diff --git a/Cargo.lock b/Cargo.lock
index e79e37b6e8..3880a9982a 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1971,6 +1971,7 @@ dependencies = [
  "ruff_python_ast",
  "rustc-hash 2.0.0",
  "salsa",
+ "thiserror",
  "tracing",
 ]
 
diff --git a/crates/red_knot/src/main.rs b/crates/red_knot/src/main.rs
index bddaa5000a..1a6a555be2 100644
--- a/crates/red_knot/src/main.rs
+++ b/crates/red_knot/src/main.rs
@@ -3,6 +3,7 @@ use std::sync::Mutex;
 
 use clap::Parser;
 use crossbeam::channel as crossbeam_channel;
+use red_knot_workspace::site_packages::site_packages_dirs_of_venv;
 use tracing::subscriber::Interest;
 use tracing::{Level, Metadata};
 use tracing_subscriber::filter::LevelFilter;
@@ -41,6 +42,17 @@ struct Args {
     )]
     current_directory: Option<SystemPathBuf>,
 
+    #[arg(
+        long,
+        help = "Path to the virtual environment the project uses",
+        long_help = "\
+Path to the virtual environment the project uses. \
+If provided, red-knot will use the `site-packages` directory of this virtual environment \
+to resolve type information for the project's third-party dependencies.",
+        value_name = "PATH"
+    )]
+    venv_path: Option<SystemPathBuf>,
+
     #[arg(
         long,
         value_name = "DIRECTORY",
@@ -87,6 +99,7 @@ pub fn main() -> anyhow::Result<()> {
         current_directory,
         custom_typeshed_dir,
         extra_search_path: extra_paths,
+        venv_path,
         target_version,
         verbosity,
         watch,
@@ -120,6 +133,17 @@ pub fn main() -> anyhow::Result<()> {
     let workspace_metadata =
         WorkspaceMetadata::from_path(system.current_directory(), &system).unwrap();
 
+    let site_packages = if let Some(venv_path) = venv_path {
+        let venv_path = system.canonicalize_path(&venv_path).unwrap_or(venv_path);
+        assert!(
+            system.is_directory(&venv_path),
+            "Provided venv-path {venv_path} is not a directory!"
+        );
+        site_packages_dirs_of_venv(&venv_path, &system).unwrap()
+    } else {
+        vec![]
+    };
+
     // TODO: Respect the settings from the workspace metadata. when resolving the program settings.
     let program_settings = ProgramSettings {
         target_version: target_version.into(),
@@ -127,7 +151,7 @@ pub fn main() -> anyhow::Result<()> {
             extra_paths,
             src_root: workspace_metadata.root().to_path_buf(),
             custom_typeshed: custom_typeshed_dir,
-            site_packages: vec![],
+            site_packages,
         },
     };
 
diff --git a/crates/red_knot_module_resolver/src/path.rs b/crates/red_knot_module_resolver/src/path.rs
index ec58973495..7dc5948386 100644
--- a/crates/red_knot_module_resolver/src/path.rs
+++ b/crates/red_knot_module_resolver/src/path.rs
@@ -446,6 +446,12 @@ impl SearchPath {
     }
 
     /// Create a new search path pointing to the `site-packages` directory on disk
+    ///
+    /// TODO: the validation done here is somewhat redundant given that `site-packages`
+    /// are already validated at a higher level by the time we get here.
+    /// However, removing the validation here breaks some file-watching tests -- and
+    /// ultimately we'll probably want all search paths to be validated before a
+    /// `Program` is instantiated, so it doesn't seem like a huge priority right now.
     pub(crate) fn site_packages(
         system: &dyn System,
         root: SystemPathBuf,
diff --git a/crates/red_knot_module_resolver/src/resolver.rs b/crates/red_knot_module_resolver/src/resolver.rs
index 56ddf68b76..8150643b54 100644
--- a/crates/red_knot_module_resolver/src/resolver.rs
+++ b/crates/red_knot_module_resolver/src/resolver.rs
@@ -128,12 +128,16 @@ fn try_resolve_module_resolution_settings(
         site_packages,
     } = program.search_paths(db.upcast());
 
+    if !extra_paths.is_empty() {
+        tracing::info!("Extra search paths: {extra_paths:?}");
+    }
+
     if let Some(custom_typeshed) = custom_typeshed {
         tracing::info!("Custom typeshed directory: {custom_typeshed}");
     }
 
-    if !extra_paths.is_empty() {
-        tracing::info!("extra search paths: {extra_paths:?}");
+    if !site_packages.is_empty() {
+        tracing::info!("Site-packages directories: {site_packages:?}");
     }
 
     let system = db.system();
diff --git a/crates/red_knot_workspace/Cargo.toml b/crates/red_knot_workspace/Cargo.toml
index 35c8cb0efa..39abc06207 100644
--- a/crates/red_knot_workspace/Cargo.toml
+++ b/crates/red_knot_workspace/Cargo.toml
@@ -24,6 +24,7 @@ crossbeam = { workspace = true }
 notify = { workspace = true }
 rustc-hash = { workspace = true }
 salsa = { workspace = true }
+thiserror = { workspace = true }
 tracing = { workspace = true }
 
 [dev-dependencies]
diff --git a/crates/red_knot_workspace/resources/test/empty-unix-venv/.gitignore b/crates/red_knot_workspace/resources/test/empty-unix-venv/.gitignore
new file mode 100644
index 0000000000..f59ec20aab
--- /dev/null
