diff --git a/src/tools/compiletest/src/compute_diff.rs b/src/tools/compiletest/src/compute_diff.rs
new file mode 100644
index 0000000000000..92c80c27de03b
--- /dev/null
+++ b/src/tools/compiletest/src/compute_diff.rs
@@ -0,0 +1,157 @@
+use std::collections::VecDeque;
+use std::fs::{File, FileType};
+use std::path::Path;
+
+#[derive(Debug, PartialEq)]
+pub enum DiffLine {
+    Context(String),
+    Expected(String),
+    Resulting(String),
+}
+
+#[derive(Debug, PartialEq)]
+pub struct Mismatch {
+    pub line_number: u32,
+    pub lines: Vec<DiffLine>,
+}
+
+impl Mismatch {
+    fn new(line_number: u32) -> Mismatch {
+        Mismatch { line_number, lines: Vec::new() }
+    }
+}
+
+// Produces a diff between the expected output and actual output.
+pub fn make_diff(expected: &str, actual: &str, context_size: usize) -> Vec<Mismatch> {
+    let mut line_number = 1;
+    let mut context_queue: VecDeque<&str> = VecDeque::with_capacity(context_size);
+    let mut lines_since_mismatch = context_size + 1;
+    let mut results = Vec::new();
+    let mut mismatch = Mismatch::new(0);
+
+    for result in diff::lines(expected, actual) {
+        match result {
+            diff::Result::Left(str) => {
+                if lines_since_mismatch >= context_size && lines_since_mismatch > 0 {
+                    results.push(mismatch);
+                    mismatch = Mismatch::new(line_number - context_queue.len() as u32);
+                }
+
+                while let Some(line) = context_queue.pop_front() {
+                    mismatch.lines.push(DiffLine::Context(line.to_owned()));
+                }
+
+                mismatch.lines.push(DiffLine::Expected(str.to_owned()));
+                line_number += 1;
+                lines_since_mismatch = 0;
+            }
+            diff::Result::Right(str) => {
+                if lines_since_mismatch >= context_size && lines_since_mismatch > 0 {
+                    results.push(mismatch);
+                    mismatch = Mismatch::new(line_number - context_queue.len() as u32);
+                }
+
+                while let Some(line) = context_queue.pop_front() {
+                    mismatch.lines.push(DiffLine::Context(line.to_owned()));
+                }
+
+                mismatch.lines.push(DiffLine::Resulting(str.to_owned()));
+                lines_since_mismatch = 0;
+            }
+            diff::Result::Both(str, _) => {
+                if context_queue.len() >= context_size {
+                    let _ = context_queue.pop_front();
+                }
+
+                if lines_since_mismatch < context_size {
+                    mismatch.lines.push(DiffLine::Context(str.to_owned()));
+                } else if context_size > 0 {
+                    context_queue.push_back(str);
+                }
+
+                line_number += 1;
+                lines_since_mismatch += 1;
+            }
+        }
+    }
+
+    results.push(mismatch);
+    results.remove(0);
+
+    results
+}
+
+pub(crate) fn write_diff(expected: &str, actual: &str, context_size: usize) -> String {
+    use std::fmt::Write;
+    let mut output = String::new();
+    let diff_results = make_diff(expected, actual, context_size);
+    for result in diff_results {
+        let mut line_number = result.line_number;
+        for line in result.lines {
+            match line {
+                DiffLine::Expected(e) => {
+                    writeln!(output, "-\t{}", e).unwrap();
+                    line_number += 1;
+                }
+                DiffLine::Context(c) => {
+                    writeln!(output, "{}\t{}", line_number, c).unwrap();
+                    line_number += 1;
+                }
+                DiffLine::Resulting(r) => {
+                    writeln!(output, "+\t{}", r).unwrap();
+                }
+            }
+        }
+        writeln!(output).unwrap();
+    }
+    output
+}
+
+/// Filters based on filetype and extension whether to diff a file.
+///
+/// Returns whether any data was actually written.
+pub(crate) fn write_filtered_diff<Filter>(
+    diff_filename: &str,
+    out_dir: &Path,
+    compare_dir: &Path,
+    verbose: bool,
+    filter: Filter,
+) -> bool
+where
+    Filter: Fn(FileType, Option<&str>) -> bool,
+{
+    use std::io::{Read, Write};
+    let mut diff_output = File::create(diff_filename).unwrap();
+    let mut wrote_data = false;
+    for entry in walkdir::WalkDir::new(out_dir) {
+        let entry = entry.expect("failed to read file");
+        let extension = entry.path().extension().and_then(|p| p.to_str());
+        if filter(entry.file_type(), extension) {
+            let expected_path = compare_dir.join(entry.path().strip_prefix(&out_dir).unwrap());
+            let expected = if let Ok(s) = std::fs::read(&expected_path) { s } else { continue };
+            let actual_path = entry.path();
+            let actual = std::fs::read(&actual_path).unwrap();
+            let diff = unified_diff::diff(
+                &expected,
+                &expected_path.to_string_lossy(),
+                &actual,
+                &actual_path.to_string_lossy(),
+                3,
+            );
+            wrote_data |= !diff.is_empty();
+            diff_output.write_all(&diff).unwrap();
+        }
+    }
+
+    if !wrote_data {
+        println!("note: diff is identical to nightly rustdoc");
+        assert!(diff_output.metadata().unwrap().len() == 0);
+        return false;
+    } else if verbose {
+        eprintln!("printing diff:");
+        let mut buf = Vec::new();
+        diff_output.read_to_end(&mut buf).unwrap();
+        std::io::stderr().lock().write_all(&mut buf).unwrap();
+    }
+    true
+}
diff --git a/src/tools/compiletest/src/main.rs b/src/tools/compiletest/src/main.rs
index 9e655557fd271..fbf3249db94d1 100644
--- a/src/tools/compiletest/src/main.rs
+++ b/src/tools/compiletest/src/main.rs
@@ -28,6 +28,7 @@ use self::header::{make_test_description, EarlyProps};
 mod tests;
 
