[ty] Add search paths info to unresolved import diagnostics (#20040)

Fixes https://github.com/astral-sh/ty/issues/457

---------

Co-authored-by: Alex Waygood <alex.waygood@gmail.com>

crates/ty_python_semantic/resources/mdtest/snapshots/basic.md_-_Structures_-_Multiple_objects_imp…_(cbfbf5ff94e6e104).snap

@@ -26,6 +26,9 @@ error[unresolved-import]: Cannot resolve imported module `does_not_exist`
 2 | from does_not_exist import foo, bar, baz
   |      ^^^^^^^^^^^^^^
   |
+info: Searched in the following paths during module resolution:
+info:   1. /src (first-party code)
+info:   2. vendored://stdlib (stdlib typeshed stubs vendored by ty)
 info: make sure your Python environment is properly configured: https://docs.astral.sh/ty/modules/#python-environment
 info: rule `unresolved-import` is enabled by default
 

crates/ty_python_semantic/resources/mdtest/snapshots/basic.md_-_Structures_-_Unresolvable_module_…_(846453deaca1071c).snap

@@ -24,6 +24,9 @@ error[unresolved-import]: Cannot resolve imported module `zqzqzqzqzqzqzq`
 1 | import zqzqzqzqzqzqzq  # error: [unresolved-import] "Cannot resolve imported module `zqzqzqzqzqzqzq`"
   |        ^^^^^^^^^^^^^^
   |
+info: Searched in the following paths during module resolution:
+info:   1. /src (first-party code)
+info:   2. vendored://stdlib (stdlib typeshed stubs vendored by ty)
 info: make sure your Python environment is properly configured: https://docs.astral.sh/ty/modules/#python-environment
 info: rule `unresolved-import` is enabled by default
 

crates/ty_python_semantic/resources/mdtest/snapshots/basic.md_-_Structures_-_Unresolvable_submodu…_(4fad4be9778578b7).snap

@@ -36,6 +36,9 @@ error[unresolved-import]: Cannot resolve imported module `a.foo`
 3 |
 4 | # Topmost component unresolvable:
   |
+info: Searched in the following paths during module resolution:
+info:   1. /src (first-party code)
+info:   2. vendored://stdlib (stdlib typeshed stubs vendored by ty)
 info: make sure your Python environment is properly configured: https://docs.astral.sh/ty/modules/#python-environment
 info: rule `unresolved-import` is enabled by default
 
@@ -49,6 +52,9 @@ error[unresolved-import]: Cannot resolve imported module `b.foo`
 5 | import b.foo  # error: [unresolved-import] "Cannot resolve imported module `b.foo`"
   |        ^^^^^
   |
+info: Searched in the following paths during module resolution:
+info:   1. /src (first-party code)
+info:   2. vendored://stdlib (stdlib typeshed stubs vendored by ty)
 info: make sure your Python environment is properly configured: https://docs.astral.sh/ty/modules/#python-environment
 info: rule `unresolved-import` is enabled by default
 

crates/ty_python_semantic/resources/mdtest/snapshots/unresolved_import.md_-_Unresolved_import_di…_-_An_unresolvable_impo…_(72d090df51ea97b8).snap

@@ -28,6 +28,9 @@ error[unresolved-import]: Cannot resolve imported module `does_not_exist`
 2 |
 3 | x = does_not_exist.foo
   |
+info: Searched in the following paths during module resolution:
+info:   1. /src (first-party code)
+info:   2. vendored://stdlib (stdlib typeshed stubs vendored by ty)
 info: make sure your Python environment is properly configured: https://docs.astral.sh/ty/modules/#python-environment
 info: rule `unresolved-import` is enabled by default
 

crates/ty_python_semantic/resources/mdtest/snapshots/unresolved_import.md_-_Unresolved_import_di…_-_Using_`from`_with_an…_(9fa713dfa17cc404).snap

@@ -28,6 +28,9 @@ error[unresolved-import]: Cannot resolve imported module `does_not_exist`
 2 |
 3 | stat = add(10, 15)
   |
+info: Searched in the following paths during module resolution:
+info:   1. /src (first-party code)
+info:   2. vendored://stdlib (stdlib typeshed stubs vendored by ty)
 info: make sure your Python environment is properly configured: https://docs.astral.sh/ty/modules/#python-environment
 info: rule `unresolved-import` is enabled by default
 

crates/ty_python_semantic/src/module_resolver/mod.rs

@@ -10,8 +10,7 @@ pub use resolver::{resolve_module, resolve_real_module};
 use ruff_db::system::SystemPath;
 
 use crate::Db;
-use crate::module_resolver::resolver::{ModuleResolveMode, search_paths};
-use resolver::SearchPathIterator;
+pub(crate) use resolver::{ModuleResolveMode, SearchPathIterator, search_paths};
 
 mod list;
 mod module;

crates/ty_python_semantic/src/module_resolver/path.rs

@@ -700,6 +700,25 @@ impl SearchPath {
             SearchPathInner::StandardLibraryVendored(_) => "std-vendored",
         }
     }
+
+    /// Returns a string suitable for describing what kind of search path this is
+    /// in user-facing diagnostics.
+    #[must_use]
+    pub(crate) fn describe_kind(&self) -> &'static str {
+        match *self.0 {
+            SearchPathInner::Extra(_) => {
+                "extra search path specified on the CLI or in your config file"
+            }
+            SearchPathInner::FirstParty(_) => "first-party code",
+            SearchPathInner::StandardLibraryCustom(_) => {
+                "custom stdlib stubs specified on the CLI or in your config file"
+            }
+            SearchPathInner::StandardLibraryReal(_) => "runtime stdlib source code",
+            SearchPathInner::SitePackages(_) => "site-packages",
+            SearchPathInner::Editable(_) => "editable install",
+            SearchPathInner::StandardLibraryVendored(_) => "stdlib typeshed stubs vendored by ty",
+        }
+    }
 }
 
 impl PartialEq<SystemPath> for SearchPath {

crates/ty_python_semantic/src/types/infer.rs

@@ -64,7 +64,9 @@ use super::string_annotation::{
 use super::subclass_of::SubclassOfInner;
 use super::{ClassBase, add_inferred_python_version_hint_to_diagnostic};
 use crate::module_name::{ModuleName, ModuleNameResolutionError};
-use crate::module_resolver::{KnownModule, file_to_module, resolve_module};
+use crate::module_resolver::{
+    KnownModule, ModuleResolveMode, file_to_module, resolve_module, search_paths,
+};
 use crate::node_key::NodeKey;
 use crate::place::{
     Boundness, ConsideredDefinitions, LookupError, Place, PlaceAndQualifiers,
@@ -4983,6 +4985,26 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                 }
             }
 
+            // Add search paths information to the diagnostic
+            // Use the same search paths function that is used in actual module resolution
+            let mut search_paths =
+                search_paths(self.db(), ModuleResolveMode::StubsAllowed).peekable();
+
+            if search_paths.peek().is_some() {
+                diagnostic.info(format_args!(
+                    "Searched in the following paths during module resolution:"
+                ));
+
+                for (index, path) in search_paths.enumerate() {
+                    diagnostic.info(format_args!(
+                        "  {}. {} ({})",
+                        index + 1,
+                        path,
+                        path.describe_kind()
+                    ));
+                }
+            }
+
             diagnostic.info(
                 "make sure your Python environment is properly configured: \
                 https://docs.astral.sh/ty/modules/#python-environment",