[clang-format] Add option to explicitly specify a config file

[zwliew]

Summary

This diff extends the -style=file option to allow a config file to be specified explicitly. This is useful (for instance) when adding IDE commands to reformat code to a personal style.

Usage: clang-format -style=file:<path/to/config/file> ...

[tnorth]

Ping.

[MyDeveloperDay]

In principle, I think this is something that might help a lot of people as I've seen people asking for a better mechanism to be able to load a centrally stored .clang-format file.

but maybe wait a couple of days for others to comment @klimek , @mitchell-stellar , @sammccall ...

Also you need to make full context diffs, plus please add a release note, and because I can't check the full diff, I'm unsure if your Format.h change means we need to regenerate anything in the ClangFormatStyleOptions.rst

[sammccall]

In this patch, relative paths are relative to the working directory (or at least the current directory of the VFS).

This makes sense for command-line args, but if I understand correctly this patch will also allow BasedOnStyle: file:some/path. Is that the case?
In that case, interpreting paths relative to the working directory seems suspicious. I'd suggest one of:

• BasedOnStyle: file:... is not allowed
• BasedOnStyle: file:... must be an absolute path (note this probably means it's not portable across OSs)
• BasedOnStyle: file:relative/path is relative to the current config file, or working directory if the current config file is inline in command line args.

include/clang/Format/Format.h

2329 ↗

(On Diff #236561)

Please upload using full context (e.g. with arc diff)

2332 ↗

(On Diff #236561)

why does this need to be a public API?

2343 ↗

(On Diff #236561)

this function isn't implemented - LoadConfigFile vs loadConfigFile

lib/Format/Format.cpp

2573 ↗

(On Diff #236561)

Expected<FormatStyle> or ErrorOr<FormatStyle> would be a better return type for this function I think.
Read isn't going to return unsuitable, so plumbing through multiple error codes doesn't seem worthwhile.

2613 ↗

(On Diff #236561)

just slice the stringref rather than copy here?

2615 ↗

(On Diff #236561)

why stat the file and check, rather than just try to read it?

unittests/Format/FormatTest.cpp

14370 ↗

(On Diff #236561)

Can you add this via a real absolute path, to show this is actually relative path handling (and not just naive string manipulation by memory file system etc)

[tnorth]

Thanks for your time and feedback.

• Update diff with context,
• remove public declaration of LoadConfigFile,
• change prototype of LoadConfigFile and simplify it,
• avoid string copy to ConfigFile, use StringRef directly,
• don't stat the file before attempting to read it.

Regarding the tests, I am unsure how to do this currenlty (I'd need to read some docs/other examples).
I didn't have in mind the use case of BasedOnStyle: file:some/path.

[tnorth]

• Add a unit-test loading a format and test file from temporary files.

[tnorth]

This makes sense for command-line args, but if I understand correctly this patch will also allow BasedOnStyle: file:some/path. Is that the case?

No, it should not, and I also think it's better not to.

I think that all points are addressed now. Looking forward to have your feedback.

[MyDeveloperDay]

Nit: please add a release note and regenerate the ClangFormatStyleOptions.rst (if there are any changes because you modified Format.h).

[tnorth]

• Add release notes
• Update ClangFormat.rst and ClangFormatStyleOption.rst

[tnorth]

In D72326#1845342, @MyDeveloperDay wrote:

Nit: please add a release note and regenerate the ClangFormatStyleOptions.rst (if there are any changes because you modified Format.h).

Hmm, I tried to run docs/tools/dump_format_style.py, but it fails at two locations (once because a comment in Format.h has two slashes instead of 3, one because of an empty line). After fixing those, it seems that the generated file contains less information than the commited one. This file was probably updated manually as well. Therefore I also did in the updated diff... one should probably fix this separately.
Release note + ClangFormat.rst files update in the diff as well.

[tnorth]

Ping.

[MyDeveloperDay]

In the absence of any other comments, This LGTM, this may help to resolve D68569: [clang-format] Also look for .{ext}.clang-format file, I think I prefer this solution.

[tnorth]

Ping. I guess we need more approval to go forward?

[lebedev.ri]

include/clang/Format/Format.h

2341 ↗

(On Diff #241155)

So is it : or =?

[tnorth]

Fix comment to file:<configfile> instead of the incorrect file=<configfile>

[tnorth]

include/clang/Format/Format.h

2341 ↗

(On Diff #241155)

: is correct, fixed.

[tnorth]

Ping.

[MyDeveloperDay]

LGTM

[tnorth]

Thanks @MyDeveloperDay . I guess more approvals are needed at this point?

