Add some guidelines on the CLI.

Author: ehuss

src/SUMMARY.md

@@ -29,6 +29,7 @@
     - [Emitting Errors and other Diagnostics](diagnostics.md)
         - [`LintStore`](./diagnostics/lintstore.md)
         - [Diagnostic Codes](./diagnostics/diagnostic-codes.md)
+    - [Command-line arguments](./cli.md)
     - [ICE-breaker teams](ice-breaker/about.md)
         - ["Cleanup Crew" ICE-breakers](ice-breaker/cleanup-crew.md)
         - [LLVM ICE-breakers](ice-breaker/llvm.md)

src/cli.md

@@ -0,0 +1,29 @@
+# Command-line Arguments
+
+Command-line flags are documented in the [rustc book][cli-docs]. All *stable*
+flags should be documented there. Unstable flags should be documented in the
+[unstable book].
+
+## Guidelines
+
+- Flags should be orthogonal to each other. For example, if we'd have a
+  json-emitting variant of multiple actions `foo` and `bar`, an additional
+  `--json` flag is better than adding `--foo-json` and `--bar-json`.
+- Avoid flags with the `no-` prefix. Instead, use the [`parse_bool`] function,
+  such as `-C embed-bitcode=no`.
+- Consider the behavior if the flag is passed multiple times. In some
+  situations, the values should be accumulated (in order!). In other
+  situations, subsequence flags should override previous flags (for example,
+  the lint-level flags). And some flags (like `-o`) should generate an error
+  if it is too ambiguous what multiple flags would mean.
+- Always give options a long descriptive name, if only for more understandable
+  compiler scripts.
+- The `--verbose` flag is for adding verbose information to `rustc` output
+  when not compiling a program. For example, using it with the `--version`
+  flag gives information about the hashes of the code.
+- Experimental flags and options must be guarded behind the `-Z
+  unstable-options` flag.
+
+[cli-docs]: https://doc.rust-lang.org/rustc/command-line-arguments.html
+[unstable book]: https://doc.rust-lang.org/nightly/unstable-book/
+[`parse_bool`]: https://github.com/rust-lang/rust/blob/e5335592e78354e33d798d20c04bcd677c1df62d/src/librustc_session/options.rs#L307-L313