[ty] Check typeshed VERSIONS for parent modules when reporting failed stdlib imports (#20908)

This is a drive-by improvement that I stumbled backwards into while looking into * astral-sh/ty#296 I was writing some simple tests for "thing not in old version of stdlib" diagnostics and checked what was added in 3.14, and saw `compression.zstd` and to my surprise discovered that `import compression.zstd` and `from compression import zstd` had completely different quality diagnostics. This is because `compression` and `compression.zstd` were *both* introduced in 3.14, and so per VERSIONS policy only an entry for `compression` was added, and so we don't actually have any definite info on `compression.zstd` and give up on producing a diagnostic. However the `from compression import zstd` form fails on looking up `compression` and we *do* have an exact match for that, so it gets a better diagnostic! (aside: I have now learned about the VERSIONS format and I *really* wish they would just enumerate all the submodules but, oh well!) The fix is, when handling an import failure, if we fail to find an exact match *we requery with the parent module*. In cases like `compression.zstd` this lets us at least identify that, hey, not even `compression` exists, and luckily that fixes the whole issue. In cases where the parent module and submodule were introduced at different times then we may discover that the parent module is in-range and that's fine, we don't produce the richer stdlib diagnostic.

Author: Gankra

crates/ty_python_semantic/resources/mdtest/import/basic.md

@@ -192,6 +192,26 @@ from string.templatelib import Template # error: [unresolved-import]
 from importlib.resources import abc # error: [unresolved-import]
 ```
 
+## Attempting to import a stdlib submodule when both parts haven't yet been added
+
+`compression` and `compression.zstd` were both added in 3.14 so there is a typeshed `VERSIONS` entry
+for `compression` but not `compression.zstd`. We can't be confident `compression.zstd` exists but we
+do know `compression` does and can still give good diagnostics about it.
+
+<!-- snapshot-diagnostics -->
+
+```toml
+[environment]
+python-version = "3.10"
+```
+
+```py
+import compression.zstd # error: [unresolved-import]
+from compression import zstd # error: [unresolved-import]
+import compression.fakebutwhocansay # error: [unresolved-import]
+from compression import fakebutwhocansay # error: [unresolved-import]
+```
+
 ## Attempting to import a stdlib module that was previously removed
 
 <!-- snapshot-diagnostics -->

crates/ty_python_semantic/resources/mdtest/snapshots/basic.md_-_Structures_-_Attempting_to_import…_(dba22bd97137ee38).snap

@@ -0,0 +1,83 @@
+---
+source: crates/ty_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: basic.md - Structures - Attempting to import a stdlib submodule when both parts haven't yet been added
+mdtest path: crates/ty_python_semantic/resources/mdtest/import/basic.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | import compression.zstd # error: [unresolved-import]
+2 | from compression import zstd # error: [unresolved-import]
+3 | import compression.fakebutwhocansay # error: [unresolved-import]
+4 | from compression import fakebutwhocansay # error: [unresolved-import]
+```
+
+# Diagnostics
+
+```
+error[unresolved-import]: Cannot resolve imported module `compression.zstd`
+ --> src/mdtest_snippet.py:1:8
+ |
+1 | import compression.zstd # error: [unresolved-import]
+ | ^^^^^^^^^^^^^^^^
+2 | from compression import zstd # error: [unresolved-import]
+3 | import compression.fakebutwhocansay # error: [unresolved-import]
+ |
+info: The stdlib module `compression` is only available on Python 3.14+
+info: Python 3.10 was assumed when resolving modules because it was specified on the command line
+info: rule `unresolved-import` is enabled by default
+
+```
+
+```
+error[unresolved-import]: Cannot resolve imported module `compression`
+ --> src/mdtest_snippet.py:2:6
+ |
+1 | import compression.zstd # error: [unresolved-import]
+2 | from compression import zstd # error: [unresolved-import]
+ | ^^^^^^^^^^^
+3 | import compression.fakebutwhocansay # error: [unresolved-import]
+4 | from compression import fakebutwhocansay # error: [unresolved-import]
+ |
+info: The stdlib module `compression` is only available on Python 3.14+
+info: Python 3.10 was assumed when resolving modules because it was specified on the command line
+info: rule `unresolved-import` is enabled by default
+
+```
+
+```
+error[unresolved-import]: Cannot resolve imported module `compression.fakebutwhocansay`
+ --> src/mdtest_snippet.py:3:8
+ |
+1 | import compression.zstd # error: [unresolved-import]
+2 | from compression import zstd # error: [unresolved-import]
+3 | import compression.fakebutwhocansay # error: [unresolved-import]
+ | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+4 | from compression import fakebutwhocansay # error: [unresolved-import]
+ |
+info: The stdlib module `compression` is only available on Python 3.14+
+info: Python 3.10 was assumed when resolving modules because it was specified on the command line
+info: rule `unresolved-import` is enabled by default
+
+```
+
+```
+error[unresolved-import]: Cannot resolve imported module `compression`
+ --> src/mdtest_snippet.py:4:6
+ |
+2 | from compression import zstd # error: [unresolved-import]
+3 | import compression.fakebutwhocansay # error: [unresolved-import]
+4 | from compression import fakebutwhocansay # error: [unresolved-import]
+ | ^^^^^^^^^^^
+ |
+info: The stdlib module `compression` is only available on Python 3.14+
+info: Python 3.10 was assumed when resolving modules because it was specified on the command line
+info: rule `unresolved-import` is enabled by default
+
+```

crates/ty_python_semantic/src/types/infer/builder.rs

@@ -4790,21 +4790,26 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
  let program = Program::get(self.db());
  let typeshed_versions = program.search_paths(self.db()).typeshed_versions();
 
- if let Some(version_range) = typeshed_versions.exact(&module_name) {
- // We know it is a stdlib module on *some* Python versions...
- let python_version = program.python_version(self.db());
- if !version_range.contains(python_version) {
- // ...But not on *this* Python version.
- diagnostic.info(format_args!(
- "The stdlib module `{module_name}` is only available on Python {version_range}",
- version_range = version_range.diagnostic_display(),
- ));
- add_inferred_python_version_hint_to_diagnostic(
- self.db(),
- &mut diagnostic,
- "resolving modules",
- );
- return;
+ // Loop over ancestors in case we have info on the parent module but not submodule
+ for module_name in module_name.ancestors() {
+ if let Some(version_range) = typeshed_versions.exact(&module_name) {
+ // We know it is a stdlib module on *some* Python versions...
+ let python_version = program.python_version(self.db());
+ if !version_range.contains(python_version) {
+ // ...But not on *this* Python version.
+ diagnostic.info(format_args!(
+ "The stdlib module `{module_name}` is only available on Python {version_range}",
+ version_range = version_range.diagnostic_display(),
+ ));
+ add_inferred_python_version_hint_to_diagnostic(
+ self.db(),
+ &mut diagnostic,
+ "resolving modules",
+ );
+ return;
+ }
+ // We found the most precise answer we could, stop searching
+ break;
  }
  }
  }