[mitchell-stellar]

It's not more approval that is needed, it's just that someone with commit access (assuming you do not) needs to find the time to commit this. For what it's worth, I'm getting a patch rejection for the 4th block in lib/Format/Format.cpp. It seems the contents of LoadConfigFile need to be updated to reflect the most recent changes, so please rebase against master when you can.

[tnorth]

Rebased on master (6d5603e2). Some refactoring was indeed needed.

[mitchell-stellar]

Reverted this commit due to an unexpected test failure:

******************** TEST 'Clang :: Format/dump-config-objc.h' FAILED ********************
Script:
--
: 'RUN: at line 1';   /b/1/clang-x86_64-debian-fast/llvm.obj/bin/clang-format -dump-config /b/1/clang-x86_64-debian-fast/llvm.src/clang/test/Format/dump-config-objc.h | /b/1/clang-x86_64-debian-fast/llvm.obj/bin/FileCheck /b/1/clang-x86_64-debian-fast/llvm.src/clang/test/Format/dump-config-objc.h
--
Exit Code: 1

Command Output (stderr):
--
/b/1/clang-x86_64-debian-fast/llvm.src/clang/test/Format/dump-config-objc.h:3:11: error: CHECK: expected string not found in input
// CHECK: Language: ObjC
          ^
<stdin>:1:1: note: scanning from here
---
^
<stdin>:2:1: note: possible intended match here
Language: Cpp
^

--

********************

I don't know enough about this patch in order to determine what the issue is, or how to proceed further. Perhaps @MyDeveloperDay will chime in.

[tnorth]

Ping @MyDeveloperDay , any clue about what's going on here?

[tnorth]

Ping?

[MyDeveloperDay]

clang/lib/Format/Format.cpp

3244

if you return here IsSuitable will be true..

3251

again here you need to set IsSutable to false

3253

maybe its better just to set this to true here *IsSuitable = true; and set it to false on line @2695

3365

this could then mean you return with something wrong here..

[tnorth]

Sorry.

[MyDeveloperDay]

clang/lib/Format/Format.cpp

3248

I think this isn't needed won't it already be false from line @2695

[MyDeveloperDay]

Were you able to rerun these tests?

[MyDeveloperDay]

These tests still fail

running the following in the build directory (if your build directory is side-by-side with the llvm-project directory):

c:/Python37/python ./bin/llvm-lit.py -v ./tools/clang/test/Format

$ ./run_format_lit_tests.sh
llvm-lit.py: C:\cygwin64\buildareas\clang\build\bin\..\..\llvm-project\llvm\utils\lit\lit\llvm\config.py:343: note: using clang: c:\cygwin64\buildareas\clang\build\bin\clang.exe
-- Testing: 21 tests, 12 workers --
PASS: Clang :: Format/cursor.cpp (1 of 21)
FAIL: Clang :: Format/dump-config-objc.h (2 of 21)
******************** TEST 'Clang :: Format/dump-config-objc.h' FAILED ********************
Script:
--
: 'RUN: at line 1';   c:\cygwin64\buildareas\clang\build\bin\clang-format.exe -dump-config C:\cygwin64\buildareas\clang\llvm-project\clang\test\Format\dump-config-objc.h | c:\cygwin64\buildareas\clang\build\bin\filecheck.exe C:\cygwin64\buildareas\clang\llvm-project\clang\test\Format\dump-config-objc.h
--
Exit Code: 1

Command Output (stdout):
--
$ ":" "RUN: at line 1"
$ "c:\cygwin64\buildareas\clang\build\bin\clang-format.exe" "-dump-config" "C:\cygwin64\buildareas\clang\llvm-project\clang\test\Format\dump-config-objc.h"
$ "c:\cygwin64\buildareas\clang\build\bin\filecheck.exe" "C:\cygwin64\buildareas\clang\llvm-project\clang\test\Format\dump-config-objc.h"
# command stderr:
C:\cygwin64\buildareas\clang\llvm-project\clang\test\Format\dump-config-objc.h:3:11: error: CHECK: expected string not found in input
// CHECK: Language: ObjC
          ^
<stdin>:1:1: note: scanning from here
---
^
<stdin>:2:1: note: possible intended match here
Language: Cpp
^

error: command failed with exit status: 1

--

********************

[MyDeveloperDay]

clang/lib/Format/Format.cpp

3237

pass Style in, don't keep assuming LLVM

3238

You cannot keep assuming the style is LLVM, Style needs to be passed in

/// Attempts to load a format file
llvm::Expected<FormatStyle> LoadConfigFile(StringRef ConfigFile,
                                           llvm::vfs::FileSystem *FS,
                                           bool *IsSuitable,
                                           FormatStyle Style) {
  *IsSuitable = false;

  llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> Text =
      FS->getBufferForFile(ConfigFile.str());
  if (std::error_code EC = Text.getError())
    return make_string_error(EC.message());
  std::error_code ParserErrorCode =
      parseConfiguration(Text.get()->getBuffer(), &Style);
  if (ParserErrorCode == ParseError::Unsuitable) {
    return Style;
  } else if (ParserErrorCode != ParseError::Success) {
    return make_string_error("Error reading " + ConfigFile + ": " +
                             ParserErrorCode.message());
  }
  *IsSuitable = true;
  return Style;
}

