stable clippy.toml: Create RFC

This PR creates a public RFC for how clippy.toml should be handled. I made some improvements
to some parts of the system. These are better to get fixed today than to wait until someone has a problem with them
and then have to make a breaking change.

Based on Philipp's RFC. This would be the first stage of 3 to completely stabilize clippy.toml

Author: blyxyas

book/src/SUMMARY.md

@@ -35,4 +35,5 @@
     - [Proposals](development/proposals/README.md)
         - [Roadmap 2021](development/proposals/roadmap-2021.md)
         - [Syntax Tree Patterns](development/proposals/syntax-tree-patterns.md)
+        - [clippy.toml stabilization](development/proposals/clippy-toml-stabilization.md)
     - [The Team](development/the_team.md)

book/src/development/proposals/clippy-toml-stabilization.md

@@ -0,0 +1,127 @@
+# Clippy.toml stabilization
+
+- Start Date: 2025-09-07
+
+**Progress**
+- [ ] Deprecating `CLIPPY_CONF_DIR` and creating `CLIPPY_CONF_PATH` with an improved algorithm
+- [ ] Create the `unstable-conf` key and allow for unknown values.
+- \[0/88\] Options stabilized
+
+# Summary
+
+Stabilization of the Clippy configuration file and other ways to change Clippy’s behaviour via values passed to the
+program at runtime.
+
+This RFC proposes a change to the algorithm that `CLIPPY_CONF_DIR` uses, a deprecation of `CLIPPY_CONF_DIR` in lieu of
+`CLIPPY_CONF_PATH` (allowing for custom-named Clippy configuration files), allowing for several configuration files
+when on a workspace.
+
+# Motivation
+
+High-stakes projects need a guarantee of stabilization in every aspect of their toolchain. Including configuration
+options and default values. One of these high-stakes projects is Rust-for-Linux, which has communicated with the
+Clippy team before about stabilizing a way to pass configuration values to Clippy.
+
+It’s our duty as maintainers to make sure that user’s workflows don’t break. And that they don’t worry about future
+problems in their pipelines because we haven’t formerly stabilized a configuration file.
+
+[Kernel thread talking about how clippy.toml being unstable is worrisome](https://lore.kernel.org/all/[email protected]/)
+[Zulip thread](https://rust-lang.zulipchat.com/#narrow/channel/257328-clippy/topic/stablization.20of.20clippy.2Etoml.20a/with/486169617)
+
+# Guide-level explanation
+
+Some Clippy lints can be tweaked via a configuration file commonly named `clippy.toml` or `.clippy.toml`.
+
+Clippy looks for this file in the current directory and all parents up to the root of the workspace if there is one.
+
+For crates in a workspace, the Clippy configuration files would be merged with the ones in their parent directories.
+A Clippy configuration file will always overwrite one in a parent directory
+
+This means, that if your workspace has `clippy.toml` and `crates/my_crate/clippy.toml`, when Clippy analyzes
+`my_crate`, it will merge both versions with `crates/my_crate/clippy.toml` taking prevalence over `clippy.toml` for
+`my_crate`.
+
+This system can be overrun via the `CLIPPY_CONF_PATH` environment variable passed to the `clippy-driver` or
+`cargo clippy` invocation. If this environment variable is declared and points to a directory, Clippy just looks up
+specifically that directory and assumes that there isn’t a Clippy configuration file if it can’t find `clippy.toml`
+or `.clippy.toml`. If `CLIPPY_CONF_PATH` points to a file, that file will be interpreted as `clippy.toml`.
+
+---
+
+The Clippy configuration file follows a subset of the TOML specification. You can declare key-value pairs within it
+in plain text. Here's an example.
+
+```toml
+unstable-conf = false
+
+msrv = "1.76.0"
+avoid-breaking-exported-api = true
+check-private-items = true
+```
+
+The list of available configurations along their default values is
+[here](https://doc.rust-lang.org/nightly/clippy/lint_configuration.html).
+You can enable unstable features via the `unsable-conf` configuration option. If this configuration is set to `true`,
+Clippy will warn about unknown or renamed keys, and will allow the modification of unstable configuration options. One
+can use `unstable-conf = true` sparingly in their development cycle to check for unknown Rust to check for typos or
+nonexistent configuration options. 
+
+While TOML tables (`[table]`) are currently ignored at the time of writing, they could be added in a future version
+for lint-specific configuration or similar.
+
+# Reference-level explanation
+
+Before Clippy initializes, it checks for `CLIPPY_CONF_PATH` environmental variable. If that exists, it checks if its
+a directory (in which it performs the known algorithm, looking for all parents up to `/`, merging them...). If
+`CLIPPY_CONF_PATH` is a file, just read it as we'd do with `clippy.toml`.
+
+`CLIPPY_CONF_PATH` takes prevalence over `CLIPPY_CONF_DIR`, but if the former doesn't exist, look for the latter. If
+none of these exist, look for the usual `clippy.toml`/`.clippy.toml` in the crate's directory.
+
+With merging, we are referring about the configuration keys with more prevalence, completely replacing the less
+prevalent ones. Configuration keys which are not mentioned in the most prevalent one would be taken from less
+prevalent ones. As a practical example,
+
+```toml
+# clippy.toml
+msrv = "1.82.0"
+disallowed-macros = [ { path = "my_crate::bad_macro" } ]
+
+# crates/my_crate/clippy.toml
+msrv = "1.76.0" # This overrides that 1.82.0 for `my_crate`
+# We adopt that disallowed-macros from the outer clippy.toml
+```
+
+## Stabilization process for keys
+
+Each key needs to be independently stabilized, or it won't be active on configurations without `unstable-conf` enabled.
+This is done via opening a Github PR marking it as no longer unstable, having fixed all issues related to that key
+beforehand.
+
+Things that need to be accounted for before stabilizing a key:
+
+- Are there any lints having problems with this configuration key?
+- Is this key mentioned in any issue? Is it a relevant factor to that issue?
+- Does this key impact unstable features of the compiler, does it rely on behaviour that only happens on nightly?
+- If the key affects macro behaviour, is it well handled?
+
+If necessary, perform some tests on well-known and stable codebases before merging, either via lintcheck, crater or
+manually.
+
+One can use @rust-rfcbot's capabilities to facilitate registering concerns and closing them with
+`@rust-rfcbot concern <concern name>`. When all concerns are fixed, a team member will use the `@rust-rfcbot fcp merge`
+command. [Refer to rust-rfcbot's documentation for more information](https://github.com/rust-lang/rfcbot-rs)
+but this should be enough for our usecase.
+
+While a key is not stable, it's marked on documentation as unstable,  and it will report an error 
+
+# Drawbacks
+
+- There isn't a good way to specify configuration values from the terminal, for the time being.
+- In stable mode, typos and/or deprecated and/or renamed configuration keys being renamed cannot be warned against.
+
+# Prior art
+
+- `rustfmt.toml`
+- `.cargo/config.toml`
+- `rust-toolchain.toml`