// Buffer diagnostics in a Vec<u8>.
+#[derive(Clone)]
+pub struct DiagnosticSink(sync::Arc<sync::Mutex<Vec<u8>>>);
+
+impl io::Write for DiagnosticSink {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        self.0.lock().unwrap().write(buf)
+    }
+    fn flush(&mut self) -> io::Result<()> {
+        self.0.lock().unwrap().flush()
+    }
+}
+
+fn main() {
+    let out = process::Command::new("rustc")
+        .arg("--print=sysroot")
+        .current_dir(".")
+        .output()
+        .unwrap();
+    let sysroot = str::from_utf8(&out.stdout).unwrap().trim();
+    let buffer = sync::Arc::new(sync::Mutex::new(Vec::new()));
+    let config = rustc_interface::Config {
+        opts: config::Options {
+            maybe_sysroot: Some(path::PathBuf::from(sysroot)),
+            // Configure the compiler to emit diagnostics in compact JSON format.
+            error_format: config::ErrorOutputType::Json {
+                pretty: false,
+                json_rendered: rustc_errors::emitter::HumanReadableErrorType::Default(
+                    rustc_errors::emitter::ColorConfig::Never,
+                ),
+            },
+            ..config::Options::default()
+        },
+        // This program contains a type error.
+        input: config::Input::Str {
+            name: source_map::FileName::Custom("main.rs".to_string()),
+            input: "fn main() { let x: &str = 1; }".to_string(),
+        },
+        // Redirect the diagnostic output of the compiler to a buffer.
+        diagnostic_output: rustc_session::DiagnosticOutput::Raw(Box::from(DiagnosticSink(
+            buffer.clone(),
+        ))),
+        crate_cfg: rustc_hash::FxHashSet::default(),
+        input_path: None,
+        output_dir: None,
+        output_file: None,
+        file_loader: None,
+        stderr: None,
+        crate_name: None,
+        lint_caps: rustc_hash::FxHashMap::default(),
+        register_lints: None,
+        override_queries: None,
+        registry: registry::Registry::new(&rustc_error_codes::DIAGNOSTICS),
+    };
+    rustc_interface::run_compiler(config, |compiler| {
+        compiler.enter(|queries| {
+            queries.global_ctxt().unwrap().take().enter(|tcx| {
+                // Run the analysis phase on the local crate to trigger the type error.
+                tcx.analysis(rustc_hir::def_id::LOCAL_CRATE);
+            });
+        });
+    });
+    // Read and print buffered diagnostics.
+    let diagnostics = String::from_utf8(buffer.lock().unwrap().clone()).unwrap();
+    println!("{}", diagnostics);
+}
+
+```