Now your code will read

// User provided clang-format file using -style=file:/path/to/format/file
// Check for explicit config filename
if (StyleName.startswith_lower("file:")) {
  auto StyleNameFile = StyleName.substr(5);
  bool IsSuitable = true;
  auto FileStyle = LoadConfigFile(StyleNameFile, FS, &IsSuitable, Style);
  if (FileStyle && !IsSuitable) {
    return make_string_error("Configuration file(s) do(es) not support " +
                             getLanguageName((*FileStyle).Language) + ": " +
                             StyleNameFile);
  }
  return FileStyle;
}

`

And then in the loop of files you pass the Style in too
if (auto ConfigStyle =

LoadConfigFile(ConfigFile, FS, &IsSuitable, Style)) {

3286

pass Style in and rename return value

3336

We should probably keep something like this so we know which config file we are loading

3358

Pass current Style in

[MyDeveloperDay]

ping @tnorth are you planning on fixing this as its currently reverted.

[MyDeveloperDay]

@tnorth Are you planning on working on this or do you mind if one of us fixes the issues enough to get it over the line. (this was previously accepted and landed but the unit tests fail, for what looks like a fairly minor issue)

[MyDeveloperDay]

After rebasing these tests seem to work (on Windows)

-- Testing: 24 tests, 16 workers --
PASS: Clang :: Format/dry-run-alias.cpp (1 of 24)
PASS: Clang :: Format/remove-duplicate-includes.cpp (2 of 24)
PASS: Clang :: Format/disable-format.cpp (3 of 24)
PASS: Clang :: Format/cursor.cpp (4 of 24)
PASS: Clang :: Format/multiple-inputs.cpp (5 of 24)
PASS: Clang :: Format/basic.cpp (6 of 24)
PASS: Clang :: Format/incomplete.cpp (7 of 24)
PASS: Clang :: Format/line-ranges.cpp (8 of 24)
PASS: Clang :: Format/adjust-indent.cpp (9 of 24)
UNSUPPORTED: Clang :: Format/style-on-command-line.cpp (10 of 24)
PASS: Clang :: Format/ranges.cpp (11 of 24)
PASS: Clang :: Format/access-modifiers.cpp (12 of 24)
PASS: Clang :: Format/multiple-inputs-error.cpp (13 of 24)
PASS: Clang :: Format/dump-config-objc.h (14 of 24)
PASS: Clang :: Format/xmloutput.cpp (15 of 24)
PASS: Clang :: Format/dump-config-cxx.h (16 of 24)
PASS: Clang :: Format/multiple-inputs-inplace.cpp (17 of 24)
PASS: Clang :: Format/error-config.cpp (18 of 24)
PASS: Clang :: Format/dry-run.cpp (19 of 24)
PASS: Clang :: Format/inplace.cpp (20 of 24)
PASS: Clang :: Format/dump-config-list-override.cpp (21 of 24)
PASS: Clang :: Format/disable-include-sorting.cpp (22 of 24)
PASS: Clang :: Format/language-detection.cpp (23 of 24)
PASS: Clang :: Format/verbose.cpp (24 of 24)

I'm going to Commandeer this revision and see if we can't get this relanded.

[MyDeveloperDay]

Rebasing previously landed but reverted commit

[MyDeveloperDay]

clang/lib/Format/Format.cpp

3281

part of me wonders if this format should be

file://<path> rather than file:<path>

User provided clang-format file using -style=file:///path/to/format/file

vs

User provided clang-format file using -style=file:/path/to/format/file

This would leave the way open to other protocols http:// or https:// or anything else.

[HazardyKnusperkeks]

clang/lib/Format/Format.cpp

3281

Maybe, but I doubt we want http. And if we use file:// a windows path D:\Path\ would not be valid, would it?

[MyDeveloperDay]

The reason I have picked this us was because of:

https://twitter.com/bruxisma/status/1462987809879257101

This slightly annoys me because :

a) What they were talking about was in my view is disrespectful and inaccurate.
b) I thought we'd already landed this (which we had)

I went looking for this review which had previously been accepted and landed, but got reverted because it seemed to fail the tests

The original author and the original-original-author has both obviously moved on and dropped it and so it didn't get fixed.

