Improve out of line module resolution

Fixes 5119

When the directory structure was laid out as follows:

```
dir
 |---mod_a
 |    |---sub_mod_1.rs
 |    |---sub_mod_2.rs
 |---mod_a.rs
```

And ``mod_a.rs`` contains the following content:

```rust
mod mod_a {
    mod sub_mod_1;
    mod sub_mod_2;
}
```

rustfmt previously tried to find ``sub_mod_1.rs`` and ``sub_mod_2.rs``
in ``./mod_a/mod_a/``. This directory does not exist and this caused
rustfmt to fail with the error message:

    Error writing files: failed to resolve mod

Now, both ``sub_mod_1.rs`` and ``sub_mod_2.rs`` are resolved correctly
and found at ``mod_a/sub_mod_1.rs`` and ``mod_a/sub_mod_2.rs``.

10 files changed, 82 insertions, 1 deletions

diff --git a/src/modules.rs b/src/modules.rs
index 9c964b274e0..70b937b0283 100644
--- a/src/modules.rs
+++ b/src/modules.rs
@@ -458,6 +458,7 @@ impl<'ast, 'sess, 'c> ModResolver<'ast, 'sess> {
             self.directory.path.push(path.as_str());
             self.directory.ownership = DirectoryOwnership::Owned { relative: None };
         } else {
+            let id = id.as_str();
             // We have to push on the current module name in the case of relative
             // paths in order to ensure that any additional module paths from inline
             // `mod x { ... }` come after the relative extension.
@@ -468,9 +469,15 @@ impl<'ast, 'sess, 'c> ModResolver<'ast, 'sess> {
                 if let Some(ident) = relative.take() {
                     // remove the relative offset
                     self.directory.path.push(ident.as_str());
+
+                    // In the case where there is an x.rs and an ./x directory we want
+                    // to prevent adding x twice. For example, ./x/x
+                    if self.directory.path.exists() && !self.directory.path.join(id).exists() {
+                        return;
+                    }
                 }
             }
-            self.directory.path.push(id.as_str());
+            self.directory.path.push(id);
         }
     }
 
diff --git a/src/test/mod_resolver.rs b/src/test/mod_resolver.rs
index ec9ed0f0b8d..fcff6d14e6f 100644
--- a/src/test/mod_resolver.rs
+++ b/src/test/mod_resolver.rs
@@ -50,3 +50,17 @@ fn skip_out_of_line_nested_inline_within_out_of_line() {
         &["tests/mod-resolver/skip-files-issue-5065/one.rs"],
     );
 }
+
+#[test]
+fn fmt_out_of_line_test_modules() {
+    // See also https://github.com/rust-lang/rustfmt/issues/5119
+    verify_mod_resolution(
+        "tests/mod-resolver/test-submodule-issue-5119/tests/test1.rs",
+        &[
+            "tests/mod-resolver/test-submodule-issue-5119/tests/test1.rs",
+            "tests/mod-resolver/test-submodule-issue-5119/tests/test1/sub1.rs",
+            "tests/mod-resolver/test-submodule-issue-5119/tests/test1/sub2.rs",
+            "tests/mod-resolver/test-submodule-issue-5119/tests/test1/sub3/sub4.rs",
+        ],
+    )
+}
diff --git a/tests/cargo-fmt/main.rs b/tests/cargo-fmt/main.rs
index 5493b09e4aa..bf81f253f69 100644
--- a/tests/cargo-fmt/main.rs
+++ b/tests/cargo-fmt/main.rs
@@ -2,6 +2,7 @@
 
 use std::env;
 use std::process::Command;
+use std::path::Path;
 
