Respect requires-python when installing tools (#10401)

## Summary

This PR revives #7827 to improve
tool resolutions such that, if the resolution fails, and the selected
interpreter doesn't match the required Python version from the solve, we
attempt to re-solve with a newly-discovered interpreter that _does_
match the required Python version.

For now, we attempt to choose a Python interpreter that's greater than
the inferred `requires-python`, but compatible with the same Python
minor. This helps avoid successive failures for cases like Posting,
where choosing Python 3.13 fails because it has a dependency that lacks
source distributions and doesn't publish any Python 3.13 wheels. We
should further improve the strategy to solve _that_ case too, but this
is at least the more conservative option...

In short, if you do `uv tool instal posting`, and we find Python 3.8 on
your machine, we'll detect that `requires-python: >=3.11`, then search
for the latest Python 3.11 interpreter and re-resolve.

Closes #6381.
Closes #10282.

## Test Plan

The following should succeed:

```
cargo run python uninstall --all
cargo run python install 3.8
cargo run tool install posting
```

In the logs, we see:

```
...
DEBUG No compatible version found for: posting
DEBUG Refining interpreter with: Python >=3.11, <3.12
DEBUG Searching for Python >=3.11, <3.12 in managed installations or search path
DEBUG Searching for managed installations at `/Users/crmarsh/.local/share/uv/python`
DEBUG Skipping incompatible managed installation `cpython-3.8.20-macos-aarch64-none`
DEBUG Found `cpython-3.13.1-macos-aarch64-none` at `/opt/homebrew/bin/python3` (search path)
DEBUG Skipping interpreter at `/opt/homebrew/opt/[email protected]/bin/python3.13` from search path: does not satisfy request `>=3.11, <3.12`
DEBUG Found `cpython-3.11.7-macos-aarch64-none` at `/opt/homebrew/bin/python3.11` (search path)
DEBUG Re-resolving with Python 3.11.7
DEBUG Using request timeout of 30s
DEBUG Solving with installed Python version: 3.11.7
DEBUG Solving with target Python version: >=3.11.7
DEBUG Adding direct dependency: posting*
DEBUG Searching for a compatible version of posting (*)
...
```

crates/uv-requirements/src/specification.rs

@@ -50,7 +50,7 @@ use uv_workspace::pyproject::PyProjectToml;
 
 use crate::RequirementsSource;
 
-#[derive(Debug, Default)]
+#[derive(Debug, Default, Clone)]
 pub struct RequirementsSpecification {
     /// The name of the project specifying requirements.
     pub project: Option<PackageName>,

crates/uv-resolver/src/error.rs

@@ -22,6 +22,7 @@ use crate::fork_urls::ForkUrls;
 use crate::prerelease::AllowPrerelease;
 use crate::pubgrub::{PubGrubPackage, PubGrubPackageInner, PubGrubReportFormatter};
 use crate::python_requirement::PythonRequirement;
+use crate::requires_python::LowerBound;
 use crate::resolution::ConflictingDistributionError;
 use crate::resolver::{
     MetadataUnavailable, ResolverEnvironment, UnavailablePackage, UnavailableReason,
@@ -294,6 +295,33 @@ impl NoSolutionError {
         strip(derivation_tree).expect("derivation tree should contain at least one term")
     }
 
+    /// Given a [`DerivationTree`], identify the largest required Python version that is missing.
+    pub fn find_requires_python(&self) -> LowerBound {
+        fn find(derivation_tree: &ErrorTree, minimum: &mut LowerBound) {
+            match derivation_tree {
+                DerivationTree::Derived(derived) => {
+                    find(derived.cause1.as_ref(), minimum);
+                    find(derived.cause2.as_ref(), minimum);
+                }
+                DerivationTree::External(External::FromDependencyOf(.., package, version)) => {
+                    if let PubGrubPackageInner::Python(_) = &**package {
+                        if let Some((lower, ..)) = version.bounding_range() {
+                            let lower = LowerBound::new(lower.cloned());
+                            if lower > *minimum {
+                                *minimum = lower;
+                            }
+                        }
+                    }
+                }
+                DerivationTree::External(_) => {}
+            }
+        }
+
+        let mut minimum = LowerBound::default();
+        find(&self.error, &mut minimum);
+        minimum
+    }
+
     /// Initialize a [`NoSolutionHeader`] for this error.
     pub fn header(&self) -> NoSolutionHeader {
         NoSolutionHeader::new(self.env.clone())

