Warn when patch isn't specified (#7959)

When patch version isn't specified and a matching version is referenced,
it will default patch to 0 which could be unclear/confusing. This PR
warns the user of that default.

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## Summary

<!-- What's the purpose of the change? What does it do, and why? -->

The first part of this issue
https://github.com/astral-sh/uv/issues/7426. Will tackle the second part
mentioned (`~=`) in a separate PR once I know this is the correct way to
warn users.

## Test Plan

<!-- How was it tested? -->

Unit tests were added

---------

Co-authored-by: Zanie Blue <contact@zanie.dev>

crates/uv-resolver/src/requires_python.rs

@@ -248,6 +248,18 @@ impl RequiresPython {
         self.range.lower().as_ref() == Bound::Unbounded
     }
 
+    /// Returns `true` if the `Requires-Python` specifier is set to an exact version
+    /// without specifying a patch version. (e.g. `==3.10`)
+    pub fn is_exact_without_patch(&self) -> bool {
+        match self.range.lower().as_ref() {
+            Bound::Included(version) => {
+                version.release().len() == 2
+                    && self.range.upper().as_ref() == Bound::Included(version)
+            }
+            _ => false,
+        }
+    }
+
     /// Returns the [`RequiresPythonBound`] truncated to the major and minor version.
     pub fn bound_major_minor(&self) -> LowerBound {
         match self.range.lower().as_ref() {

crates/uv-resolver/src/requires_python/tests.rs

@@ -134,3 +134,25 @@ fn upper_bound_ordering() {
         }
     }
 }
+
+#[test]
+fn is_exact_without_patch() {
+    let test_cases = [
+        ("==3.12", true),
+        ("==3.10, <3.11", true),
+        ("==3.10, <=3.11", true),
+        ("==3.12.1", false),
+        ("==3.12.*", false),
+        ("==3.*", false),
+        (">=3.10", false),
+        (">3.9", false),
+        ("<4.0", false),
+        (">=3.10, <3.11", false),
+        ("", false),
+    ];
+    for (version, expected) in test_cases {
+        let version_specifiers = VersionSpecifiers::from_str(version).unwrap();
+        let requires_python = RequiresPython::from_specifiers(&version_specifiers).unwrap();
+        assert_eq!(requires_python.is_exact_without_patch(), expected);
+    }
+}

crates/uv/src/commands/project/lock.rs

@@ -335,6 +335,8 @@ async fn do_lock(
             let default =
                 RequiresPython::greater_than_equal_version(&interpreter.python_minor_version());
             warn_user_once!("The workspace `requires-python` value does not contain a lower bound: `{requires_python}`. Set a lower bound to indicate the minimum compatible Python version (e.g., `{default}`).");
+        } else if requires_python.is_exact_without_patch() {
+            warn_user_once!("The workspace `requires-python` value contains an exact match without a patch version: `{requires_python}`. When omitted, the patch version is implicitly `0`, e.g., `{requires_python}.0`. Did you mean `{requires_python}.*`?");
         }
         requires_python
     } else {

crates/uv/tests/it/lock.rs

@@ -3530,6 +3530,7 @@ fn lock_requires_python_exact() -> Result<()> {
 
     ----- stderr -----
     Using CPython 3.12.[X]
+    warning: The workspace `requires-python` value contains an exact match without a patch version: `==3.12`. When omitted, the patch version is implicitly `0`, e.g., `==3.12.[X]`. Did you mean `==3.12.*`?
     Resolved 2 packages in [TIME]
     "###);
 
@@ -3586,6 +3587,7 @@ fn lock_requires_python_exact() -> Result<()> {
 
     ----- stderr -----
     Using CPython 3.12.[X]
+    warning: The workspace `requires-python` value contains an exact match without a patch version: `==3.12`. When omitted, the patch version is implicitly `0`, e.g., `==3.12.[X]`. Did you mean `==3.12.*`?
     Resolved 2 packages in [TIME]
     "###);