fix: direct people to the right repository

[rust.git] / docs / user / manual.adoc

= User Manual
:toc: preamble
:sectanchors:
:page-layout: post
:icons: font
:source-highlighter: rouge
:experimental:

At its core, rust-analyzer is a *library* for semantic analysis of Rust code as it changes over time.
This manual focuses on a specific usage of the library -- running it as part of a server that implements the
https://microsoft.github.io/language-server-protocol/[Language Server Protocol] (LSP).
The LSP allows various code editors, like VS Code, Emacs or Vim, to implement semantic features like completion or goto definition by talking to an external language server process.

[TIP]
====
[.lead]
To improve this document, send a pull request: +
https://github.com/rust-analyzer/rust-analyzer/blob/master/docs/user/manual.adoc[https://github.com/rust-analyzer/.../manual.adoc]

The manual is written in https://asciidoc.org[AsciiDoc] and includes some extra files which are generated from the source code. Run `cargo test` and `cargo test -p xtask` to create these and then `asciidoctor manual.adoc` to create an HTML copy.

====

If you have questions about using rust-analyzer, please ask them in the https://users.rust-lang.org/c/ide/14["`IDEs and Editors`"] topic of Rust users forum.

== Installation

In theory, one should be able to just install the <<rust-analyzer-language-server-binary,`rust-analyzer` binary>> and have it automatically work with any editor.
We are not there yet, so some editor specific setup is required.

Additionally, rust-analyzer needs the sources of the standard library.
If the source code is not present, rust-analyzer will attempt to install it automatically.

To add the sources manually, run the following command:

```bash
$ rustup component add rust-src
```

=== VS Code

This is the best supported editor at the moment.
The rust-analyzer plugin for VS Code is maintained
https://github.com/rust-analyzer/rust-analyzer/tree/master/editors/code[in tree].

You can install the latest release of the plugin from
https://marketplace.visualstudio.com/items?itemName=matklad.rust-analyzer[the marketplace].

Note that the plugin may cause conflicts with the
https://marketplace.visualstudio.com/items?itemName=rust-lang.rust[official Rust plugin].
It is recommended to disable the Rust plugin when using the rust-analyzer extension.

By default, the plugin will prompt you to download the matching version of the server as well:

image::https://user-images.githubusercontent.com/9021944/75067008-17502500-54ba-11ea-835a-f92aac50e866.png[]

[NOTE]
====
To disable this notification put the following to `settings.json`

[source,json]
----
{ "rust-analyzer.updates.askBeforeDownload": false }
----
====

The server binary is stored in:

* Linux: `~/.config/Code/User/globalStorage/matklad.rust-analyzer`
* Linux (Remote, such as WSL): `~/.vscode-server/data/User/globalStorage/matklad.rust-analyzer`
* macOS: `~/Library/Application\ Support/Code/User/globalStorage/matklad.rust-analyzer`
* Windows: `%APPDATA%\Code\User\globalStorage\matklad.rust-analyzer`

Note that we only support two most recent versions of VS Code.

==== Updates

The extension will be updated automatically as new versions become available.
It will ask your permission to download the matching language server version binary if needed.

===== Nightly

We ship nightly releases for VS Code.
To help us out with testing the newest code and follow the bleeding edge of our `master`, please use the following config:

[source,json]
----
{ "rust-analyzer.updates.channel": "nightly" }
----

You will be prompted to install the `nightly` extension version.
Just click `Download now` and from that moment you will get automatic updates every 24 hours.

If you don't want to be asked for `Download now` every day when the new nightly version is released add the following to your `settings.json`:
[source,json]
----
{ "rust-analyzer.updates.askBeforeDownload": false }
----

NOTE: Nightly extension should **only** be installed via the `Download now` action from VS Code.

==== Manual installation

Alternatively, procure both `rust-analyzer.vsix` and your platform's matching `rust-analyzer-{platform}`, for example from the
https://github.com/rust-analyzer/rust-analyzer/releases[releases] page.

Install the extension with the `Extensions: Install from VSIX` command within VS Code, or from the command line via:
[source]
----
$ code --install-extension /path/to/rust-analyzer.vsix
----

Copy the `rust-analyzer-{platform}` binary anywhere, then add the path to your settings.json, for example:
[source,json]
----
{ "rust-analyzer.server.path": "~/.local/bin/rust-analyzer-linux" }
----

==== Building From Source

Alternatively, both the server and the Code plugin can be installed from source:

[source]
----
$ git clone https://github.com/rust-analyzer/rust-analyzer.git && cd rust-analyzer
$ cargo xtask install
----

You'll need Cargo, nodejs and npm for this.