I don't like wasting all that effort, especailly if we are going to get grief for it. So I rebased the review so we can land it again, (and checked both the unit tests and lit tests)

I hope we don't have to go around the houses on this too much.

clang/lib/Format/Format.cpp

3281

yes file://C:\Windows\Kernel32.dll is a valid file url.

but I on reflection I think file: will be ok.

[tnorth]

Hi there,
Yes, sorry for the long silence, I ended up not having time to come back to this anymore for personal reasons. I also lack familiarity with the codebase and tests, and I couldn't invest more time into it; I had the impression that this last failing test would require major changes.
I'm happy if this ends up being merged. Thanks for your time and guidance again.

[MyDeveloperDay]

Ping! Could we see this previously approved patch over the line I’d like to use it to build a regression suit of code and format files without the need for every test to be in its own directory

[HazardyKnusperkeks]

clang/lib/Format/Format.cpp

3249

else after return

But I prefer a switch on ParseErrorCode.

3280

Why move that, it it's not used here?

3286

Why did you not follow your own comment?

[zwliew]

Hi, I'd like to help to get this patch accepted and merged. I have a few suggestions/questions below, and I can help make any changes to the patch if needed!

clang/docs/ClangFormatStyleOptions.rst

35

I think two backticks are missing here.

clang/include/clang/Format/Format.h

4069

To be consistent with the ClangFormatOptions.rst docs, I think configfile should be changed to format_file_path.

clang/lib/Format/Format.cpp

3184

Similar to above, configfile should be changed to format_file_path.

3237

I think it's still a good idea to pass Style into the function instead.

3237

I think LoadConfigFile could be refactored to return std::error_code instead, similar to parseConfiguration(), like so:

// note I renamed LoadConfigFile to loadConfigFile to follow the existing function naming style.
std::error_code loadConfigFile(StringRef ConfigFile, llvm::vfs::FileSystem *FS, FormatStyle *Style) {
    llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> Text = FS->getBufferForFile(ConfigFile.str());
    if (auto ec = Text.getError()) {
        return ec;
    }
    return parseConfiguration(Text.get()->getBuffer(), Style);
}

And the part in getStyle() would look like this:

// Check for explicit config filename
if (StyleName.startswith_insensitive("file:")) {
    auto StyleFileName = StyleName.substr(5);
    if (auto ec = loadConfigFile(StyleFileName, FS, &Style)) {
        return make_string_error(ec.message());
    }
    LLVM_DEBUG(llvm::dbgs() << "Using configuration file " << StyleFileName << '\n');
    return Style;
}

What do you think?

3280

Yup, I think this shouldn't be moved.

3283

Following my code suggestion above, I think this code block could be refactored to this:

// Check for explicit config filename
if (StyleName.startswith_insensitive("file:")) {
    auto StyleFileName = StyleName.substr(5);
    if (auto ec = loadConfigFile(StyleFileName, FS, &Style)) {
        return make_string_error(ec.message());
    }
    LLVM_DEBUG(llvm::dbgs() << "Using configuration file " << StyleFileName << '\n');
    return Style;
}

What do you think?

Also, on another note, I feel like the -style option is too overloaded. Would it be better to use a separate option like -style-file instead?

[zwliew]

On further thought, the logic for loadConfigFile() looks incomplete. It does not properly handle BasedOnStyle: InheritParentConfig. In fact, the logic for -style=file can be pulled out and merged into loadConfigFile().

I'll look into making this change, and possibly refactoring the code to reduce code duplication.

[MyDeveloperDay]

Please go ahead

[zwliew]

Support inheritance from parent configs via BasedOnStyle: InheritParentConfig

I made the following changes:

1. Don't return early when BasedOnStyle: InheritParentConfig is set. Also search for parent configs starting from the parent directory of the specified config file.

2. Remove the LoadConfigFile() function. There isn't a clean way to refactor the --style=file logic to use it and The function is small and only used in 1/2 places, so I don't think is much of a reason to keep it.

Please have a look and let me know your thoughts, thanks! Also, this is my first time using Phabricator, so let me know if I did anything wrong.

Side question: Is there a reason that the number of children configs in the fallback case is limited to 1 (that is, when all the configs have BasedOnStyle: InheritParentConfig set)?

[zwliew]

clang/lib/Format/Format.cpp

3401–3402

Is there a reason for this to be limited to 1? I came across this case during testing, but I can't think of why this should be so.

[zwliew]

Add a test for inheritance from parent config with base

[HazardyKnusperkeks]

clang/lib/Format/Format.cpp

3283

You can certainly refactor code.

What happens with -style="Google" -style-file="Foo/Bar" is it different from -style-file="Foo/Bar" -style="Google"?
I do not perse disagree, but I think it makes stuff clearer if there is only one option.