+++ b/crates/red_knot_workspace/resources/test/empty-unix-venv/.gitignore
@@ -0,0 +1 @@
+*
\ No newline at end of file
diff --git a/crates/red_knot_workspace/resources/test/empty-unix-venv/CACHEDIR.TAG b/crates/red_knot_workspace/resources/test/empty-unix-venv/CACHEDIR.TAG
new file mode 100644
index 0000000000..bc1ecb967a
--- /dev/null
+++ b/crates/red_knot_workspace/resources/test/empty-unix-venv/CACHEDIR.TAG
@@ -0,0 +1 @@
+Signature: 8a477f597d28d172789f06886806bc55
\ No newline at end of file
diff --git a/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/activate b/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/activate
new file mode 100644
index 0000000000..06480874a0
--- /dev/null
+++ b/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/activate
@@ -0,0 +1,108 @@
+# Copyright (c) 2020-202x The virtualenv developers
+#
+# Permission is hereby granted, free of charge, to any person obtaining
+# a copy of this software and associated documentation files (the
+# "Software"), to deal in the Software without restriction, including
+# without limitation the rights to use, copy, modify, merge, publish,
+# distribute, sublicense, and/or sell copies of the Software, and to
+# permit persons to whom the Software is furnished to do so, subject to
+# the following conditions:
+#
+# The above copyright notice and this permission notice shall be
+# included in all copies or substantial portions of the Software.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+# LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+# OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+# WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+# This file must be used with "source bin/activate" *from bash*
+# you cannot run it directly
+
+
+if [ "${BASH_SOURCE-}" = "$0" ]; then
+    echo "You must source this script: \$ source $0" >&2
+    exit 33
+fi
+
+deactivate () {
+    unset -f pydoc >/dev/null 2>&1 || true
+
+    # reset old environment variables
+    # ! [ -z ${VAR+_} ] returns true if VAR is declared at all
+    if ! [ -z "${_OLD_VIRTUAL_PATH:+_}" ] ; then
+        PATH="$_OLD_VIRTUAL_PATH"
+        export PATH
+        unset _OLD_VIRTUAL_PATH
+    fi
+    if ! [ -z "${_OLD_VIRTUAL_PYTHONHOME+_}" ] ; then
+        PYTHONHOME="$_OLD_VIRTUAL_PYTHONHOME"
+        export PYTHONHOME
+        unset _OLD_VIRTUAL_PYTHONHOME
+    fi
+
+    # The hash command must be called to get it to forget past
+    # commands. Without forgetting past commands the $PATH changes
+    # we made may not be respected
+    hash -r 2>/dev/null
+
+    if ! [ -z "${_OLD_VIRTUAL_PS1+_}" ] ; then
+        PS1="$_OLD_VIRTUAL_PS1"
+        export PS1
+        unset _OLD_VIRTUAL_PS1
+    fi
+
+    unset VIRTUAL_ENV
+    unset VIRTUAL_ENV_PROMPT
+    if [ ! "${1-}" = "nondestructive" ] ; then
+    # Self destruct!
+        unset -f deactivate
+    fi
+}
+
+# unset irrelevant variables
+deactivate nondestructive
+
+VIRTUAL_ENV='/Users/alexw/dev/ruff/crates/red_knot_workspace/resources/test/empty-test-venv'
+if ([ "$OSTYPE" = "cygwin" ] || [ "$OSTYPE" = "msys" ]) && $(command -v cygpath &> /dev/null) ; then
+    VIRTUAL_ENV=$(cygpath -u "$VIRTUAL_ENV")
+fi
+export VIRTUAL_ENV
+
+_OLD_VIRTUAL_PATH="$PATH"
+PATH="$VIRTUAL_ENV/bin:$PATH"
+export PATH
+
+if [ "x" != x ] ; then
+    VIRTUAL_ENV_PROMPT=""
+else
+    VIRTUAL_ENV_PROMPT=$(basename "$VIRTUAL_ENV")
+fi
+export VIRTUAL_ENV_PROMPT
+
+# unset PYTHONHOME if set
+if ! [ -z "${PYTHONHOME+_}" ] ; then
+    _OLD_VIRTUAL_PYTHONHOME="$PYTHONHOME"
+    unset PYTHONHOME
+fi
+
+if [ -z "${VIRTUAL_ENV_DISABLE_PROMPT-}" ] ; then
+    _OLD_VIRTUAL_PS1="${PS1-}"
+    PS1="(${VIRTUAL_ENV_PROMPT}) ${PS1-}"
+    export PS1
+fi
+
+# Make sure to unalias pydoc if it's already there
+alias pydoc 2>/dev/null >/dev/null && unalias pydoc || true
+
+pydoc () {
+    python -m pydoc "$@"
+}
+
+# The hash command must be called to get it to forget past
+# commands. Without forgetting past commands the $PATH changes
+# we made may not be respected
+hash -r 2>/dev/null
diff --git a/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/activate.bat b/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/activate.bat
new file mode 100644
index 0000000000..4ed9b55308
--- /dev/null
+++ b/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/activate.bat
@@ -0,0 +1,59 @@
+@REM Copyright (c) 2020-202x The virtualenv developers
+@REM
+@REM Permission is hereby granted, free of charge, to any person obtaining
+@REM a copy of this software and associated documentation files (the
+@REM "Software"), to deal in the Software without restriction, including
+@REM without limitation the rights to use, copy, modify, merge, publish,
+@REM distribute, sublicense, and/or sell copies of the Software, and to
+@REM permit persons to whom the Software is furnished to do so, subject to
+@REM the following conditions:
+@REM
+@REM The above copyright notice and this permission notice shall be
+@REM included in all copies or substantial portions of the Software.
