Merge #10157

10157: Add section on configuring compilation errors when using `rust-project.json` r=matklad a=dozzman

When using `rust-project.json` to specify the project workspace, flychecks are disabled. It is not mentioned that they can be re-enabled by specifying a custom checking command using the `checkOnSave.overrideCommand` configuration. This additional section makes it clear that using `rust-project.json` disables flychecks and how to enable them either using `cargo check` (as an example) or (more likely) a custom command which emits json error messages.

Further information can be found at this forum thread:

https://users.rust-lang.org/t/rust-analyzer-doesnt-show-cargo-check-compilation-errors-warnings-when-using-rust-project-json/64412

Co-authored-by: Dorian Peake <[email protected]>

Author: bors[bot]

docs/user/manual.adoc

@@ -616,6 +616,15 @@ See https://github.com/rust-analyzer/rust-project.json-example for a small examp
 
 You can set `RA_LOG` environmental variable to `rust_analyzer=info` to inspect how rust-analyzer handles config and project loading.
 
+Note that calls to `cargo check` are disabled when using `rust-project.json` by default, so compilation errors and warnings will no longer be sent to your LSP client. To enable these compilation errors you will need to specify explicitly what command rust-analyzer should run to perform the checks using the `checkOnSave.overrideCommand` configuration. As an example, the following configuration explicitly sets `cargo check` as the `checkOnSave` command.
+
+[source,json]
+----
+{ "rust-analyzer.checkOnSave.overrideCommand": ["cargo", "check", "--message-format=json"] }
+----
+
+The `checkOnSave.overrideCommand` requires the command specified to output json error messages for rust-analyzer to consume. The `--message-format=json` flag does this for `cargo check` so whichever command you use must also output errors in this format. See the <<Configuration>> section for more information. 
+
 == Security
 
 At the moment, rust-analyzer assumes that all code is trusted.