Add a Stdin Specification

This completes step 1 of
#573 (comment).

Author: jfly

docs/site/reference/formatter-spec.md

@@ -30,8 +30,8 @@ The formatter's CLI must be of the form:
 Where:
 
 - `<command>` is the name of the formatter executable.
-- `[options]` is any number of flags and options that the formatter accepts.
-- `[...<files>]` is one or more files given to the formatter for processing.
+- `[options]` are any number of flags and options that the formatter accepts.
+- `[...<files>]` are one or more files given to the formatter for processing.
 
 Example:
 

docs/site/reference/stdin-spec.md

@@ -0,0 +1,51 @@
+---
+outline: deep
+---
+
+# Stdin Specification
+
+Formatters **MAY** also implement the Stdin Specification, which allows
+formatting "virtual files" passed via stdin.
+
+A formatter **MUST** implement the Stdin Specification if its formatting behavior
+can depend on the name of the file being formatted.
+
+## Rules
+
+In order for the formatter to comply with this spec, it **MUST** implement the
+vanilla [Formatter Specification](/reference/formatter-spec), and additionally
+satisfy the following:
+
+### 1. `--stdin-filepath` flag
+
+The formatter's CLI **MUST** be of the form:
+
+```
+<command> [options] [--stdin-filepath <path>]
+```
+
+Where:
+
+- `<command>` is the name of the formatting tool.
+- `[options]` are any number of flags and options that the formatter accepts.
+- `--stdin-filepath <path>` is an optional flag that puts the formatter in
+  "stdin mode". In stdin mode, the formatter reads file contents from stdin
+  rather than the filesystem.
+    - The formatter _MAY_ understand `--stdin-filepath=<path>` as well, but **MUST**
+      understand the space separated variant.
+
+Example:
+
+```
+$ echo "{}" | nixfmt --stdin-filepath path/to/file.nix
+```
+
+### 2. Print to stdout, do not assume file is present on filesystem
+
+When in stdin mode, the formatter:
+
+1. **MUST** print the formatted file to stdout.
+2. **MUST NOT** attempt to read the file on the filesystem. Instead, it
+   **MUST** read from stdin.
+3. **MUST NOT** write to the given path on the filesytem. It _MAY_ write to
+   temporary files elsewhere on disk, but _SHOULD_ clean them up when done.