3401–3402

See comment on D93844.

[zwliew]

clang/lib/Format/Format.cpp

3283

Fair enough. Some ideas off the top of my head that resolve the ambiguity sound needlessly complicated. Thanks!

[HazardyKnusperkeks]

Do you plan to refactor something for this review, or do you think you are done? Then I will look at it again as a whole.

[zwliew]

In D72326#3211701, @HazardyKnusperkeks wrote:

Do you plan to refactor something for this review, or do you think you are done? Then I will look at it again as a whole.

I'm going to try refactoring the code for loading and parsing the config file into a separate function. I'll update the diff in no longer than a day.

[zwliew]

I am done. Please review the latest changes. Thanks!

I made the following changes:

1. Refactor the code for loading and parsing configs into a separate function

2. Add a new test case (9.1.2) to test the case mentioned in https://reviews.llvm.org/D93844#inline-1112285.

3. Fix the test case failure for the newly added test 9.1.2.

[zwliew]

clang/lib/Format/Format.cpp

3283

I've refactored the code in the latest revision (under loadAndParseConfigFile().

3401–3402

I've added a new test case (9.1.2) to illustrate the test failure, and also fixed the code to fix the test case.

[curdeius]

LGTM but please wait for the green light from other involved reviewers.
Also, as noted in a comment, I'd like to see unrelated changes in a different patch (unless you convince me that it's overly complicated or too much dependent on this patch).

clang/lib/Format/Format.cpp

3298

Here and elsewhere: please end the comments with a full stop.

clang/unittests/Format/FormatTest.cpp

21466–21467

?

21483

Can we split changes unrelated to the specific configuration file (fixing/testing the inherited config) to another patch?

[curdeius]

Oh, and please rename the patch before landing.

[zwliew]

Addressed the comments on the previous diff:

1. Adjusted some comments; suffixed each comment with a period.
2. Removed the changes unrelated to this revision (the ones the issue mentioned in the latest comments in https://reviews.llvm.org/D93844)

[zwliew]

Thanks for the review. I've moved the unrelated change to https://reviews.llvm.org/D116371 instead.

[MyDeveloperDay]

LGTM

[HazardyKnusperkeks]

Although there is no context. Maybe still update the uploaded diff for the archive.

[zwliew]

Rebased on master for context

[zwliew]

Oh no...I'm so sorry. Didn't mean to cause this large diff. Trying to fix it now.

[zwliew]

Fixed rebase and diff, and tried to restore the reviewers and subscribers.

[HazardyKnusperkeks]

Do you have commit access, or do you need someone to commit this and your other changes? If the latter we need a name and an email for the commit. Then someone will come along and commit it.

[zwliew]

In D72326#3214265, @HazardyKnusperkeks wrote:

Do you have commit access, or do you need someone to commit this and your other changes? If the latter we need a name and an email for the commit. Then someone will come along and commit it.

I don't think I have commit access, so I would be grateful if someone helps me do it.

Name:
Zhao Wei Liew

Email:
[e-mail]

clang/docs/ClangFormat.rst

Clang-format options:
     --sort-includes            - If set, overrides the include sorting behavior
                                  determined by the SortIncludes style flag
     --style=<string>           - Coding style, currently supports:
                                    LLVM, Google, Chromium, Mozilla, WebKit.
                                  Use -style=file to load style configuration from
                                  .clang-format file located in one of the parent
                                  directories of the source file (or current
                                  directory for stdin).
+                                 Use -style=file:<format_file_path> to load style
+                                 configuration from a format file located at
+                                 <format_file_path>. This path can be absolute or
+                                 relative to the working directory.
                                  Use -style="{key: value, ...}" to set specific
                                  parameters, e.g.:
                                    -style="{BasedOnStyle: llvm, IndentWidth: 8}"
     --verbose                  - If set, shows the list of processed files
 
   Generic Options:
 
     --help                     - Display available options (--help-hidden for more)

clang/docs/ClangFormatStyleOptions.rst

 use ``-style=file`` and put style configuration in the ``.clang-format`` or
 ``_clang-format`` file in the project directory.
 
 When using ``-style=file``, :program:`clang-format` for each input file will
 try to find the ``.clang-format`` file located in the closest parent directory
 of the input file. When the standard input is used, the search is started from
 the current directory.
 
+When using ``-style=file:<format_file_path>``, :program:`clang-format` for
+each input file will use the format file located at `<format_file_path>`.
+The path may be absolute or relative to the working directory.
+
 The ``.clang-format`` file uses YAML format:
 
 .. code-block:: yaml
 
   key1: value1
   key2: value2
   # A comment.
   ...

clang/docs/ReleaseNotes.rst

 - Option ``QualifierOrder`` has been added to allow the order
   `const` `volatile` `static` `inline` `constexpr` `restrict`
   to be controlled relative to the `type`.
 
