docs: explain how to get code completion to work when hacking on clippy

[matthiaskrgr]

There have been several people in discord now asking how to get code-completion for rustc code when working on clippy code.

We should see if we can figure something out that works (using rust-analyzer?) and perhaps have some kind of guideline for getting it running for everyone, similar to the "how to get the rustc-master toolchain" section in the contribution guide. https://github.com/rust-lang/rust-clippy/blob/master/CONTRIBUTING.md#fixing-build-failures-caused-by-rust

[flip1995]

I hope, that this will be easier, once Clippy is a subtree in rustc and we can fix nightlies.

I have the most success with this setup (nvim + coc.nvim* + rust-analyzer):

Use master toolchain with -c rustc-dev (installed with setup-toolchain.sh) and -c rust-src + -c rustfmt (both not installed in setup-toolchain.sh)

+ errors are displayed for the latest rustc internals

+ code completion for Clippy internals

+ code completion for deps from Cargo.toml (not rustc-internals)

(color scheme is a bit broken, since this is a WSL instance in powershell...)

- no code completion for rustc internals
- no goto definition/... for rustc internals
- no auto format, when rustfmt is not available in master toolchain
- the rustfmt of the master toolchain may differ from the one used in CI (nightly)

*the coc.nvim plugin kind of emulates the LSP of vscode, so this should also work with vscode or other editors, that have a LSP implementation/plugin.

[matthiaskrgr]

likely relevant: rust-lang/rust-analyzer#3517 (comment)

[tnielens]

another one: intellij-rust/intellij-rust#1618

[matthiaskrgr]

My current workaround is to redirect all the rustc dep paths to respective absolute paths to my rustc-repo. This blows up compiletime since I need to build 70% of rustc now but it makes rust-analyzer find rustc sources for analysis :)
You should also make sure that you have around 20-25 gigabyte of free space for the target dir.

diff --git a/Cargo.toml b/Cargo.toml
index 6999b6bd..027749c0 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -35,6 +35,11 @@ semver = "0.9"
 rustc_tools_util = { version = "0.2.0", path = "rustc_tools_util"}
 tempfile = { version = "3.1.0", optional = true }
 lazy_static = "1.0"
+# make rustc deps visible to rust analyzer
+rustc_driver = { path = "/home/matthias/vcs/github/rust/src/librustc_driver" }
+rustc_errors = { path = "/home/matthias/vcs/github/rust/src/librustc_errors" }
+rustc_interface = { path = "/home/matthias/vcs/github/rust/src/librustc_interface" }
+rustc_middle = { path = "/home/matthias/vcs/github/rust/src/librustc_middle" }

 [dev-dependencies]
 cargo_metadata = "0.9.0"
diff --git a/clippy_lints/Cargo.toml b/clippy_lints/Cargo.toml
index 1c0be727..4f8b7e62 100644
--- a/clippy_lints/Cargo.toml
+++ b/clippy_lints/Cargo.toml
@@ -33,5 +33,28 @@ semver = "0.9.0"
 # see <https://github.com/rust-lang/rust/pull/63587#issuecomment-522343864>
 url = { version =  "2.1.0", features = ["serde"] }

+# make rustc deps visible to rust analyzer
+fmt_macros = { path = "/home/matthias/vcs/github/rust/src/libfmt_macros" }
+rustc_ast = { path = "/home/matthias/vcs/github/rust/src/librustc_ast" }
+rustc_ast_pretty = { path = "/home/matthias/vcs/github/rust/src/librustc_ast_pretty" }
+rustc_attr = { path = "/home/matthias/vcs/github/rust/src/librustc_attr" }
+rustc_data_structures = { path = "/home/matthias/vcs/github/rust/src/librustc_data_structures" }
+rustc_driver = { path = "/home/matthias/vcs/github/rust/src/librustc_driver" }
+rustc_errors = { path = "/home/matthias/vcs/github/rust/src/librustc_errors" }
+rustc_hir = { path = "/home/matthias/vcs/github/rust/src/librustc_hir" }
+rustc_hir_pretty = { path = "/home/matthias/vcs/github/rust/src/librustc_hir_pretty" }
+rustc_index = { path = "/home/matthias/vcs/github/rust/src/librustc_index" }
+rustc_infer = { path = "/home/matthias/vcs/github/rust/src/librustc_infer" }
+rustc_lexer = { path = "/home/matthias/vcs/github/rust/src/librustc_lexer" }
+rustc_lint = { path = "/home/matthias/vcs/github/rust/src/librustc_lint" }
+rustc_middle = { path = "/home/matthias/vcs/github/rust/src/librustc_middle" }
+rustc_mir = { path = "/home/matthias/vcs/github/rust/src/librustc_mir" }
+rustc_parse = { path = "/home/matthias/vcs/github/rust/src/librustc_parse" }
+rustc_session = { path = "/home/matthias/vcs/github/rust/src/librustc_session" }
+rustc_span = { path = "/home/matthias/vcs/github/rust/src/librustc_span" }
+rustc_target = { path = "/home/matthias/vcs/github/rust/src/librustc_target" }
+rustc_trait_selection = { path = "/home/matthias/vcs/github/rust/src/librustc_trait_selection" }
+rustc_typeck = { path = "/home/matthias/vcs/github/rust/src/librustc_typeck" }
+
 [features]
 deny-warnings = []

[tesuji]

Why don't you use upstream libs?

[matthiaskrgr]

Looks like there is no component that actually supplies the rustc source code (only libcore/liballoc/libstc etc)