Note that installing via `xtask install` does not work for VS Code Remote, instead you'll need to install the `.vsix` manually.

If you're not using Code, you can compile and install only the LSP server:

[source]
----
$ cargo xtask install --server
----

==== Troubleshooting

Here are some useful self-diagnostic commands:

* **Rust Analyzer: Show RA Version** shows the version of `rust-analyzer` binary.
* **Rust Analyzer: Status** prints some statistics about the server, and dependency information for the current file.
* To enable server-side logging, run with `env RA_LOG=info` and see `Output > Rust Analyzer Language Server` in VS Code's panel.
* To log project loading (sysroot & `cargo metadata`), set `RA_LOG=project_model=debug`.
* To log all LSP requests, add `"rust-analyzer.trace.server": "verbose"` to the settings and look for `Rust Analyzer Language Server Trace` in the panel.
* To enable client-side logging, add `"rust-analyzer.trace.extension": true` to the settings and open `Output > Rust Analyzer Client` in the panel.

=== rust-analyzer Language Server Binary

Other editors generally require the `rust-analyzer` binary to be in `$PATH`.
You can download the pre-built binary from the https://github.com/rust-analyzer/rust-analyzer/releases[releases] page.
Typically, you then need to rename the binary for your platform, e.g. `rust-analyzer-mac` if you're on Mac OS, to `rust-analyzer` and make it executable in addition to moving it into a directory in your `$PATH`.

On Linux to install the `rust-analyzer` binary into `~/.local/bin`, this commands could be used

[source,bash]
----
$ curl -L https://github.com/rust-analyzer/rust-analyzer/releases/latest/download/rust-analyzer-linux -o ~/.local/bin/rust-analyzer
$ chmod +x ~/.local/bin/rust-analyzer
----

Ensure `~/.local/bin` is listed in the `$PATH` variable.

Alternatively, you can install it from source using the command below.
You'll need the latest stable version of the Rust toolchain.

[source,bash]
----
$ git clone https://github.com/rust-analyzer/rust-analyzer.git && cd rust-analyzer
$ cargo xtask install --server
----

If your editor can't find the binary even though the binary is on your `$PATH`, the likely explanation is that it doesn't see the same `$PATH` as the shell, see https://github.com/rust-analyzer/rust-analyzer/issues/1811[this issue].
On Unix, running the editor from a shell or changing the `.desktop` file to set the environment should help.

==== `rustup`

`rust-analyzer` is available in `rustup`, but only in the nightly toolchain:

[source,bash]
---
$ rustup +nightly component add rust-analyzer-preview
---

==== Arch Linux

The `rust-analyzer` binary can be installed from the repos or AUR (Arch User Repository):

- https://www.archlinux.org/packages/community/x86_64/rust-analyzer/[`rust-analyzer`] (built from latest tagged source)
- https://aur.archlinux.org/packages/rust-analyzer-git[`rust-analyzer-git`] (latest Git version)

Install it with pacman, for example:

[source,bash]
----
$ pacman -S rust-analyzer
----

=== Emacs

Note this excellent https://robert.kra.hn/posts/2021-02-07_rust-with-emacs/[guide] from https://github.com/rksm[@rksm].

Prerequisites: You have installed the <<rust-analyzer-language-server-binary,`rust-analyzer` binary>>.

Emacs support is maintained as part of the https://github.com/emacs-lsp/lsp-mode[Emacs-LSP] package in https://github.com/emacs-lsp/lsp-mode/blob/master/lsp-rust.el[lsp-rust.el].

1. Install the most recent version of `emacs-lsp` package by following the https://github.com/emacs-lsp/lsp-mode[Emacs-LSP instructions].
2. Set `lsp-rust-server` to `'rust-analyzer`.
3. Run `lsp` in a Rust buffer.
4. (Optionally) bind commands like `lsp-rust-analyzer-join-lines`, `lsp-extend-selection` and `lsp-rust-analyzer-expand-macro` to keys.

=== Vim/NeoVim

Prerequisites: You have installed the <<rust-analyzer-language-server-binary,`rust-analyzer` binary>>.
Not needed if the extension can install/update it on its own, coc-rust-analyzer is one example.