 - Add a ``Custom`` style to ``SpaceBeforeParens``, to better configure the
   space before parentheses. The custom options can be set using
   ``SpaceBeforeParensOptions``.
 
+- The command line argument `-style=<string>` has been extended so that a specific
+  format file at location <format_file_path> can be selected. This is supported
+  via the syntax: `-style=file:<format_file_path>`.
+
 - Improved C++20 Modules and Coroutines support.
 
 libclang
 --------
 
 - ...
 
 Static Analyzer

clang/include/clang/Format/Format.h

 ///
 /// ``StyleName`` can take several forms:
 /// * "{<key>: <value>, ...}" - Set specic style parameters.
 /// * "<style name>" - One of the style names supported by
 /// getPredefinedStyle().
 /// * "file" - Load style configuration from a file called ``.clang-format``
 /// located in one of the parent directories of ``FileName`` or the current
 /// directory if ``FileName`` is empty.
+/// * "file:<format_file_path>" to explicitly specify the configuration file to
+/// use.
 ///
 /// \param[in] StyleName Style name to interpret according to the description
 /// above.
 /// \param[in] FileName Path to start search for .clang-format if ``StyleName``
 /// == "file".
 /// \param[in] FallbackStyle The name of a predefined style used to fallback to
 /// in case \p StyleName is "file" and no file can be found.
 /// \param[in] Code The actual code to be formatted. Used to determine the

clang/lib/Format/Format.cpp

 
 const char *StyleOptionHelpDescription =
     "Coding style, currently supports:\n"
     "  LLVM, GNU, Google, Chromium, Microsoft, Mozilla, WebKit.\n"
     "Use -style=file to load style configuration from\n"
     ".clang-format file located in one of the parent\n"
     "directories of the source file (or current\n"
     "directory for stdin).\n"
+    "Use -style=file:<format_file_path> to explicitly specify"
+    "the configuration file.\n"
     "Use -style=\"{key: value, ...}\" to set specific\n"
     "parameters, e.g.:\n"
     "  -style=\"{BasedOnStyle: llvm, IndentWidth: 8}\"";
 
 static FormatStyle::LanguageKind getLanguageByFileName(StringRef FileName) {
   if (FileName.endswith(".java"))
     return FormatStyle::LK_Java;
   if (FileName.endswith_insensitive(".js") ||
if (GuessedLanguage == FormatStyle::LK_Cpp) {
     }
   }
   return GuessedLanguage;
 }
 
 const char *DefaultFormatStyle = "file";
 
 const char *DefaultFallbackStyle = "LLVM";
 
+llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>>
+loadAndParseConfigFile(StringRef ConfigFile, llvm::vfs::FileSystem *FS,
+                       FormatStyle *Style, bool AllowUnknownOptions) {
+  llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> Text =
+      FS->getBufferForFile(ConfigFile.str());
+  if (auto EC = Text.getError())
+    return EC;
+  if (auto EC = parseConfiguration(*Text.get(), Style, AllowUnknownOptions))
+    return EC;
+  return Text;
+}
+
 llvm::Expected<FormatStyle> getStyle(StringRef StyleName, StringRef FileName,
                                      StringRef FallbackStyleName,
                                      StringRef Code, llvm::vfs::FileSystem *FS,
                                      bool AllowUnknownOptions) {
   if (!FS) {
     FS = llvm::vfs::getRealFileSystem().get();
   }
   FormatStyle Style = getLLVMStyle(guessLanguage(FileName, Code));
 
   FormatStyle FallbackStyle = getNoStyle();
   if (!getPredefinedStyle(FallbackStyleName, Style.Language, &FallbackStyle))
     return make_string_error("Invalid fallback style \"" + FallbackStyleName);
if (std::error_code ec =
       return make_string_error("Error parsing -style: " + ec.message());
     if (Style.InheritsParentConfig)
       ChildFormatTextToApply.emplace_back(
           llvm::MemoryBuffer::getMemBuffer(StyleName, Source, false));
     else
       return Style;
   }
 