+@REM
+@REM THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+@REM EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+@REM MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+@REM NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+@REM LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+@REM OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+@REM WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+@for %%i in ("/Users/alexw/dev/ruff/crates/red_knot_workspace/resources/test/empty-test-venv") do @set "VIRTUAL_ENV=%%~fi"
+
+@set "VIRTUAL_ENV_PROMPT="
+@if NOT DEFINED VIRTUAL_ENV_PROMPT (
+    @for %%d in ("%VIRTUAL_ENV%") do @set "VIRTUAL_ENV_PROMPT=%%~nxd"
+)
+
+@if defined _OLD_VIRTUAL_PROMPT (
+    @set "PROMPT=%_OLD_VIRTUAL_PROMPT%"
+) else (
+    @if not defined PROMPT (
+        @set "PROMPT=$P$G"
+    )
+    @if not defined VIRTUAL_ENV_DISABLE_PROMPT (
+        @set "_OLD_VIRTUAL_PROMPT=%PROMPT%"
+    )
+)
+@if not defined VIRTUAL_ENV_DISABLE_PROMPT (
+    @set "PROMPT=(%VIRTUAL_ENV_PROMPT%) %PROMPT%"
+)
+
+@REM Don't use () to avoid problems with them in %PATH%
+@if defined _OLD_VIRTUAL_PYTHONHOME @goto ENDIFVHOME
+    @set "_OLD_VIRTUAL_PYTHONHOME=%PYTHONHOME%"
+:ENDIFVHOME
+
+@set PYTHONHOME=
+
+@REM if defined _OLD_VIRTUAL_PATH (
+@if not defined _OLD_VIRTUAL_PATH @goto ENDIFVPATH1
+    @set "PATH=%_OLD_VIRTUAL_PATH%"
+:ENDIFVPATH1
+@REM ) else (
+@if defined _OLD_VIRTUAL_PATH @goto ENDIFVPATH2
+    @set "_OLD_VIRTUAL_PATH=%PATH%"
+:ENDIFVPATH2
+
+@set "PATH=%VIRTUAL_ENV%\bin;%PATH%"
diff --git a/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/activate.csh b/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/activate.csh
new file mode 100644
index 0000000000..e0e8bc4876
--- /dev/null
+++ b/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/activate.csh
@@ -0,0 +1,76 @@
+# Copyright (c) 2020-202x The virtualenv developers
+#
+# Permission is hereby granted, free of charge, to any person obtaining
+# a copy of this software and associated documentation files (the
+# "Software"), to deal in the Software without restriction, including
+# without limitation the rights to use, copy, modify, merge, publish,
+# distribute, sublicense, and/or sell copies of the Software, and to
+# permit persons to whom the Software is furnished to do so, subject to
+# the following conditions:
+#
+# The above copyright notice and this permission notice shall be
+# included in all copies or substantial portions of the Software.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+# LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+# OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+# WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+# This file must be used with "source bin/activate.csh" *from csh*.
+# You cannot run it directly.
+# Created by Davide Di Blasi <davidedb@gmail.com>.
+
+set newline='\
+'
+
+alias deactivate 'test $?_OLD_VIRTUAL_PATH != 0 && setenv PATH "$_OLD_VIRTUAL_PATH:q" && unset _OLD_VIRTUAL_PATH; rehash; test $?_OLD_VIRTUAL_PROMPT != 0 && set prompt="$_OLD_VIRTUAL_PROMPT:q" && unset _OLD_VIRTUAL_PROMPT; unsetenv VIRTUAL_ENV; unsetenv VIRTUAL_ENV_PROMPT; test "\!:*" != "nondestructive" && unalias deactivate && unalias pydoc'
+
+# Unset irrelevant variables.
+deactivate nondestructive
+
+setenv VIRTUAL_ENV '/Users/alexw/dev/ruff/crates/red_knot_workspace/resources/test/empty-test-venv'
+
+set _OLD_VIRTUAL_PATH="$PATH:q"
+setenv PATH "$VIRTUAL_ENV:q/bin:$PATH:q"
+
+
+
+if ('' != "") then
+    setenv VIRTUAL_ENV_PROMPT ''
+else
+    setenv VIRTUAL_ENV_PROMPT "$VIRTUAL_ENV:t:q"
+endif
+
+if ( $?VIRTUAL_ENV_DISABLE_PROMPT ) then
+    if ( $VIRTUAL_ENV_DISABLE_PROMPT == "" ) then
+        set do_prompt = "1"
+    else
+        set do_prompt = "0"
+    endif
+else
+    set do_prompt = "1"
+endif
+
+if ( $do_prompt == "1" ) then
+    # Could be in a non-interactive environment,
+    # in which case, $prompt is undefined and we wouldn't
+    # care about the prompt anyway.
+    if ( $?prompt ) then
+        set _OLD_VIRTUAL_PROMPT="$prompt:q"
+        if ( "$prompt:q" =~ *"$newline:q"* ) then
+            :
+        else
+            set prompt = '('"$VIRTUAL_ENV_PROMPT:q"') '"$prompt:q"
+        endif
+    endif
+endif
+
+unset env_name
+unset do_prompt
+
+alias pydoc python -m pydoc
+
+rehash
diff --git a/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/activate.fish b/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/activate.fish
new file mode 100644
index 0000000000..a9044de83e
--- /dev/null
+++ b/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/activate.fish
@@ -0,0 +1,124 @@
+# Copyright (c) 2020-202x The virtualenv developers
+#
+# Permission is hereby granted, free of charge, to any person obtaining
+# a copy of this software and associated documentation files (the
+# "Software"), to deal in the Software without restriction, including
+# without limitation the rights to use, copy, modify, merge, publish,
+# distribute, sublicense, and/or sell copies of the Software, and to
+# permit persons to whom the Software is furnished to do so, subject to
+# the following conditions:
+#
+# The above copyright notice and this permission notice shall be
+# included in all copies or substantial portions of the Software.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+# LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+# OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+# WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+# This file must be used using `source bin/activate.fish` *within a running fish ( http://fishshell.com ) session*.
+# Do not run it directly.
+
+function _bashify_path -d "Converts a fish path to something bash can recognize"
+    set fishy_path $argv
+    set bashy_path $fishy_path[1]
+    for path_part in $fishy_path[2..-1]
+        set bashy_path "$bashy_path:$path_part"
+    end
+    echo $bashy_path
+end
+
+function _fishify_path -d "Converts a bash path to something fish can recognize"
+    echo $argv | tr ':' '\n'
+end
+
+function deactivate -d 'Exit virtualenv mode and return to the normal environment.'