 pub mod common;
+pub mod compute_diff;
 pub mod errors;
 pub mod header;
 mod json;
diff --git a/src/tools/compiletest/src/runtest.rs b/src/tools/compiletest/src/runtest.rs
index 2a4bb9eb88b30..0821e279d2485 100644
--- a/src/tools/compiletest/src/runtest.rs
+++ b/src/tools/compiletest/src/runtest.rs
@@ -8,6 +8,7 @@ use crate::common::{CompareMode, FailMode, PassMode};
 use crate::common::{Config, TestPaths};
 use crate::common::{Pretty, RunPassValgrind};
 use crate::common::{UI_RUN_STDERR, UI_RUN_STDOUT};
+use crate::compute_diff::{write_diff, write_filtered_diff};
 use crate::errors::{self, Error, ErrorKind};
 use crate::header::TestProps;
 use crate::json;
@@ -18,7 +19,7 @@ use regex::{Captures, Regex};
 use rustfix::{apply_suggestions, get_suggestions_from_json, Filter};
 
 use std::collections::hash_map::DefaultHasher;
-use std::collections::{HashMap, HashSet, VecDeque};
+use std::collections::{HashMap, HashSet};
 use std::env;
 use std::ffi::{OsStr, OsString};
 use std::fs::{self, create_dir_all, File, OpenOptions};
@@ -100,111 +101,6 @@ pub fn get_lib_name(lib: &str, dylib: bool) -> String {
     }
 }
 