+  // User provided clang-format file using -style=file:path/to/format/file.
+  if (!Style.InheritsParentConfig &&
+      StyleName.startswith_insensitive("file:")) {
+    auto ConfigFile = StyleName.substr(5);
+    llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> Text =
+        loadAndParseConfigFile(ConfigFile, FS, &Style, AllowUnknownOptions);
+    if (auto EC = Text.getError())
+      return make_string_error("Error reading " + ConfigFile + ": " +
+                               EC.message());
+
+    LLVM_DEBUG(llvm::dbgs()
+               << "Using configuration file " << ConfigFile << "\n");
+
+    if (!Style.InheritsParentConfig)
+      return Style;
+
+    // Search for parent configs starting from the parent directory of
+    // ConfigFile.
+    FileName = ConfigFile;
+    ChildFormatTextToApply.emplace_back(std::move(*Text));
+  }
+
   // If the style inherits the parent configuration it is a command line
   // configuration, which wants to inherit, so we have to skip the check of the
   // StyleName.
   if (!Style.InheritsParentConfig && !StyleName.equals_insensitive("file")) {
     if (!getPredefinedStyle(StyleName, Style.Language, &Style))
       return make_string_error("Invalid value for -style");
     if (!Style.InheritsParentConfig)
       return Style;
for (const auto &F : FilesToLookFor) {
       llvm::sys::path::append(ConfigFile, F);
       LLVM_DEBUG(llvm::dbgs() << "Trying " << ConfigFile << "...\n");
 
       Status = FS->status(ConfigFile.str());
 
       if (Status &&
           (Status->getType() == llvm::sys::fs::file_type::regular_file)) {
         llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> Text =
-            FS->getBufferForFile(ConfigFile.str());
-        if (std::error_code EC = Text.getError())
-          return make_string_error(EC.message());
-        if (std::error_code ec =
-                parseConfiguration(*Text.get(), &Style, AllowUnknownOptions)) {
-          if (ec == ParseError::Unsuitable) {
+            loadAndParseConfigFile(ConfigFile, FS, &Style, AllowUnknownOptions);
+        if (auto EC = Text.getError()) {
+          if (EC == ParseError::Unsuitable) {
             if (!UnsuitableConfigFiles.empty())
               UnsuitableConfigFiles.append(", ");
             UnsuitableConfigFiles.append(ConfigFile);
             continue;
           }
           return make_string_error("Error reading " + ConfigFile + ": " +
-                                   ec.message());
+                                   EC.message());
         }
         LLVM_DEBUG(llvm::dbgs()
                    << "Using configuration file " << ConfigFile << "\n");
 
         if (!Style.InheritsParentConfig) {
           if (ChildFormatTextToApply.empty())
             return Style;
 
           LLVM_DEBUG(llvm::dbgs() << "Applying child configurations\n");
           applyChildFormatTexts(&Style);
 
for (const auto &F : FilesToLookFor) {
       }
     }
   }
   if (!UnsuitableConfigFiles.empty())
     return make_string_error("Configuration file(s) do(es) not support " +
                              getLanguageName(Style.Language) + ": " +
                              UnsuitableConfigFiles);
 
   if (!ChildFormatTextToApply.empty()) {
     LLVM_DEBUG(llvm::dbgs()
                << "Applying child configurations on fallback style\n");
     applyChildFormatTexts(&FallbackStyle);
   }
 
   return FallbackStyle;
 }
 
 } // namespace format
 } // namespace clang

clang/unittests/Format/FormatTest.cpp

TEST(FormatStyle, GetStyleOfFile) {
 
   auto Style7b = getStyle("file", "/d/.clang-format", "LLVM", "", &FS, true);
   ASSERT_TRUE((bool)Style7b);
 
   // Test 8: inferred per-language defaults apply.
   auto StyleTd = getStyle("file", "x.td", "llvm", "", &FS);
   ASSERT_TRUE((bool)StyleTd);
   ASSERT_EQ(*StyleTd, getLLVMStyle(FormatStyle::LK_TableGen));
 
   // Test 9.1.1: overwriting a file style, when no parent file exists with no
   // fallback style.
   ASSERT_TRUE(FS.addFile(
       "/e/sub/.clang-format", 0,
       llvm::MemoryBuffer::getMemBuffer("BasedOnStyle: InheritParentConfig\n"
                                        "ColumnLimit: 20")));
   ASSERT_TRUE(FS.addFile("/e/sub/code.cpp", 0,
                          llvm::MemoryBuffer::getMemBuffer("int i;")));
   auto Style9 = getStyle("file", "/e/sub/code.cpp", "none", "", &FS);
   ASSERT_TRUE(static_cast<bool>(Style9));
   ASSERT_EQ(*Style9, [] {
     auto Style = getNoStyle();
     Style.ColumnLimit = 20;
     return Style;
   }());
 