+    # reset old environment variables
+    if test -n "$_OLD_VIRTUAL_PATH"
+        # https://github.com/fish-shell/fish-shell/issues/436 altered PATH handling
+        if test (echo $FISH_VERSION | head -c 1) -lt 3
+            set -gx PATH (_fishify_path "$_OLD_VIRTUAL_PATH")
+        else
+            set -gx PATH $_OLD_VIRTUAL_PATH
+        end
+        set -e _OLD_VIRTUAL_PATH
+    end
+
+    if test -n "$_OLD_VIRTUAL_PYTHONHOME"
+        set -gx PYTHONHOME "$_OLD_VIRTUAL_PYTHONHOME"
+        set -e _OLD_VIRTUAL_PYTHONHOME
+    end
+
+    if test -n "$_OLD_FISH_PROMPT_OVERRIDE"
+       and functions -q _old_fish_prompt
+        # Set an empty local `$fish_function_path` to allow the removal of `fish_prompt` using `functions -e`.
+        set -l fish_function_path
+
+        # Erase virtualenv's `fish_prompt` and restore the original.
+        functions -e fish_prompt
+        functions -c _old_fish_prompt fish_prompt
+        functions -e _old_fish_prompt
+        set -e _OLD_FISH_PROMPT_OVERRIDE
+    end
+
+    set -e VIRTUAL_ENV
+    set -e VIRTUAL_ENV_PROMPT
+
+    if test "$argv[1]" != 'nondestructive'
+        # Self-destruct!
+        functions -e pydoc
+        functions -e deactivate
+        functions -e _bashify_path
+        functions -e _fishify_path
+    end
+end
+
+# Unset irrelevant variables.
+deactivate nondestructive
+
+set -gx VIRTUAL_ENV '/Users/alexw/dev/ruff/crates/red_knot_workspace/resources/test/empty-test-venv'
+
+# https://github.com/fish-shell/fish-shell/issues/436 altered PATH handling
+if test (echo $FISH_VERSION | head -c 1) -lt 3
+    set -gx _OLD_VIRTUAL_PATH (_bashify_path $PATH)
+else
+    set -gx _OLD_VIRTUAL_PATH $PATH
+end
+set -gx PATH "$VIRTUAL_ENV"'/bin' $PATH
+
+# Prompt override provided?
+# If not, just use the environment name.
+if test -n ''
+    set -gx VIRTUAL_ENV_PROMPT ''
+else
+    set -gx VIRTUAL_ENV_PROMPT (basename "$VIRTUAL_ENV")
+end
+
+# Unset `$PYTHONHOME` if set.
+if set -q PYTHONHOME
+    set -gx _OLD_VIRTUAL_PYTHONHOME $PYTHONHOME
+    set -e PYTHONHOME
+end
+
+function pydoc
+    python -m pydoc $argv
+end
+
+if test -z "$VIRTUAL_ENV_DISABLE_PROMPT"
+    # Copy the current `fish_prompt` function as `_old_fish_prompt`.
+    functions -c fish_prompt _old_fish_prompt
+
+    function fish_prompt
+        # Run the user's prompt first; it might depend on (pipe)status.
+        set -l prompt (_old_fish_prompt)
+
+        printf '(%s) ' $VIRTUAL_ENV_PROMPT
+
+        string join -- \n $prompt # handle multi-line prompts
+    end
+
+    set -gx _OLD_FISH_PROMPT_OVERRIDE "$VIRTUAL_ENV"
+end
diff --git a/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/activate.nu b/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/activate.nu
new file mode 100644
index 0000000000..1de75538f4
--- /dev/null
+++ b/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/activate.nu
@@ -0,0 +1,117 @@
+# Copyright (c) 2020-202x The virtualenv developers
+#
+# Permission is hereby granted, free of charge, to any person obtaining
+# a copy of this software and associated documentation files (the
+# "Software"), to deal in the Software without restriction, including
+# without limitation the rights to use, copy, modify, merge, publish,
+# distribute, sublicense, and/or sell copies of the Software, and to
+# permit persons to whom the Software is furnished to do so, subject to
+# the following conditions:
+#
+# The above copyright notice and this permission notice shall be
+# included in all copies or substantial portions of the Software.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+# LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+# OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+# WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+# virtualenv activation module
+# Activate with `overlay use activate.nu`
+# Deactivate with `deactivate`, as usual
+#
+# To customize the overlay name, you can call `overlay use activate.nu as foo`,
+# but then simply `deactivate` won't work because it is just an alias to hide
+# the "activate" overlay. You'd need to call `overlay hide foo` manually.
+
+export-env {
+    def is-string [x] {
+        ($x | describe) == 'string'
+    }
+
+    def has-env [...names] {
+        $names | each {|n|
+            $n in $env
+        } | all {|i| $i == true}
+    }
+
+    # Emulates a `test -z`, but btter as it handles e.g 'false'
+    def is-env-true [name: string] {
+      if (has-env $name) {
+        # Try to parse 'true', '0', '1', and fail if not convertible
+        let parsed = (do -i { $env | get $name | into bool })
+        if ($parsed | describe) == 'bool' {
+          $parsed
+        } else {
+          not ($env | get -i $name | is-empty)
+        }
+      } else {
+        false
+      }
+    }
+
+    let virtual_env = '/Users/alexw/dev/ruff/crates/red_knot_workspace/resources/test/empty-test-venv'
+    let bin = 'bin'
+
+    let is_windows = ($nu.os-info.family) == 'windows'
+    let path_name = (if (has-env 'Path') {
+            'Path'
+        } else {
+            'PATH'
+        }
+    )
+
+    let venv_path = ([$virtual_env $bin] | path join)
+    let new_path = ($env | get $path_name | prepend $venv_path)
+
+    # If there is no default prompt, then use the env name instead
+    let virtual_env_prompt = (if ('' | is-empty) {
+        ($virtual_env | path basename)
+    } else {
+        ''
+    })
+
+    let new_env = {
+        $path_name         : $new_path