The are several LSP client implementations for vim or neovim:

==== coc-rust-analyzer

1. Install coc.nvim by following the instructions at
   https://github.com/neoclide/coc.nvim[coc.nvim]
   (Node.js required)
2. Run `:CocInstall coc-rust-analyzer` to install
   https://github.com/fannheyward/coc-rust-analyzer[coc-rust-analyzer],
   this extension implements _most_ of the features supported in the VSCode extension:
   * automatically install and upgrade stable/nightly releases
   * same configurations as VSCode extension, `rust-analyzer.server.path`, `rust-analyzer.cargo.features` etc.
   * same commands too, `rust-analyzer.analyzerStatus`, `rust-analyzer.ssr` etc.
   * inlay hints for variables and method chaining, _Neovim Only_
   * semantic highlighting is not implemented yet

Note: for code actions, use `coc-codeaction-cursor` and `coc-codeaction-selected`; `coc-codeaction` and `coc-codeaction-line` are unlikely to be useful.

==== LanguageClient-neovim

1. Install LanguageClient-neovim by following the instructions
   https://github.com/autozimu/LanguageClient-neovim[here]
   * The GitHub project wiki has extra tips on configuration

2. Configure by adding this to your vim/neovim config file (replacing the existing Rust-specific line if it exists):
+
[source,vim]
----
let g:LanguageClient_serverCommands = {
\ 'rust': ['rust-analyzer'],
\ }
----

==== YouCompleteMe

Install YouCompleteMe by following the instructions
  https://github.com/ycm-core/YouCompleteMe#installation[here].

rust-analyzer is the default in ycm, it should work out of the box.

==== ALE

To use the LSP server in https://github.com/dense-analysis/ale[ale]:

[source,vim]
----
let g:ale_linters = {'rust': ['analyzer']}
----

==== nvim-lsp

NeoVim 0.5 (not yet released) has built-in language server support.
For a quick start configuration of rust-analyzer, use https://github.com/neovim/nvim-lspconfig#rust_analyzer[neovim/nvim-lspconfig].
Once `neovim/nvim-lspconfig` is installed, use `+lua require'lspconfig'.rust_analyzer.setup({})+` in your `init.vim`.

You can also pass LSP settings to the server:

[source,vim]
----
lua << EOF
local nvim_lsp = require'lspconfig'

local on_attach = function(client)
    require'completion'.on_attach(client)
end

nvim_lsp.rust_analyzer.setup({
    on_attach=on_attach,
    settings = {
        ["rust-analyzer"] = {
            assist = {
                importMergeBehavior = "last",
                importPrefix = "by_self",
            },
            cargo = {
                loadOutDirsFromCheck = true
            },
            procMacro = {
                enable = true
            },
        }
    }
})
EOF
----

See https://sharksforarms.dev/posts/neovim-rust/ for more tips on getting started.

==== vim-lsp

vim-lsp is installed by following https://github.com/prabirshrestha/vim-lsp[the plugin instructions].
It can be as simple as adding this line to your `.vimrc`:

[source,vim]
----
Plug 'prabirshrestha/vim-lsp'
----

Next you need to register the `rust-analyzer` binary.
If it is available in `$PATH`, you may want to add this to your `.vimrc`:

[source,vim]
----
if executable('rust-analyzer')
  au User lsp_setup call lsp#register_server({
        \   'name': 'Rust Language Server',
        \   'cmd': {server_info->['rust-analyzer']},
        \   'whitelist': ['rust'],
        \ })
endif
----

There is no dedicated UI for the server configuration, so you would need to send any options as a value of the `initialization_options` field, as described in the <<_configuration,Configuration>> section.
Here is an example of how to enable the proc-macro support:

[source,vim]
----
if executable('rust-analyzer')
  au User lsp_setup call lsp#register_server({
        \   'name': 'Rust Language Server',
        \   'cmd': {server_info->['rust-analyzer']},
        \   'whitelist': ['rust'],
        \   'initialization_options': {
        \     'cargo': {
        \       'loadOutDirsFromCheck': v:true,
        \     },
        \     'procMacro': {
        \       'enable': v:true,
        \     },
        \   },
        \ })
endif
----

=== Sublime Text 3

Prerequisites: You have installed the <<rust-analyzer-language-server-binary,`rust-analyzer` binary>>.