 /// Run the cargo-fmt executable and return its output.
 fn cargo_fmt(args: &[&str]) -> (String, String) {
@@ -71,3 +72,26 @@ fn rustfmt_help() {
     assert_that!(&["--", "-h"], contains("Format Rust code"));
     assert_that!(&["--", "--help=config"], contains("Configuration Options:"));
 }
+
+#[test]
+fn cargo_fmt_out_of_line_test_modules() {
+    // See also https://github.com/rust-lang/rustfmt/issues/5119
+    let expected_modified_files = [
+        "tests/mod-resolver/test-submodule-issue-5119/src/lib.rs",
+        "tests/mod-resolver/test-submodule-issue-5119/tests/test1.rs",
+        "tests/mod-resolver/test-submodule-issue-5119/tests/test1/sub1.rs",
+        "tests/mod-resolver/test-submodule-issue-5119/tests/test1/sub2.rs",
+        "tests/mod-resolver/test-submodule-issue-5119/tests/test1/sub3/sub4.rs",
+    ];
+    let args = [
+        "-v",
+        "--check",
+        "--manifest-path",
+        "tests/mod-resolver/test-submodule-issue-5119/Cargo.toml",
+    ];
+    let (stdout, _) = cargo_fmt(&args);
+    for file in expected_modified_files {
+        let path = Path::new(file).canonicalize().unwrap();
+        assert!(stdout.contains(&format!("Diff in {}", path.display())))
+    }
+}
diff --git a/tests/mod-resolver/test-submodule-issue-5119/Cargo.toml b/tests/mod-resolver/test-submodule-issue-5119/Cargo.toml
new file mode 100644
index 00000000000..0993f127959
--- /dev/null
+++ b/tests/mod-resolver/test-submodule-issue-5119/Cargo.toml
@@ -0,0 +1,8 @@
+[package]
+name = "rustfmt-test-submodule-issue"
+version = "0.1.0"
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
diff --git a/tests/mod-resolver/test-submodule-issue-5119/src/lib.rs b/tests/mod-resolver/test-submodule-issue-5119/src/lib.rs
new file mode 100644
index 00000000000..3f7ddba8a28
--- /dev/null
+++ b/tests/mod-resolver/test-submodule-issue-5119/src/lib.rs
@@ -0,0 +1,7 @@
+pub fn foo() -> i32 {
+3
+}
+
+pub fn bar() -> i32 {
+4
+}
diff --git a/tests/mod-resolver/test-submodule-issue-5119/tests/test1.rs b/tests/mod-resolver/test-submodule-issue-5119/tests/test1.rs
new file mode 100644
index 00000000000..da4e86169ad
--- /dev/null
+++ b/tests/mod-resolver/test-submodule-issue-5119/tests/test1.rs
@@ -0,0 +1,8 @@
+mod test1 {
+#[cfg(unix)]
+mod sub1;
+#[cfg(not(unix))]
+mod sub2;
+
+mod sub3;
+}
diff --git a/tests/mod-resolver/test-submodule-issue-5119/tests/test1/sub1.rs b/tests/mod-resolver/test-submodule-issue-5119/tests/test1/sub1.rs
new file mode 100644
index 00000000000..b760ba23cd2
--- /dev/null
+++ b/tests/mod-resolver/test-submodule-issue-5119/tests/test1/sub1.rs
@@ -0,0 +1,6 @@
+use rustfmt_test_submodule_issue::foo;
+
+#[test]
+fn test_foo() {
+assert_eq!(3, foo());
+}
diff --git a/tests/mod-resolver/test-submodule-issue-5119/tests/test1/sub2.rs b/tests/mod-resolver/test-submodule-issue-5119/tests/test1/sub2.rs
new file mode 100644
index 00000000000..4fd8286eac4
--- /dev/null
+++ b/tests/mod-resolver/test-submodule-issue-5119/tests/test1/sub2.rs
@@ -0,0 +1,6 @@
+use rustfmt_test_submodule_issue::bar;
+
+#[test]
+fn test_bar() {
+assert_eq!(4, bar());
+}
diff --git a/tests/mod-resolver/test-submodule-issue-5119/tests/test1/sub3/mod.rs b/tests/mod-resolver/test-submodule-issue-5119/tests/test1/sub3/mod.rs
new file mode 100644
index 00000000000..e029785bc24
--- /dev/null
+++ b/tests/mod-resolver/test-submodule-issue-5119/tests/test1/sub3/mod.rs
@@ -0,0 +1 @@
+mod sub4;
diff --git a/tests/mod-resolver/test-submodule-issue-5119/tests/test1/sub3/sub4.rs b/tests/mod-resolver/test-submodule-issue-5119/tests/test1/sub3/sub4.rs
new file mode 100644
index 00000000000..e69de29bb2d
--- /dev/null
+++ b/tests/mod-resolver/test-submodule-issue-5119/tests/test1/sub3/sub4.rs