feat: add option to set lower priority for flycheck

crates/rust-analyzer/Cargo.toml

@@ -51,6 +51,7 @@ toml.workspace = true
 walkdir = "2.5.0"
 semver.workspace = true
 memchr = "2.7.5"
+libc.workspace = true
 cargo_metadata.workspace = true
 process-wrap.workspace = true
 dhat = { version = "0.3.3", optional = true }

crates/rust-analyzer/src/config.rs

@@ -888,6 +888,11 @@ config_data! {
         /// Note: The option must be specified as an array of command line arguments, with
         /// the first argument being the name of the command to run.
         check_overrideCommand | checkOnSave_overrideCommand: Option<Vec<String>>             = None,
+        /// Set the priority of the check command.
+        ///
+        /// On Unix, this will use `nice` to set the priority.
+        /// On Windows, this will use `BELOW_NORMAL_PRIORITY_CLASS`.
+        check_priority | checkOnSave_priority: ProcessPriority = ProcessPriority::Normal,
         /// Check for specific targets. Defaults to `#rust-analyzer.cargo.target#` if empty.
         ///
         /// Can be a single target, e.g. `"x86_64-unknown-linux-gnu"` or a list of targets, e.g.
@@ -2414,6 +2419,7 @@ impl Config {
             extra_env: self.extra_env(source_root).clone(),
             target_dir_config: self.target_dir_from_config(source_root),
             set_test: true,
+            priority: crate::flycheck::ProcessPriority::Normal,
         }
     }
 
@@ -2432,6 +2438,10 @@ impl Config {
                             crate::flycheck::InvocationStrategy::PerWorkspace
                         }
                     },
+                    priority: match self.check_priority(source_root) {
+                        ProcessPriority::Normal => crate::flycheck::ProcessPriority::Normal,
+                        ProcessPriority::Low => crate::flycheck::ProcessPriority::Low,
+                    },
                 }
             }
             Some(_) | None => FlycheckConfig::CargoCommand {
@@ -2472,6 +2482,10 @@ impl Config {
                     extra_env: self.check_extra_env(source_root),
                     target_dir_config: self.target_dir_from_config(source_root),
                     set_test: *self.cfg_setTest(source_root),
+                    priority: match self.check_priority(source_root) {
+                        ProcessPriority::Normal => crate::flycheck::ProcessPriority::Normal,
+                        ProcessPriority::Low => crate::flycheck::ProcessPriority::Low,
+                    },
                 },
                 ansi_color_output: self.color_diagnostic_output(),
             },
@@ -2865,6 +2879,13 @@ enum CargoFeaturesDef {
     Selected(Vec<String>),
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, Copy)]
+#[serde(rename_all = "snake_case")]
+pub(crate) enum ProcessPriority {
+    Normal,
+    Low,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone)]
 #[serde(rename_all = "snake_case")]
 pub(crate) enum InvocationStrategy {
@@ -3787,6 +3808,14 @@ fn field_props(field: &str, ty: &str, doc: &[&str], default: &str) -> serde_json
                 "The command will be executed once with the opened project as the working directory."
             ],
         },
+        "ProcessPriority" => set! {
+            "type": "string",
+            "enum": ["normal", "low"],
+            "enumDescriptions": [
+                "Run with normal priority.",
+                "Run with low priority (e.g., nice on Unix)."
+            ],
+        },
         "Option<CheckOnSaveTargets>" => set! {
             "anyOf": [
                 {

crates/rust-analyzer/src/flycheck.rs

@@ -36,6 +36,13 @@ pub(crate) enum InvocationStrategy {
     PerWorkspace,
 }
 
+#[derive(Clone, Copy, Debug, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
+#[serde(rename_all = "snake_case")]
+pub(crate) enum ProcessPriority {
+    Normal,
+    Low,
+}
+
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub(crate) struct CargoOptions {
     pub(crate) target_tuples: Vec<String>,
@@ -48,6 +55,7 @@ pub(crate) struct CargoOptions {
     pub(crate) extra_test_bin_args: Vec<String>,
     pub(crate) extra_env: FxHashMap<String, Option<String>>,
     pub(crate) target_dir_config: TargetDirectoryConfig,
+    pub(crate) priority: ProcessPriority,
 }
 