You also need the `LSP` package.
To install it:

1. If you've never installed a Sublime Text package, install Package Control:
   * Open the command palette (Win/Linux: `ctrl+shift+p`, Mac: `cmd+shift+p`)
   * Type `Install Package Control`, press enter
2. In the command palette, run `Package control: Install package`, and in the list that pops up, type `LSP` and press enter.

Finally, with your Rust project open, in the command palette, run `LSP: Enable Language Server In Project` or `LSP: Enable Language Server Globally`, then select `rust-analyzer` in the list that pops up to enable the rust-analyzer LSP.
The latter means that rust-analyzer is enabled by default in Rust projects.

If it worked, you should see "rust-analyzer, Line X, Column Y" on the left side of the bottom bar, and after waiting a bit, functionality like tooltips on hovering over variables should become available.

If you get an error saying `No such file or directory: 'rust-analyzer'`, see the <<rust-analyzer-language-server-binary,`rust-analyzer` binary>> section on installing the language server binary.

=== GNOME Builder

GNOME Builder 3.37.1 and newer has native `rust-analyzer` support.
If the LSP binary is not available, GNOME Builder can install it when opening a Rust file.


=== Eclipse IDE

Support for Rust development in the Eclipse IDE is provided by link:https://github.com/eclipse/corrosion[Eclipse Corrosion].
If available in PATH or in some standard location, `rust-analyzer` is detected and powers editing of Rust files without further configuration.
If `rust-analyzer` is not detected, Corrosion will prompt you for configuration of your Rust toolchain and language server with a link to the __Window > Preferences > Rust__ preference page; from here a button allows to download and configure `rust-analyzer`, but you can also reference another installation.
You'll need to close and reopen all .rs and Cargo files, or to restart the IDE, for this change to take effect.

== Configuration

**Source:** https://github.com/rust-analyzer/rust-analyzer/blob/master/crates/rust-analyzer/src/config.rs[config.rs]

The <<_installation,Installation>> section contains details on configuration for some of the editors.
In general `rust-analyzer` is configured via LSP messages, which means that it's up to the editor to decide on the exact format and location of configuration files.

Some clients, such as <<vs-code,VS Code>> or <<coc-rust-analyzer,COC plugin in Vim>> provide `rust-analyzer` specific configuration UIs. Others may require you to know a bit more about the interaction with `rust-analyzer`.

For the later category, it might help to know that the initial configuration is specified as a value of the `initializationOptions` field of the https://microsoft.github.io/language-server-protocol/specifications/specification-current/#initialize[`InitializeParams` message, in the LSP protocol].
The spec says that the field type is `any?`, but `rust-analyzer` is looking for a JSON object that is constructed using settings from the list below.
Name of the setting, ignoring the `rust-analyzer.` prefix, is used as a path, and value of the setting becomes the JSON property value.

For example, a very common configuration is to enable proc-macro support, can be achieved by sending this JSON:

[source,json]
----
{
  "cargo": {
    "loadOutDirsFromCheck": true,
  },
  "procMacro": {
    "enable": true,
  }
}
----

Please consult your editor's documentation to learn more about how to configure https://microsoft.github.io/language-server-protocol/[LSP servers].

To verify which configuration is actually used by `rust-analyzer`, set `RA_LOG` environment variable to `rust_analyzer=info` and look for config-related messages.
Logs should show both the JSON that `rust-analyzer` sees as well as the updated config.

This is the list of config options `rust-analyzer` supports:

include::./generated_config.adoc[]

== Non-Cargo Based Projects

rust-analyzer does not require Cargo.
However, if you use some other build system, you'll have to describe the structure of your project for rust-analyzer in the `rust-project.json` format:

[source,TypeScript]
----
interface JsonProject {
    /// Path to the directory with *source code* of
    /// sysroot crates.
    ///
    /// It should point to the directory where std,
    /// core, and friends can be found:
    ///
    /// https://github.com/rust-lang/rust/tree/master/library.
    ///
    /// If provided, rust-analyzer automatically adds
    /// dependencies on sysroot crates. Conversely,
    /// if you omit this path, you can specify sysroot
    /// dependencies yourself and, for example, have
    /// several different "sysroots" in one graph of
    /// crates.
    sysroot_src?: string;
    /// The set of crates comprising the current
    /// project. Must include all transitive
    /// dependencies as well as sysroot crate (libstd,
    /// libcore and such).
    crates: Crate[];
}