+        VIRTUAL_ENV        : $virtual_env
+        VIRTUAL_ENV_PROMPT : $virtual_env_prompt
+    }
+
+    let new_env = (if (is-env-true 'VIRTUAL_ENV_DISABLE_PROMPT') {
+      $new_env
+    } else {
+      # Creating the new prompt for the session
+      let virtual_prefix = $'(char lparen)($virtual_env_prompt)(char rparen) '
+
+      # Back up the old prompt builder
+      let old_prompt_command = (if (has-env 'PROMPT_COMMAND') {
+              $env.PROMPT_COMMAND
+          } else {
+              ''
+        })
+
+      let new_prompt = (if (has-env 'PROMPT_COMMAND') {
+          if 'closure' in ($old_prompt_command | describe) {
+              {|| $'($virtual_prefix)(do $old_prompt_command)' }
+          } else {
+              {|| $'($virtual_prefix)($old_prompt_command)' }
+          }
+      } else {
+          {|| $'($virtual_prefix)' }
+      })
+
+      $new_env | merge {
+        PROMPT_COMMAND      : $new_prompt
+        VIRTUAL_PREFIX      : $virtual_prefix
+      }
+    })
+
+    # Environment variables that will be loaded as the virtual env
+    load-env $new_env
+}
+
+export alias pydoc = python -m pydoc
+export alias deactivate = overlay hide activate
diff --git a/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/activate.ps1 b/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/activate.ps1
new file mode 100644
index 0000000000..2d2bc9ab7b
--- /dev/null
+++ b/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/activate.ps1
@@ -0,0 +1,82 @@
+# Copyright (c) 2020-202x The virtualenv developers
+#
+# Permission is hereby granted, free of charge, to any person obtaining
+# a copy of this software and associated documentation files (the
+# "Software"), to deal in the Software without restriction, including
+# without limitation the rights to use, copy, modify, merge, publish,
+# distribute, sublicense, and/or sell copies of the Software, and to
+# permit persons to whom the Software is furnished to do so, subject to
+# the following conditions:
+#
+# The above copyright notice and this permission notice shall be
+# included in all copies or substantial portions of the Software.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+# LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+# OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+# WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+$script:THIS_PATH = $myinvocation.mycommand.path
+$script:BASE_DIR = Split-Path (Resolve-Path "$THIS_PATH/..") -Parent
+
+function global:deactivate([switch] $NonDestructive) {
+    if (Test-Path variable:_OLD_VIRTUAL_PATH) {
+        $env:PATH = $variable:_OLD_VIRTUAL_PATH
+        Remove-Variable "_OLD_VIRTUAL_PATH" -Scope global
+    }
+
+    if (Test-Path function:_old_virtual_prompt) {
+        $function:prompt = $function:_old_virtual_prompt
+        Remove-Item function:\_old_virtual_prompt
+    }
+
+    if ($env:VIRTUAL_ENV) {
+        Remove-Item env:VIRTUAL_ENV -ErrorAction SilentlyContinue
+    }
+
+    if ($env:VIRTUAL_ENV_PROMPT) {
+        Remove-Item env:VIRTUAL_ENV_PROMPT -ErrorAction SilentlyContinue
+    }
+
+    if (!$NonDestructive) {
+        # Self destruct!
+        Remove-Item function:deactivate
+        Remove-Item function:pydoc
+    }
+}
+
+function global:pydoc {
+    python -m pydoc $args
+}
+
+# unset irrelevant variables
+deactivate -nondestructive
+
+$VIRTUAL_ENV = $BASE_DIR
+$env:VIRTUAL_ENV = $VIRTUAL_ENV
+
+if ("" -ne "") {
+    $env:VIRTUAL_ENV_PROMPT = ""
+}
+else {
+    $env:VIRTUAL_ENV_PROMPT = $( Split-Path $env:VIRTUAL_ENV -Leaf )
+}
+
+New-Variable -Scope global -Name _OLD_VIRTUAL_PATH -Value $env:PATH
+
+$env:PATH = "$env:VIRTUAL_ENV/bin:" + $env:PATH
+if (!$env:VIRTUAL_ENV_DISABLE_PROMPT) {
+    function global:_old_virtual_prompt {
+        ""
+    }
+    $function:_old_virtual_prompt = $function:prompt
+
+    function global:prompt {
+        # Add the custom prefix to the existing prompt
+        $previous_prompt_value = & $function:_old_virtual_prompt
+        ("(" + $env:VIRTUAL_ENV_PROMPT + ") " + $previous_prompt_value)
+    }
+}
diff --git a/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/activate_this.py b/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/activate_this.py
new file mode 100644
index 0000000000..b3d0821f45
--- /dev/null
+++ b/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/activate_this.py
@@ -0,0 +1,59 @@
+# Copyright (c) 2020-202x The virtualenv developers
+#
+# Permission is hereby granted, free of charge, to any person obtaining
+# a copy of this software and associated documentation files (the
+# "Software"), to deal in the Software without restriction, including
+# without limitation the rights to use, copy, modify, merge, publish,
+# distribute, sublicense, and/or sell copies of the Software, and to
+# permit persons to whom the Software is furnished to do so, subject to
+# the following conditions:
+#
+# The above copyright notice and this permission notice shall be
+# included in all copies or substantial portions of the Software.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+# LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+# OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+# WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+"""
+Activate virtualenv for current interpreter:
+
+import runpy
+runpy.run_path(this_file)
+
+This can be used when you must use an existing Python interpreter, not the virtualenv bin/python.
