Add more rust-analyzer configuration examples in the docs (#13189)

Release Notes:

- N/A

docs/src/languages/rust.md

@@ -57,3 +57,118 @@ The `rust-analyzer` target directory can be set in `initialization_options`:
 ```
 
 A `true` setting will set the target directory to `target/rust-analyzer`. You can set a custom directory with a string like `"target/analyzer"` instead of `true`.
+
+## More server configuration
+
+Rust-analyzer [manual](https://rust-analyzer.github.io/manual.html) describes various features and configuration options for rust-analyzer language server.
+Rust-analyzer in Zed runs with the default parameters.
+
+### Large projects and performance
+
+One of the main caveats that might cause extensive resource usage on large projects, is the combination of the following features:
+
+```
+rust-analyzer.checkOnSave (default: true)
+    Run the check command for diagnostics on save.
+```
+
+```
+rust-analyzer.check.workspace (default: true)
+    Whether --workspace should be passed to cargo check. If false, -p <package> will be passed instead.
+```
+
+```
+rust-analyzer.cargo.allTargets (default: true)
+    Pass --all-targets to cargo invocation
+```
+
+Which would mean that every time Zed [auto]saves, a `cargo check --workspace --all-targets` command is run, checking the entire project (workspace), lib, doc, test, bin, bench and [other targets](https://doc.rust-lang.org/cargo/reference/cargo-targets.html).
+
+While that works fine on small projects, it does not scale well.
+
+The alternatives would be to use [tasks](../tasks.md), as Zed already provides a `cargo check --workspace --all-targets` task and the ability to cmd/ctrl-click on the terminal output to navigate to the error, and limit or turn off the check on save feature entirely.
+
+Check on save feature is responsible for returning part of the diagnostics based on cargo check output, so turning it off will limit rust-analyzer with its own [diagnostics](https://rust-analyzer.github.io/manual.html#diagnostics).
+
+Consider more `rust-analyzer.cargo.` and `rust-analyzer.check.` and `rust-analyzer.diagnostics.` settings from the manual for more fine-grained configuration.
+Here's a snippet for Zed settings.json (the language server will restart automatically after the `lsp.rust-analyzer` section is edited and saved):
+
+```json5
+"lsp": {
+    "rust-analyzer": {
+        "initialization_options": {
+            // get more cargo-less diagnostics from rust-analyzer,
+            // which might include false-positives (those can be turned off by their names)
+            "diagnostics": {
+                "experimental": {
+                    "enable": true
+                }
+            },
+            // To disable the checking entirely
+            // (ignores all cargo and check settings below)
+            "checkOnSave": false,
+            // To check the `lib` target only.
+            "cargo": {
+                "allTargets": false
+            },
+            // Use `-p` instead of `--workspace` for cargo check
+            "check": {
+              "workspace": false
+            }
+        }
+    }
+}
+```
+
+### Snippets
+
+There's a way get custom completion items from rust-analyzer, that will transform the code according to the snippet body:
+
+```json
+"lsp": {
+    "rust-analyzer": {
+        "initialization_options": {
+            "completion": {
+                "snippets": {
+                    "custom": {
+                        "Arc::new": {
+                            "postfix": "arc",
+                            "body": ["Arc::new(${receiver})"],
+                            "requires": "std::sync::Arc",
+                            "scope": "expr"
+                        },
+                        "Some": {
+                            "postfix": "some",
+                            "body": ["Some(${receiver})"],
+                            "scope": "expr"
+                        },
+                        "Ok": {
+                            "postfix": "ok",
+                            "body": ["Ok(${receiver})"],
+                            "scope": "expr"
+                        },
+                        "Rc::new": {
+                            "postfix": "rc",
+                            "body": ["Rc::new(${receiver})"],
+                            "requires": "std::rc::Rc",
+                            "scope": "expr"
+                        },
+                        "Box::pin": {
+                            "postfix": "boxpin",
+                            "body": ["Box::pin(${receiver})"],
+                            "requires": "std::boxed::Box",
+                            "scope": "expr"
+                        },
+                        "vec!": {
+                            "postfix": "vec",
+                            "body": ["vec![${receiver}]"],
+                            "description": "vec![]",
+                            "scope": "expr"
+                        }
+                    }
+                }
+            }
+        }
+    }
+}
+```