-#[derive(Debug, PartialEq)]
-pub enum DiffLine {
-    Context(String),
-    Expected(String),
-    Resulting(String),
-}
-
-#[derive(Debug, PartialEq)]
-pub struct Mismatch {
-    pub line_number: u32,
-    pub lines: Vec<DiffLine>,
-}
-
-impl Mismatch {
-    fn new(line_number: u32) -> Mismatch {
-        Mismatch { line_number, lines: Vec::new() }
-    }
-}
-
-// Produces a diff between the expected output and actual output.
-pub fn make_diff(expected: &str, actual: &str, context_size: usize) -> Vec<Mismatch> {
-    let mut line_number = 1;
-    let mut context_queue: VecDeque<&str> = VecDeque::with_capacity(context_size);
-    let mut lines_since_mismatch = context_size + 1;
-    let mut results = Vec::new();
-    let mut mismatch = Mismatch::new(0);
-
-    for result in diff::lines(expected, actual) {
-        match result {
-            diff::Result::Left(str) => {
-                if lines_since_mismatch >= context_size && lines_since_mismatch > 0 {
-                    results.push(mismatch);
-                    mismatch = Mismatch::new(line_number - context_queue.len() as u32);
-                }
-
-                while let Some(line) = context_queue.pop_front() {
-                    mismatch.lines.push(DiffLine::Context(line.to_owned()));
-                }
-
-                mismatch.lines.push(DiffLine::Expected(str.to_owned()));
-                line_number += 1;
-                lines_since_mismatch = 0;
-            }
-            diff::Result::Right(str) => {
-                if lines_since_mismatch >= context_size && lines_since_mismatch > 0 {
-                    results.push(mismatch);
-                    mismatch = Mismatch::new(line_number - context_queue.len() as u32);
-                }
-
-                while let Some(line) = context_queue.pop_front() {
-                    mismatch.lines.push(DiffLine::Context(line.to_owned()));
-                }
-
-                mismatch.lines.push(DiffLine::Resulting(str.to_owned()));
-                lines_since_mismatch = 0;
-            }
-            diff::Result::Both(str, _) => {
-                if context_queue.len() >= context_size {
-                    let _ = context_queue.pop_front();
-                }
-
-                if lines_since_mismatch < context_size {
-                    mismatch.lines.push(DiffLine::Context(str.to_owned()));
-                } else if context_size > 0 {
-                    context_queue.push_back(str);
-                }
-
-                line_number += 1;
-                lines_since_mismatch += 1;
-            }
-        }
-    }
-
-    results.push(mismatch);
-    results.remove(0);
-
-    results
-}
-
-fn write_diff(expected: &str, actual: &str, context_size: usize) -> String {
-    use std::fmt::Write;
-    let mut output = String::new();
-    let diff_results = make_diff(expected, actual, context_size);
-    for result in diff_results {
-        let mut line_number = result.line_number;
-        for line in result.lines {
-            match line {
-                DiffLine::Expected(e) => {
-                    writeln!(output, "-\t{}", e).unwrap();
-                    line_number += 1;
-                }
-                DiffLine::Context(c) => {
-                    writeln!(output, "{}\t{}", line_number, c).unwrap();
-                    line_number += 1;
-                }
-                DiffLine::Resulting(r) => {
-                    writeln!(output, "+\t{}", r).unwrap();
-                }
-            }
-        }
-        writeln!(output).unwrap();
-    }
-    output
-}
-
 pub fn run(config: Config, testpaths: &TestPaths, revision: Option<&str>) {
     match &*config.target {
         "arm-linux-androideabi"
@@ -2507,43 +2403,17 @@ impl<'test> TestCx<'test> {
 
         let diff_filename = format!("build/tmp/rustdoc-compare-{}.diff", std::process::id());
 
-        {
-            let mut diff_output = File::create(&diff_filename).unwrap();
-            let mut wrote_data = false;
-            for entry in walkdir::WalkDir::new(out_dir) {
-                let entry = entry.expect("failed to read file");
-                let extension = entry.path().extension().and_then(|p| p.to_str());
-                if entry.file_type().is_file()
+        if !write_filtered_diff(
+            &diff_filename,
+            out_dir,
+            &compare_dir,
+            self.config.verbose,
+            |file_type, extension| {
+                file_type.is_file()
                     && (extension == Some("html".into()) || extension == Some("js".into()))
-                {
-                    let expected_path =
-                        compare_dir.join(entry.path().strip_prefix(&out_dir).unwrap());
-                    let expected =
-                        if let Ok(s) = std::fs::read(&expected_path) { s } else { continue };
-                    let actual_path = entry.path();
-                    let actual = std::fs::read(&actual_path).unwrap();
-                    let diff = unified_diff::diff(
-                        &expected,
-                        &expected_path.to_string_lossy(),
-                        &actual,
-                        &actual_path.to_string_lossy(),
-                        3,
-                    );
-                    wrote_data |= !diff.is_empty();
-                    diff_output.write_all(&diff).unwrap();
-                }
-            }
-
-            if !wrote_data {
-                println!("note: diff is identical to nightly rustdoc");
-                assert!(diff_output.metadata().unwrap().len() == 0);
-                return;
-            } else if self.config.verbose {
-                eprintln!("printing diff:");
-                let mut buf = Vec::new();
-                diff_output.read_to_end(&mut buf).unwrap();
-                std::io::stderr().lock().write_all(&mut buf).unwrap();
-            }
+            },
+        ) {
+            return;
         }
 
         match self.config.color {