   // Test 9.1.2: propagate more than one level with no parent file.
   ASSERT_TRUE(FS.addFile("/e/sub/sub/code.cpp", 0,
                          llvm::MemoryBuffer::getMemBuffer("int i;")));
   ASSERT_TRUE(FS.addFile("/e/sub/sub/.clang-format", 0,
                          llvm::MemoryBuffer::getMemBuffer(
                              "BasedOnStyle: InheritParentConfig\n"
                              "WhitespaceSensitiveMacros: ['FOO', 'BAR']")));
   std::vector<std::string> NonDefaultWhiteSpaceMacros{"FOO", "BAR"};
 
TEST(FormatStyle, GetStyleOfFile) {
   Style9 = getStyle("file", "/e/withoutbase/sub/code.cpp", "google", "", &FS);
   ASSERT_TRUE(static_cast<bool>(Style9));
   ASSERT_EQ(*Style9, [] {
     auto Style = getLLVMStyle();
     Style.ColumnLimit = 123;
     Style.IndentWidth = 7;
     return Style;
   }());
+
+  // Test 9.9: use inheritance from a specific config file.
+  Style9 = getStyle("file:/e/sub/sub/.clang-format", "/e/sub/sub/code.cpp",
+                    "none", "", &FS);
+  ASSERT_TRUE(static_cast<bool>(Style9));
+  ASSERT_EQ(*Style9, SubSubStyle);
+}
+
+TEST(FormatStyle, GetStyleOfSpecificFile) {
+  llvm::vfs::InMemoryFileSystem FS;
+  // Specify absolute path to a format file in a parent directory.
+  ASSERT_TRUE(
+      FS.addFile("/e/.clang-format", 0,
+                 llvm::MemoryBuffer::getMemBuffer("BasedOnStyle: LLVM")));
+  ASSERT_TRUE(
+      FS.addFile("/e/explicit.clang-format", 0,
+                 llvm::MemoryBuffer::getMemBuffer("BasedOnStyle: Google")));
+  ASSERT_TRUE(FS.addFile("/e/sub/sub/sub/test.cpp", 0,
+                         llvm::MemoryBuffer::getMemBuffer("int i;")));
+  auto Style = getStyle("file:/e/explicit.clang-format",
+                        "/e/sub/sub/sub/test.cpp", "LLVM", "", &FS);
+  ASSERT_TRUE(static_cast<bool>(Style));
+  ASSERT_EQ(*Style, getGoogleStyle());
+
+  // Specify relative path to a format file.
+  ASSERT_TRUE(
+      FS.addFile("../../e/explicit.clang-format", 0,
+                 llvm::MemoryBuffer::getMemBuffer("BasedOnStyle: Google")));
+  Style = getStyle("file:../../e/explicit.clang-format",
+                   "/e/sub/sub/sub/test.cpp", "LLVM", "", &FS);
+  ASSERT_TRUE(static_cast<bool>(Style));
+  ASSERT_EQ(*Style, getGoogleStyle());
+
+  // Specify path to a format file that does not exist.
+  Style = getStyle("file:/e/missing.clang-format", "/e/sub/sub/sub/test.cpp",
+                   "LLVM", "", &FS);
+  ASSERT_FALSE(static_cast<bool>(Style));
+  llvm::consumeError(Style.takeError());
+
+  // Specify path to a file on the filesystem.
+  SmallString<128> FormatFilePath;
+  std::error_code ECF = llvm::sys::fs::createTemporaryFile(
+      "FormatFileTest", "tpl", FormatFilePath);
+  EXPECT_FALSE((bool)ECF);
+  llvm::raw_fd_ostream FormatFileTest(FormatFilePath, ECF);
+  EXPECT_FALSE((bool)ECF);
+  FormatFileTest << "BasedOnStyle: Google\n";
+  FormatFileTest.close();
+
+  SmallString<128> TestFilePath;
+  std::error_code ECT =
+      llvm::sys::fs::createTemporaryFile("CodeFileTest", "cc", TestFilePath);
+  EXPECT_FALSE((bool)ECT);
+  llvm::raw_fd_ostream CodeFileTest(TestFilePath, ECT);
+  CodeFileTest << "int i;\n";
+  CodeFileTest.close();
+
+  std::string format_file_arg = std::string("file:") + FormatFilePath.c_str();
+  Style = getStyle(format_file_arg, TestFilePath, "LLVM", "", nullptr);
+
+  llvm::sys::fs::remove(FormatFilePath.c_str());
+  llvm::sys::fs::remove(TestFilePath.c_str());
+  ASSERT_TRUE(static_cast<bool>(Style));
+  ASSERT_EQ(*Style, getGoogleStyle());
 }
 
 TEST_F(ReplacementTest, FormatCodeAfterReplacements) {
   // Column limit is 20.
   std::string Code = "Type *a =\n"
                      "    new Type();\n"
                      "g(iiiii, 0, jjjjj,\n"
                      "  0, kkkkk, 0, mm);\n"