crates/uv-resolver/src/requires_python.rs

@@ -586,6 +586,17 @@ impl SimplifiedMarkerTree {
 pub struct LowerBound(Bound<Version>);
 
 impl LowerBound {
+    /// Initialize a [`LowerBound`] with the given bound.
+    ///
+    /// These bounds use release-only semantics when comparing versions.
+    pub fn new(bound: Bound<Version>) -> Self {
+        Self(match bound {
+            Bound::Included(version) => Bound::Included(version.only_release()),
+            Bound::Excluded(version) => Bound::Excluded(version.only_release()),
+            Bound::Unbounded => Bound::Unbounded,
+        })
+    }
+
     /// Return the [`LowerBound`] truncated to the major and minor version.
     fn major_minor(&self) -> Self {
         match &self.0 {
@@ -600,6 +611,15 @@ impl LowerBound {
             Bound::Unbounded => Self(Bound::Unbounded),
         }
     }
+
+    /// Returns `true` if the lower bound contains the given version.
+    pub fn contains(&self, version: &Version) -> bool {
+        match self.0 {
+            Bound::Included(ref bound) => bound <= version,
+            Bound::Excluded(ref bound) => bound < version,
+            Bound::Unbounded => true,
+        }
+    }
 }
 
 impl PartialOrd for LowerBound {
@@ -668,19 +688,6 @@ impl Default for LowerBound {
     }
 }
 
-impl LowerBound {
-    /// Initialize a [`LowerBound`] with the given bound.
-    ///
-    /// These bounds use release-only semantics when comparing versions.
-    pub fn new(bound: Bound<Version>) -> Self {
-        Self(match bound {
-            Bound::Included(version) => Bound::Included(version.only_release()),
-            Bound::Excluded(version) => Bound::Excluded(version.only_release()),
-            Bound::Unbounded => Bound::Unbounded,
-        })
-    }
-}
-
 impl Deref for LowerBound {
     type Target = Bound<Version>;
 
@@ -699,6 +706,17 @@ impl From<LowerBound> for Bound<Version> {
 pub struct UpperBound(Bound<Version>);
 
 impl UpperBound {
+    /// Initialize a [`UpperBound`] with the given bound.
+    ///
+    /// These bounds use release-only semantics when comparing versions.
+    pub fn new(bound: Bound<Version>) -> Self {
+        Self(match bound {
+            Bound::Included(version) => Bound::Included(version.only_release()),
+            Bound::Excluded(version) => Bound::Excluded(version.only_release()),
+            Bound::Unbounded => Bound::Unbounded,
+        })
+    }
+
     /// Return the [`UpperBound`] truncated to the major and minor version.
     fn major_minor(&self) -> Self {
         match &self.0 {
@@ -721,6 +739,15 @@ impl UpperBound {
             Bound::Unbounded => Self(Bound::Unbounded),
         }
     }
+
+    /// Returns `true` if the upper bound contains the given version.
+    pub fn contains(&self, version: &Version) -> bool {
+        match self.0 {
+            Bound::Included(ref bound) => bound >= version,
+            Bound::Excluded(ref bound) => bound > version,
+            Bound::Unbounded => true,
+        }
+    }
 }
 
 impl PartialOrd for UpperBound {
@@ -787,19 +814,6 @@ impl Default for UpperBound {
     }
 }
 