interface Crate {
    /// Optional crate name used for display purposes,
    /// without affecting semantics. See the `deps`
    /// key for semantically-significant crate names.
    display_name?: string;
    /// Path to the root module of the crate.
    root_module: string;
    /// Edition of the crate.
    edition: "2015" | "2018" | "2021";
    /// Dependencies
    deps: Dep[];
    /// Should this crate be treated as a member of
    /// current "workspace".
    ///
    /// By default, inferred from the `root_module`
    /// (members are the crates which reside inside
    /// the directory opened in the editor).
    ///
    /// Set this to `false` for things like standard
    /// library and 3rd party crates to enable
    /// performance optimizations (rust-analyzer
    /// assumes that non-member crates don't change).
    is_workspace_member?: boolean;
    /// Optionally specify the (super)set of `.rs`
    /// files comprising this crate.
    ///
    /// By default, rust-analyzer assumes that only
    /// files under `root_module.parent` can belong
    /// to a crate. `include_dirs` are included
    /// recursively, unless a subdirectory is in
    /// `exclude_dirs`.
    ///
    /// Different crates can share the same `source`.
    ///
    /// If two crates share an `.rs` file in common,
    /// they *must* have the same `source`.
    /// rust-analyzer assumes that files from one
    /// source can't refer to files in another source.
    source?: {
        include_dirs: string[],
        exclude_dirs: string[],
    },
    /// The set of cfgs activated for a given crate, like
    /// `["unix", "feature=\"foo\"", "feature=\"bar\""]`.
    cfg: string[];
    /// Target triple for this Crate.
    ///
    /// Used when running `rustc --print cfg`
    /// to get target-specific cfgs.
    target?: string;
    /// Environment variables, used for
    /// the `env!` macro
    env: : { [key: string]: string; },

    /// For proc-macro crates, path to compiled
    /// proc-macro (.so file).
    proc_macro_dylib_path?: string;
}

interface Dep {
    /// Index of a crate in the `crates` array.
    crate: number,
    /// Name as should appear in the (implicit)
    /// `extern crate name` declaration.
    name: string,
}
----

This format is provisional and subject to change.
Specifically, the `roots` setup will be different eventually.

There are tree ways to feed `rust-project.json` to rust-analyzer:

* Place `rust-project.json` file at the root of the project, and rust-anlayzer will discover it.
* Specify `"rust-analyzer.linkedProjects": [ "path/to/rust-project.json" ]` in the settings (and make sure that your LSP client sends settings as a part of initialize request).
* Specify `"rust-analyzer.linkedProjects": [ { "roots": [...], "crates": [...] }]` inline.

Relative paths are interpreted relative to `rust-project.json` file location or (for inline JSON) relative to `rootUri`.

See https://github.com/rust-analyzer/rust-project.json-example for a small example.

You can set `RA_LOG` environmental variable to `rust_analyzer=info` to inspect how rust-analyzer handles config and project loading.

== Security

At the moment, rust-analyzer assumes that all code is trusted.
Here is a **non-exhaustive** list of ways to make rust-analyzer execute arbitrary code:

* proc macros and build scripts are executed by default
* `.cargo/config` can override `rustc` with an arbitrary executable
* VS Code plugin reads configuration from project directory, and that can be used to override paths to various executables, like `rustfmt` or `rust-analyzer` itself.
* rust-analyzer's syntax trees library uses a lot of `unsafe` and hasn't been properly audited for memory safety.

rust-analyzer itself doesn't access the network.
The VS Code plugin doesn't access the network unless the nightly channel is selected in the settings.
In that case, the plugin uses the GitHub API to check for and download updates.

