Response Files

Response files are a way for compilers to accept arguments that would otherwise overflow the character limit in the command line. On Windows in particular, the character limit per command is 8191 characters. These files can contain additional options that the compiler will read and process as if they were provided in the original command. Each compiler that supports response files has different formats/expectations and implementations. Support for response files are also re-implemented per compiler by sccache so it can cache compilations accurately. There is currently support for response files on the gcc and msvc implementations in sccache.

GCC

As defined by the gcc docs:

Options in a response file are inserted in-place in the original command line. If the file does not exist or cannot be read, the option will be treated literally, and not removed.
Options in a response file are separated by whitespace.
Single or double quotes can be used to include whitespace in an option.
Any character (including a backslash) may be included by prefixing the character to be included with a backslash (e.g. \\, \?, \@, etc).
The response file may itself contain additional @file options; any such options will be processed recursively.

Implementation details:

The gcc implementation in sccache supports all of these except #3. If a response file contains any quotations (" or '), the @file arg is treated literally and not removed (and its content not processed).
Additionally, sccache will not expand concatenated arguments such as -include@foo (see #150 for more on this).
Recursive files are processed depth-first; when an @file option is encountered, its contents are read and each option is evaluated in-place before continuing to options following the @file.

MSVC

Per the MSVC docs:

The contents of a response file are inserted in-place in the original command.
Response files can contain multiple lines of options, but each option must begin and end on the same line.
Backslashes (\) cannot be used to combine options across multiple lines.
The /link directive has special treatment:
1. Entering an @file: if the /link option is provided prior to an @file in the command line, the /link directive does not affect any options within the @file.
2. Newlines: A /link directive provided in an @file on one line does not affect the next line.
3. Exiting an @file: A /link directive on the final line of a response file does not affect options following the @file option in the command line.
A response file cannot contain additional @file options, they are not recursive. (found in a separate doc)
(implied) options can be wrapped in double-quotes ("), which allows whitespace to be preserved within the option

The msvc implementation in sccache supports all of these except #4, because sccache doesn't accept the /link directive.

Additionally, because msbuild generates response files using an encoding other than utf-8, all text files under the WHATWG encoding standard are supported. This includes both utf-8 and utf-16.

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