-impl UpperBound {
-    /// Initialize a [`UpperBound`] with the given bound.
-    ///
-    /// These bounds use release-only semantics when comparing versions.
-    pub fn new(bound: Bound<Version>) -> Self {
-        Self(match bound {
-            Bound::Included(version) => Bound::Included(version.only_release()),
-            Bound::Excluded(version) => Bound::Excluded(version.only_release()),
-            Bound::Unbounded => Bound::Unbounded,
-        })
-    }
-}
-
 impl Deref for UpperBound {
     type Target = Bound<Version>;
 

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

@@ -29,7 +29,7 @@ impl CachedEnvironment {
     /// interpreter.
     pub(crate) async fn get_or_create(
         spec: EnvironmentSpecification<'_>,
-        interpreter: Interpreter,
+        interpreter: &Interpreter,
         settings: &ResolverInstallerSettings,
         state: &SharedState,
         resolve: Box<dyn ResolveLogger>,
@@ -56,7 +56,7 @@ impl CachedEnvironment {
                 "Caching via interpreter: `{}`",
                 interpreter.sys_executable().display()
             );
-            interpreter
+            interpreter.clone()
         };
 
         // Resolve the requirements with the interpreter.

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

@@ -1086,7 +1086,7 @@ pub(crate) async fn resolve_names(
     Ok(requirements)
 }
 
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub(crate) struct EnvironmentSpecification<'lock> {
     /// The requirements to include in the environment.
     requirements: RequirementsSpecification,

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

@@ -296,7 +296,7 @@ pub(crate) async fn run(
                 RequirementsSpecification::from_overrides(requirements, constraints, overrides);
             let result = CachedEnvironment::get_or_create(
                 EnvironmentSpecification::from(spec),
-                interpreter,
+                &interpreter,
                 &settings,
                 &state,
                 if show_resolution {
@@ -852,7 +852,7 @@ pub(crate) async fn run(
                         lock.as_ref()
                             .map(|(lock, install_path)| (lock, install_path.as_ref())),
                     ),
-                    base_interpreter.clone(),
+                    &base_interpreter,
                     &settings,
                     &state,
                     if show_resolution {

crates/uv/src/commands/tool/common.rs

@@ -1,25 +1,32 @@
-use std::fmt::Write;
-use std::{collections::BTreeSet, ffi::OsString};
-
 use anyhow::{bail, Context};
 use itertools::Itertools;
 use owo_colors::OwoColorize;
+use std::collections::Bound;
+use std::fmt::Write;
+use std::{collections::BTreeSet, ffi::OsString};
 use tracing::{debug, warn};
-
+use uv_cache::Cache;
+use uv_client::BaseClientBuilder;
 use uv_distribution_types::{InstalledDist, Name};
 #[cfg(unix)]
 use uv_fs::replace_symlink;
 use uv_fs::Simplified;
 use uv_installer::SitePackages;
+use uv_pep440::{Version, VersionSpecifier, VersionSpecifiers};
 use uv_pep508::PackageName;
 use uv_pypi_types::Requirement;
-use uv_python::PythonEnvironment;
-use uv_settings::ToolOptions;
+use uv_python::{
+    EnvironmentPreference, Interpreter, PythonDownloads, PythonEnvironment, PythonInstallation,
+    PythonPreference, PythonRequest, PythonVariant, VersionRequest,
+};
+use uv_settings::{PythonInstallMirrors, ToolOptions};
 use uv_shell::Shell;
 use uv_tool::{entrypoint_paths, tool_executable_dir, InstalledTools, Tool, ToolEntrypoint};
 use uv_warnings::warn_user;
 
-use crate::commands::ExitStatus;
+use crate::commands::project::ProjectError;
+use crate::commands::reporters::PythonDownloadReporter;
+use crate::commands::{pip, ExitStatus};
 use crate::printer::Printer;
 
 /// Return all packages which contain an executable with the given name.
@@ -61,6 +68,95 @@ pub(crate) fn remove_entrypoints(tool: &Tool) {
     }
 }
 