 #[derive(Clone, Debug)]
@@ -101,6 +109,7 @@ pub(crate) enum FlycheckConfig {
         args: Vec<String>,
         extra_env: FxHashMap<String, Option<String>>,
         invocation_strategy: InvocationStrategy,
+        priority: ProcessPriority,
     },
 }
 
@@ -700,9 +709,16 @@ impl FlycheckActor {
                     self.ws_target_dir.as_ref().map(Utf8PathBuf::as_path),
                 );
                 cmd.args(&options.extra_args);
+                set_priority(&mut cmd, options.priority);
                 Some(cmd)
             }
-            FlycheckConfig::CustomCommand { command, args, extra_env, invocation_strategy } => {
+            FlycheckConfig::CustomCommand {
+                command,
+                args,
+                extra_env,
+                invocation_strategy,
+                priority,
+            } => {
                 let root = match invocation_strategy {
                     InvocationStrategy::Once => &*self.root,
                     InvocationStrategy::PerWorkspace => {
@@ -734,6 +750,7 @@ impl FlycheckActor {
                     }
                 }
 
+                set_priority(&mut cmd, *priority);
                 Some(cmd)
             }
         }
@@ -745,6 +762,33 @@ impl FlycheckActor {
     }
 }
 
+fn set_priority(cmd: &mut Command, priority: ProcessPriority) {
+    match priority {
+        ProcessPriority::Normal => (),
+        ProcessPriority::Low => {
+            #[cfg(unix)]
+            {
+                use std::os::unix::process::CommandExt;
+                unsafe {
+                    cmd.pre_exec(|| {
+                        if libc::setpriority(libc::PRIO_PROCESS, 0, 10) != 0 {
+                            return Err(std::io::Error::last_os_error());
+                        }
+                        Ok(())
+                    });
+                }
+            }
+            #[cfg(windows)]
+            {
+                use std::os::windows::process::CommandExt;
+                cmd.creation_flags(
+                    windows_sys::Win32::System::Threading::BELOW_NORMAL_PRIORITY_CLASS,
+                );
+            }
+        }
+    }
+}
+
 #[allow(clippy::large_enum_variant)]
 enum CargoCheckMessage {
     CompilerArtifact(cargo_metadata::Artifact),

docs/book/src/configuration_generated.md

@@ -338,6 +338,16 @@ Note: The option must be specified as an array of command line arguments, with
 the first argument being the name of the command to run.
 
 
+## rust-analyzer.check.priority {#check.priority}
+
+Default: `"normal"`
+
+Set the priority of the check command.
+
+On Unix, this will use `nice` to set the priority.
+On Windows, this will use `BELOW_NORMAL_PRIORITY_CLASS`.
+
+
 ## rust-analyzer.check.targets {#check.targets}
 
 Default: `null`

editors/code/package.json

@@ -1225,6 +1225,24 @@
                     }
                 }
             },
+            {
+                "title": "Check",
+                "properties": {
+                    "rust-analyzer.check.priority": {
+                        "markdownDescription": "Set the priority of the check command.\n\nOn Unix, this will use `nice` to set the priority.\nOn Windows, this will use `BELOW_NORMAL_PRIORITY_CLASS`.",
+                        "default": "normal",
+                        "type": "string",
+                        "enum": [
+                            "normal",
+                            "low"
+                        ],
+                        "enumDescriptions": [
+                            "Run with normal priority.",
+                            "Run with low priority (e.g., nice on Unix)."
+                        ]
+                    }
+                }
+            },
             {
                 "title": "Check",
                 "properties": {