+"""  # noqa: D415
+
+from __future__ import annotations
+
+import os
+import site
+import sys
+
+try:
+    abs_file = os.path.abspath(__file__)
+except NameError as exc:
+    msg = "You must use import runpy; runpy.run_path(this_file)"
+    raise AssertionError(msg) from exc
+
+bin_dir = os.path.dirname(abs_file)
+base = bin_dir[: -len("bin") - 1]  # strip away the bin part from the __file__, plus the path separator
+
+# prepend bin to PATH (this file is inside the bin directory)
+os.environ["PATH"] = os.pathsep.join([bin_dir, *os.environ.get("PATH", "").split(os.pathsep)])
+os.environ["VIRTUAL_ENV"] = base  # virtual env is right above bin directory
+os.environ["VIRTUAL_ENV_PROMPT"] = "" or os.path.basename(base)  # noqa: SIM222
+
+# add the virtual environments libraries to the host python import mechanism
+prev_length = len(sys.path)
+for lib in "../lib/python3.12/site-packages".split(os.pathsep):
+    path = os.path.realpath(os.path.join(bin_dir, lib))
+    site.addsitedir(path)
+sys.path[:] = sys.path[prev_length:] + sys.path[0:prev_length]
+
+sys.real_prefix = sys.prefix
+sys.prefix = base
diff --git a/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/deactivate.bat b/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/deactivate.bat
new file mode 100644
index 0000000000..95af1351b0
--- /dev/null
+++ b/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/deactivate.bat
@@ -0,0 +1,39 @@
+@REM Copyright (c) 2020-202x The virtualenv developers
+@REM
+@REM Permission is hereby granted, free of charge, to any person obtaining
+@REM a copy of this software and associated documentation files (the
+@REM "Software"), to deal in the Software without restriction, including
+@REM without limitation the rights to use, copy, modify, merge, publish,
+@REM distribute, sublicense, and/or sell copies of the Software, and to
+@REM permit persons to whom the Software is furnished to do so, subject to
+@REM the following conditions:
+@REM
+@REM The above copyright notice and this permission notice shall be
+@REM included in all copies or substantial portions of the Software.
+@REM
+@REM THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+@REM EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+@REM MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+@REM NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+@REM LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+@REM OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+@REM WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+@set VIRTUAL_ENV=
+@set VIRTUAL_ENV_PROMPT=
+
+@REM Don't use () to avoid problems with them in %PATH%
+@if not defined _OLD_VIRTUAL_PROMPT @goto ENDIFVPROMPT
+    @set "PROMPT=%_OLD_VIRTUAL_PROMPT%"
+    @set _OLD_VIRTUAL_PROMPT=
+:ENDIFVPROMPT
+
+@if not defined _OLD_VIRTUAL_PYTHONHOME @goto ENDIFVHOME
+    @set "PYTHONHOME=%_OLD_VIRTUAL_PYTHONHOME%"
+    @set _OLD_VIRTUAL_PYTHONHOME=
+:ENDIFVHOME
+
+@if not defined _OLD_VIRTUAL_PATH @goto ENDIFVPATH
+    @set "PATH=%_OLD_VIRTUAL_PATH%"
+    @set _OLD_VIRTUAL_PATH=
+:ENDIFVPATH
\ No newline at end of file
diff --git a/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/pydoc.bat b/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/pydoc.bat
new file mode 100644
index 0000000000..8a8d590d22
--- /dev/null
+++ b/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/pydoc.bat
@@ -0,0 +1,22 @@
+@REM Copyright (c) 2020-202x The virtualenv developers
+@REM
+@REM Permission is hereby granted, free of charge, to any person obtaining
+@REM a copy of this software and associated documentation files (the
+@REM "Software"), to deal in the Software without restriction, including
+@REM without limitation the rights to use, copy, modify, merge, publish,
+@REM distribute, sublicense, and/or sell copies of the Software, and to
+@REM permit persons to whom the Software is furnished to do so, subject to
+@REM the following conditions:
+@REM
+@REM The above copyright notice and this permission notice shall be
+@REM included in all copies or substantial portions of the Software.
+@REM
+@REM THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+@REM EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+@REM MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+@REM NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+@REM LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+@REM OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+@REM WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+python.exe -m pydoc %*
\ No newline at end of file
diff --git a/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/python b/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/python
new file mode 120000
index 0000000000..f14ea3e16c
--- /dev/null
+++ b/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/python
@@ -0,0 +1 @@
+/Users/alexw/.pyenv/versions/3.12.4/bin/python3.12
\ No newline at end of file
diff --git a/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/python3 b/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/python3
new file mode 120000
index 0000000000..d8654aa0e2
--- /dev/null
+++ b/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/python3
@@ -0,0 +1 @@
+python
\ No newline at end of file
diff --git a/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/python3.12 b/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/python3.12
new file mode 120000
index 0000000000..d8654aa0e2
--- /dev/null
+++ b/crates/red_knot_workspace/resources/test/empty-unix-venv/bin/python3.12
@@ -0,0 +1 @@
+python
\ No newline at end of file
diff --git a/crates/red_knot_workspace/resources/test/empty-unix-venv/lib/python3.12/site-packages/_virtualenv.pth b/crates/red_knot_workspace/resources/test/empty-unix-venv/lib/python3.12/site-packages/_virtualenv.pth
new file mode 100644
index 0000000000..1c3ff99867
--- /dev/null
+++ b/crates/red_knot_workspace/resources/test/empty-unix-venv/lib/python3.12/site-packages/_virtualenv.pth
@@ -0,0 +1 @@
+import _virtualenv
\ No newline at end of file
diff --git a/crates/red_knot_workspace/resources/test/empty-unix-venv/lib/python3.12/site-packages/_virtualenv.py b/crates/red_knot_workspace/resources/test/empty-unix-venv/lib/python3.12/site-packages/_virtualenv.py
new file mode 100644
index 0000000000..f5a0280481
--- /dev/null
+++ b/crates/red_knot_workspace/resources/test/empty-unix-venv/lib/python3.12/site-packages/_virtualenv.py
@@ -0,0 +1,103 @@
+"""Patches that are applied at runtime to the virtual environment."""
+
+from __future__ import annotations
+
+import os
+import sys
+
+VIRTUALENV_PATCH_FILE = os.path.join(__file__)
+
+
+def patch_dist(dist):
+    """
+    Distutils allows user to configure some arguments via a configuration file:
+    https://docs.python.org/3/install/index.html#distutils-configuration-files.
+
+    Some of this arguments though don't make sense in context of the virtual environment files, let's fix them up.
+    """  # noqa: D205
+    # we cannot allow some install config as that would get packages installed outside of the virtual environment
+    old_parse_config_files = dist.Distribution.parse_config_files
+
+    def parse_config_files(self, *args, **kwargs):
+        result = old_parse_config_files(self, *args, **kwargs)
+        install = self.get_option_dict("install")
+
+        if "prefix" in install:  # the prefix governs where to install the libraries
+            install["prefix"] = VIRTUALENV_PATCH_FILE, os.path.abspath(sys.prefix)
+        for base in ("purelib", "platlib", "headers", "scripts", "data"):
+            key = f"install_{base}"
+            if key in install:  # do not allow global configs to hijack venv paths
+                install.pop(key, None)
+        return result
+
+    dist.Distribution.parse_config_files = parse_config_files
+
+
+# Import hook that patches some modules to ignore configuration values that break package installation in case
+# of virtual environments.