+/// Given a no-solution error and the [`Interpreter`] that was used during the solve, attempt to
+/// discover an alternate [`Interpreter`] that satisfies the `requires-python` constraint.
+pub(crate) async fn refine_interpreter(
+    interpreter: &Interpreter,
+    python_request: Option<&PythonRequest>,
+    err: &pip::operations::Error,
+    client_builder: &BaseClientBuilder<'_>,
+    reporter: &PythonDownloadReporter,
+    install_mirrors: &PythonInstallMirrors,
+    python_preference: PythonPreference,
+    python_downloads: PythonDownloads,
+    cache: &Cache,
+) -> anyhow::Result<Option<Interpreter>, ProjectError> {
+    let pip::operations::Error::Resolve(uv_resolver::ResolveError::NoSolution(ref no_solution_err)) =
+        err
+    else {
+        return Ok(None);
+    };
+
+    // Infer the `requires-python` constraint from the error.
+    let requires_python = no_solution_err.find_requires_python();
+
+    // If the existing interpreter already satisfies the `requires-python` constraint, we don't need
+    // to refine it. We'd expect to fail again anyway.
+    if requires_python.contains(interpreter.python_version()) {
+        return Ok(None);
+    }
+
+    // If the user passed a `--python` request, and the refined interpreter is incompatible, we
+    // can't use it.
+    if let Some(python_request) = python_request {
+        if !python_request.satisfied(interpreter, cache) {
+            return Ok(None);
+        }
+    }
+
+    // We want an interpreter that's as close to the required version as possible. If we choose the
+    // "latest" Python, we risk choosing a version that lacks wheels for the tool's requirements
+    // (assuming those requirements don't publish source distributions).
+    //
+    // TODO(charlie): Solve for the Python version iteratively (or even, within the resolver
+    // itself). The current strategy can also fail if the tool's requirements have greater
+    // `requires-python` constraints, and we didn't see them in the initial solve. It can also fail
+    // if the tool's requirements don't publish wheels for this interpreter version, though that's
+    // rarer.
+    let lower_bound = match requires_python.as_ref() {
+        Bound::Included(version) => VersionSpecifier::greater_than_equal_version(version.clone()),
+        Bound::Excluded(version) => VersionSpecifier::greater_than_version(version.clone()),
+        Bound::Unbounded => unreachable!("`requires-python` should never be unbounded"),
+    };
+
+    let upper_bound = match requires_python.as_ref() {
+        Bound::Included(version) => {
+            let major = version.release().first().copied().unwrap_or(0);
+            let minor = version.release().get(1).copied().unwrap_or(0);
+            VersionSpecifier::less_than_version(Version::new([major, minor + 1]))
+        }
+        Bound::Excluded(version) => {
+            let major = version.release().first().copied().unwrap_or(0);
+            let minor = version.release().get(1).copied().unwrap_or(0);
+            VersionSpecifier::less_than_version(Version::new([major, minor + 1]))
+        }
+        Bound::Unbounded => unreachable!("`requires-python` should never be unbounded"),
+    };
+
+    let python_request = PythonRequest::Version(VersionRequest::Range(
+        VersionSpecifiers::from_iter([lower_bound, upper_bound]),
+        PythonVariant::default(),
+    ));
+
+    debug!("Refining interpreter with: {python_request}");
+
+    let interpreter = PythonInstallation::find_or_download(
+        Some(&python_request),
+        EnvironmentPreference::OnlySystem,
+        python_preference,
+        python_downloads,
+        client_builder,
+        cache,
+        Some(reporter),
+        install_mirrors.python_install_mirror.as_deref(),
+        install_mirrors.pypy_install_mirror.as_deref(),
+    )
+    .await?
+    .into_interpreter();
+
+    Ok(Some(interpreter))
+}
+
 /// Installs tool executables for a given package and handles any conflicts.
 pub(crate) fn install_executables(
     environment: &PythonEnvironment,