diff --git a/rustfmt.toml b/rustfmt.toml
index 0a90c89bffe..5eab1944645 100644
--- a/rustfmt.toml
+++ b/rustfmt.toml
@@ -12,47 +12,6 @@ ignore = [
 
     # tidy issues (line length, etc.)
     # to be fixed shortly
-    "src/libcore/iter/adapters/mod.rs",
-    "src/libcore/iter/traits/iterator.rs",
-    "src/librustc/hir/lowering.rs",
-    "src/librustc/infer/error_reporting/nice_region_error/outlives_closure.rs",
-    "src/librustc/lint/mod.rs",
-    "src/librustc/middle/resolve_lifetime.rs",
-    "src/librustc/traits/mod.rs",
-    "src/librustc/ty/constness.rs",
-    "src/librustc/ty/context.rs",
-    "src/librustc/ty/wf.rs",
-    "src/librustc_codegen_llvm/back/write.rs",
-    "src/librustc_codegen_llvm/consts.rs",
-    "src/librustc_codegen_llvm/debuginfo/metadata.rs",
-    "src/librustc_codegen_ssa/base.rs",
-    "src/librustc_codegen_ssa/mir/place.rs",
-    "src/librustc_codegen_utils/symbol_names/v0.rs",
-    "src/librustc_errors/emitter.rs",
-    "src/librustc_mir/borrow_check/diagnostics/mutability_errors.rs",
-    "src/librustc_mir/borrow_check/type_check/mod.rs",
-    "src/librustc_mir/build/expr/as_rvalue.rs",
-    "src/librustc_mir/build/matches/mod.rs",
-    "src/librustc_mir/build/mod.rs",
-    "src/librustc_mir/const_eval.rs",
-    "src/librustc_mir/interpret/place.rs",
-    "src/librustc_mir/monomorphize/collector.rs",
-    "src/librustc_passes/ast_validation.rs",
-    "src/librustc_resolve/lib.rs",
-    "src/librustc_resolve/resolve_imports.rs",
-    "src/librustc_typeck/astconv.rs",
-    "src/librustc_typeck/check/_match.rs",
-    "src/librustc_typeck/check/coercion.rs",
-    "src/librustc_typeck/check/method/confirm.rs",
-    "src/librustc_typeck/check/mod.rs",
-    "src/librustc_typeck/check/wfcheck.rs",
-    "src/librustdoc/html/markdown/tests.rs",
-    "src/libstd/sys/sgx/abi/mem.rs",
-    "src/libstd/sys/unix/os.rs",
-    "src/libsyntax_expand/parse/lexer/tests.rs",
-    "src/libsyntax_expand/parse/tests.rs",
-    "src/libsyntax_ext/test.rs",
-    "src/tools/build-manifest/src/main.rs",
     "src/librustc_feature",
 
     # do not format submodules
@@ -71,4 +30,7 @@ ignore = [
     "src/tools/rls",
     "src/tools/rust-installer",
     "src/tools/rustfmt",
+
+    # We do not format this file as it is externally sourced and auto-generated.
+    "src/libstd/sys/cloudabi/abi/cloudabi.rs",
 ]
diff --git a/src/tools/tidy/src/style.rs b/src/tools/tidy/src/style.rs
index addee1108e8..b15c29921d2 100644
--- a/src/tools/tidy/src/style.rs
+++ b/src/tools/tidy/src/style.rs
@@ -2,14 +2,17 @@
 //!
 //! Example checks are:
 //!
-//! * No lines over 100 characters.
-//! * No files with over 3000 lines.
+//! * No lines over 100 characters (in non-Rust files).
+//! * No files with over 3000 lines (in non-Rust files).
 //! * No tabs.
 //! * No trailing whitespace.
 //! * No CR characters.
 //! * No `TODO` or `XXX` directives.
 //! * No unexplained ` ```ignore ` or ` ```rust,ignore ` doc tests.
 //!
+//! Note that some of these rules are excluded from Rust files because we enforce rustfmt. It is
+//! preferable to be formatted rather than tidy-clean.
+//!
 //! A number of these checks can be opted-out of with various directives of the form:
 //! `// ignore-tidy-CHECK-NAME`.
 
@@ -142,6 +145,15 @@ pub fn check(path: &Path, bad: &mut bool) {
             return;
         }
 
+        let under_rustfmt = filename.ends_with(".rs") &&
+            // This list should ideally be sourced from rustfmt.toml but we don't want to add a toml
+            // parser to tidy.
+            !file.ancestors().any(|a| {
+                a.ends_with("src/test") ||
+                    a.ends_with("src/libstd/sys/cloudabi") ||
+                    a.ends_with("src/doc/book")
+            });
+
         if filename.ends_with(".md")
             && file.parent().unwrap().file_name().unwrap().to_string_lossy() != "error_codes"
         {
@@ -181,7 +193,10 @@ pub fn check(path: &Path, bad: &mut bool) {
             let mut err = |msg: &str| {
                 tidy_error!(bad, "{}:{}: {}", file.display(), i + 1, msg);
             };
-            if line.chars().count() > max_columns && !long_line_is_ok(max_columns, line) {
+            if !under_rustfmt
+                && line.chars().count() > max_columns
+                && !long_line_is_ok(max_columns, line)
+            {
                 suppressible_tidy_err!(
                     err,
                     skip_line_length,