+_DISTUTILS_PATCH = "distutils.dist", "setuptools.dist"
+# https://docs.python.org/3/library/importlib.html#setting-up-an-importer
+
+
+class _Finder:
+    """A meta path finder that allows patching the imported distutils modules."""
+
+    fullname = None
+
+    # lock[0] is threading.Lock(), but initialized lazily to avoid importing threading very early at startup,
+    # because there are gevent-based applications that need to be first to import threading by themselves.
+    # See https://github.com/pypa/virtualenv/issues/1895 for details.
+    lock = []  # noqa: RUF012
+
+    def find_spec(self, fullname, path, target=None):  # noqa: ARG002
+        if fullname in _DISTUTILS_PATCH and self.fullname is None:
+            # initialize lock[0] lazily
+            if len(self.lock) == 0:
+                import threading
+
+                lock = threading.Lock()
+                # there is possibility that two threads T1 and T2 are simultaneously running into find_spec,
+                # observing .lock as empty, and further going into hereby initialization. However due to the GIL,
+                # list.append() operation is atomic and this way only one of the threads will "win" to put the lock
+                # - that every thread will use - into .lock[0].
+                # https://docs.python.org/3/faq/library.html#what-kinds-of-global-value-mutation-are-thread-safe
+                self.lock.append(lock)
+
+            from functools import partial
+            from importlib.util import find_spec
+
+            with self.lock[0]:
+                self.fullname = fullname
+                try:
+                    spec = find_spec(fullname, path)
+                    if spec is not None:
+                        # https://www.python.org/dev/peps/pep-0451/#how-loading-will-work
+                        is_new_api = hasattr(spec.loader, "exec_module")
+                        func_name = "exec_module" if is_new_api else "load_module"
+                        old = getattr(spec.loader, func_name)
+                        func = self.exec_module if is_new_api else self.load_module
+                        if old is not func:
+                            try:  # noqa: SIM105
+                                setattr(spec.loader, func_name, partial(func, old))
+                            except AttributeError:
+                                pass  # C-Extension loaders are r/o such as zipimporter with <3.7
+                        return spec
+                finally:
+                    self.fullname = None
+        return None
+
+    @staticmethod
+    def exec_module(old, module):
+        old(module)
+        if module.__name__ in _DISTUTILS_PATCH:
+            patch_dist(module)
+
+    @staticmethod
+    def load_module(old, name):
+        module = old(name)
+        if module.__name__ in _DISTUTILS_PATCH:
+            patch_dist(module)
+        return module
+
+
+sys.meta_path.insert(0, _Finder())
diff --git a/crates/red_knot_workspace/resources/test/empty-unix-venv/pyvenv.cfg b/crates/red_knot_workspace/resources/test/empty-unix-venv/pyvenv.cfg
new file mode 100644
index 0000000000..b044f0a820
--- /dev/null
+++ b/crates/red_knot_workspace/resources/test/empty-unix-venv/pyvenv.cfg
@@ -0,0 +1,6 @@
+home = /Users/alexw/.pyenv/versions/3.12.4/bin
+implementation = CPython
+uv = 0.2.32
+version_info = 3.12.4
+include-system-site-packages = false
+relocatable = false
diff --git a/crates/red_knot_workspace/src/lib.rs b/crates/red_knot_workspace/src/lib.rs
index f0b3f62a98..45a27012fc 100644
--- a/crates/red_knot_workspace/src/lib.rs
+++ b/crates/red_knot_workspace/src/lib.rs
@@ -1,4 +1,5 @@
 pub mod db;
 pub mod lint;
+pub mod site_packages;
 pub mod watch;
 pub mod workspace;
diff --git a/crates/red_knot_workspace/src/site_packages.rs b/crates/red_knot_workspace/src/site_packages.rs
new file mode 100644
index 0000000000..b457d6daca
--- /dev/null
+++ b/crates/red_knot_workspace/src/site_packages.rs
@@ -0,0 +1,170 @@
+//! Utilities for finding the `site-packages` directory,
+//! into which third-party packages are installed.
+//!
+//! The routines exposed by this module have different behaviour depending
+//! on the platform of the *host machine*, which may be
+//! different from the *target platform for type checking*. (A user
+//! might be running red-knot on a Windows machine, but might
+//! reasonably ask us to type-check code assuming that the code runs
+//! on Linux.)
+
+use std::io;
+
+use ruff_db::system::{System, SystemPath, SystemPathBuf};
+
+/// Attempt to retrieve the `site-packages` directory
+/// associated with a given Python installation.
+///
+/// `sys_prefix_path` is equivalent to the value of [`sys.prefix`]
+/// at runtime in Python. For the case of a virtual environment, where a
+/// Python binary is at `/.venv/bin/python`, `sys.prefix` is the path to
+/// the virtual environment the Python binary lies inside, i.e. `/.venv`,
+/// and `site-packages` will be at `.venv/Lib/site-packages`. System
+/// Python installations generally work the same way: if a system Python
+/// installation lies at `/opt/homebrew/bin/python`, `sys.prefix` will be
+/// `/opt/homebrew`, and `site-packages` will be at
+/// `/opt/homebrew/Lib/site-packages`.
+///
+/// This routine does not verify that `sys_prefix_path` points
+/// to an existing directory on disk; it is assumed that this has already
+/// been checked.
+///
+/// [`sys.prefix`]: https://docs.python.org/3/library/sys.html#sys.prefix
+#[cfg(target_os = "windows")]
+fn site_packages_dir_from_sys_prefix(
+    sys_prefix_path: &SystemPath,
+    system: &dyn System,
+) -> Result<SystemPathBuf, SitePackagesDiscoveryError> {
+    let site_packages = venv_path.join("Lib/site-packages");
+    system
+        .is_directory(&site_packages)
+        .then_some(site_packages)
+        .ok_or(SitePackagesDiscoveryError::NoSitePackagesDirFound)
+}
+
+/// Attempt to retrieve the `site-packages` directory
+/// associated with a given Python installation.