== Features

include::./generated_features.adoc[]

== Assists (Code Actions)

Assists, or code actions, are small local refactorings, available in a particular context.
They are usually triggered by a shortcut or by clicking a light bulb icon in the editor.
Cursor position or selection is signified by `┃` character.

include::./generated_assists.adoc[]

== Diagnostics

While most errors and warnings provided by rust-analyzer come from the `cargo check` integration, there's a growing number of diagnostics implemented using rust-analyzer's own analysis.
Some of these diagnostics don't respect `\#[allow]` or `\#[deny]` attributes yet, but can be turned off using the `rust-analyzer.diagnostics.enable`, `rust-analyzer.diagnostics.enableExperimental` or `rust-analyzer.diagnostics.disabled` settings.

include::./generated_diagnostic.adoc[]

== Editor Features
=== VS Code

==== Color configurations

It is possible to change the foreground/background color of inlay hints.
Just add this to your `settings.json`:

[source,jsonc]
----
{
  "workbench.colorCustomizations": {
    // Name of the theme you are currently using
    "[Default Dark+]": {
      "rust_analyzer.inlayHints.foreground": "#868686f0",
      "rust_analyzer.inlayHints.background": "#3d3d3d48",

      // Overrides for specific kinds of inlay hints
      "rust_analyzer.inlayHints.foreground.typeHints": "#fdb6fdf0",
      "rust_analyzer.inlayHints.foreground.paramHints": "#fdb6fdf0",
      "rust_analyzer.inlayHints.background.chainingHints": "#6b0c0c81"
    }
  }
}
----

==== Semantic style customizations

You can customize the look of different semantic elements in the source code.
For example, mutable bindings are underlined by default and you can override this behavior by adding the following section to your `settings.json`:

[source,jsonc]
----
{
  "editor.semanticTokenColorCustomizations": {
    "rules": {
      "*.mutable": {
        "fontStyle": "", // underline is the default
      },
    }
  },
}
----

Most themes doesn't support styling unsafe operations differently yet. You can fix this by adding overrides for the rules `operator.unsafe`, `function.unsafe`, and `method.unsafe`:

[source,jsonc]
----
{
   "editor.semanticTokenColorCustomizations": {
         "rules": {
             "operator.unsafe": "#ff6600",
             "function.unsafe": "#ff6600"
             "method.unsafe": "#ff6600"
         }
    },
}
----

In addition to the top-level rules you can specify overrides for specific themes. For example, if you wanted to use a darker text color on a specific light theme, you might write:

[source,jsonc]
----
{
   "editor.semanticTokenColorCustomizations": {
         "rules": {
             "operator.unsafe": "#ff6600"
         },
         "[Ayu Light]": {
            "rules": {
               "operator.unsafe": "#572300"
            }
         }
    },
}
----

Make sure you include the brackets around the theme name. For example, use `"[Ayu Light]"` to customize the theme Ayu Light.

==== Special `when` clause context for keybindings.
You may use `inRustProject` context to configure keybindings for rust projects only.
For example:

[source,json]
----
{
  "key": "ctrl+i",
  "command": "rust-analyzer.toggleInlayHints",
  "when": "inRustProject"
}
----
More about `when` clause contexts https://code.visualstudio.com/docs/getstarted/keybindings#_when-clause-contexts[here].

==== Setting runnable environment variables
You can use "rust-analyzer.runnableEnv" setting to define runnable environment-specific substitution variables.
The simplest way for all runnables in a bunch:
```jsonc
"rust-analyzer.runnableEnv": {
    "RUN_SLOW_TESTS": "1"
}
```

Or it is possible to specify vars more granularly:
```jsonc
"rust-analyzer.runnableEnv": [
    {
        // "mask": null, // null mask means that this rule will be applied for all runnables
        env: {
             "APP_ID": "1",
             "APP_DATA": "asdf"
        }
    },
    {
        "mask": "test_name",
        "env": {
             "APP_ID": "2", // overwrites only APP_ID
        }
    }
]
```

You can use any valid regular expression as a mask.
Also note that a full runnable name is something like *run bin_or_example_name*, *test some::mod::test_name* or *test-mod some::mod*, so it is possible to distinguish binaries, single tests, and test modules with this masks: `"^run"`, `"^test "` (the trailing space matters!), and `"^test-mod"` respectively.

==== Compiler feedback from external commands

Instead of relying on the built-in `cargo check`, you can configure Code to run a command in the background and use the `$rustc-watch` problem matcher to generate inline error markers from its output.

To do this you need to create a new https://code.visualstudio.com/docs/editor/tasks[VS Code Task] and set `rust-analyzer.checkOnSave.enable: false` in preferences.

For example, if you want to run https://crates.io/crates/cargo-watch[`cargo watch`] instead, you might add the following to `.vscode/tasks.json`:

```json
{
    "label": "Watch",
    "group": "build",
    "type": "shell",
    "command": "cargo watch",
    "problemMatcher": "$rustc-watch",
    "isBackground": true
}
```