+///
+/// `sys_prefix_path` is equivalent to the value of [`sys.prefix`]
+/// at runtime in Python. For the case of a virtual environment, where a
+/// Python binary is at `/.venv/bin/python`, `sys.prefix` is the path to
+/// the virtual environment the Python binary lies inside, i.e. `/.venv`,
+/// and `site-packages` will be at `.venv/lib/python3.X/site-packages`.
+/// System Python installations generally work the same way: if a system
+/// Python installation lies at `/opt/homebrew/bin/python`, `sys.prefix`
+/// will be `/opt/homebrew`, and `site-packages` will be at
+/// `/opt/homebrew/lib/python3.X/site-packages`.
+///
+/// This routine does not verify that `sys_prefix_path` points
+/// to an existing directory on disk; it is assumed that this has already
+/// been checked.
+///
+/// [`sys.prefix`]: https://docs.python.org/3/library/sys.html#sys.prefix
+#[cfg(not(target_os = "windows"))]
+fn site_packages_dir_from_sys_prefix(
+    sys_prefix_path: &SystemPath,
+    system: &dyn System,
+) -> Result<SystemPathBuf, SitePackagesDiscoveryError> {
+    // In the Python standard library's `site.py` module (used for finding `site-packages`
+    // at runtime), we can find this in [the non-Windows branch]:
+    //
+    // ```py
+    // libdirs = [sys.platlibdir]
+    // if sys.platlibdir != "lib":
+    //     libdirs.append("lib")
+    // ```
+    //
+    // Pyright therefore searches for both a `lib/python3.X/site-packages` directory
+    // and a `lib64/python3.X/site-packages` directory on non-MacOS Unix systems,
+    // since `sys.platlibdir` can sometimes be equal to `"lib64"`.
+    //
+    // However, we only care about the `site-packages` directory insofar as it allows
+    // us to discover Python source code that can be used for inferring type
+    // information regarding third-party dependencies. That means that we don't need
+    // to care about any possible `lib64/site-packages` directories, since
+    // [the `sys`-module documentation] states that `sys.platlibdir` is *only* ever
+    // used for C extensions, never for pure-Python modules.
+    //
+    // [the non-Windows branch]: https://github.com/python/cpython/blob/a8be8fc6c4682089be45a87bd5ee1f686040116c/Lib/site.py#L401-L410
+    // [the `sys`-module documentation]: https://docs.python.org/3/library/sys.html#sys.platlibdir
+    for entry_result in system.read_directory(&sys_prefix_path.join("lib"))? {
+        let Ok(entry) = entry_result else {
+            continue;
+        };
+        if !entry.file_type().is_directory() {
+            continue;
+        }
+
+        let path = entry.path();
+
+        // The `python3.x` part of the `site-packages` path can't be computed from
+        // the `--target-version` the user has passed, as they might be running Python 3.12 locally
+        // even if they've requested that we type check their code "as if" they're running 3.8.
+        //
+        // The `python3.x` part of the `site-packages` path *could* be computed
+        // by parsing the virtual environment's `pyvenv.cfg` file.
+        // Right now that seems like overkill, but in the future we may need to parse
+        // the `pyvenv.cfg` file anyway, in which case we could switch to that method
+        // rather than iterating through the whole directory until we find
+        // an entry where the last component of the path starts with `python3.`
+        if !path
+            .components()
+            .next_back()
+            .is_some_and(|last_part| last_part.as_str().starts_with("python3."))
+        {
+            continue;
+        }
+
+        let site_packages_candidate = path.join("site-packages");
+        if system.is_directory(&site_packages_candidate) {
+            return Ok(site_packages_candidate);
+        }
+    }
+    Err(SitePackagesDiscoveryError::NoSitePackagesDirFound)
+}
+
+#[derive(Debug, thiserror::Error)]
+pub enum SitePackagesDiscoveryError {
+    #[error("Failed to search the virtual environment directory for `site-packages` due to {0}")]
+    CouldNotReadLibDirectory(#[from] io::Error),
+    #[error("Could not find the `site-packages` directory in the virtual environment")]
+    NoSitePackagesDirFound,
+}
+
+/// Given a validated, canonicalized path to a virtual environment,
+/// return a list of `site-packages` directories that are available from that environment.
+///
+/// See the documentation for `site_packages_dir_from_sys_prefix` for more details.
+///
+/// TODO: Currently we only ever return 1 path from this function:
+/// the `site-packages` directory that is actually inside the virtual environment.
+/// Some `site-packages` directories are able to also access system `site-packages` and
+/// user `site-packages`, however.
+pub fn site_packages_dirs_of_venv(
+    venv_path: &SystemPath,
+    system: &dyn System,
+) -> Result<Vec<SystemPathBuf>, SitePackagesDiscoveryError> {
+    Ok(vec![site_packages_dir_from_sys_prefix(venv_path, system)?])
+}
+
+#[cfg(test)]
+mod tests {
+    use ruff_db::system::{OsSystem, System, SystemPath};
+
+    use crate::site_packages::site_packages_dirs_of_venv;
+
+    #[test]
+    // Windows venvs have different layouts, and we only have a Unix venv committed for now.
+    // This test is skipped on Windows until we commit a Windows venv.
+    #[cfg(not(target_os = "windows"))]
+    fn can_find_site_packages_dir_in_committed_venv() {
+        let path_to_venv = SystemPath::new("resources/test/empty-unix-venv");
+        let system = OsSystem::default();
+
+        // if this doesn't hold true, the premise of the test is incorrect.
+        assert!(system.is_directory(path_to_venv));
+
+        let site_packages_dirs = site_packages_dirs_of_venv(path_to_venv, &system).unwrap();
+        assert_eq!(site_packages_dirs.len(), 1);
+    }
+}