Improve handling of missing interpreters during discovery (#4218)

Cherry-picked from https://github.com/astral-sh/uv/pull/4214

The first commit gets us some context on an IO error during queries:

Previously:

```
failed to canonicalize path `[VENV]/bin/python3`
    Caused by: No such file or directory (os error 2)
```

Now:

```
Failed to query Python interpreter
    Caused by: failed to canonicalize path `[VENV]/bin/python3`
    Caused by: No such file or directory (os error 2)
```

but really we shouldn't attempt to query a missing interpreter during
discovery anyway, so we improve handling of that too.

crates/uv-toolchain/src/discovery.rs

@@ -438,6 +438,10 @@ fn should_stop_discovery(err: &Error) -> bool {
                 trace!("Skipping bad interpreter at {}", path.display());
                 false
             }
+            InterpreterError::NotFound(path) => {
+                trace!("Skipping missing interpreter at {}", path.display());
+                false
+            }
         },
         _ => true,
     }

crates/uv-toolchain/src/interpreter.rs

@@ -454,8 +454,10 @@ impl ExternallyManaged {
 
 #[derive(Debug, Error)]
 pub enum Error {
-    #[error(transparent)]
+    #[error("Failed to query Python interpreter")]
     Io(#[from] io::Error),
+    #[error("Python interpreter not found at `{0}`")]
+    NotFound(PathBuf),
     #[error("Failed to query Python interpreter at `{path}`")]
     SpawnFailed {
         path: PathBuf,
@@ -645,7 +647,14 @@ impl InterpreterInfo {
 
         // We check the timestamp of the canonicalized executable to check if an underlying
         // interpreter has been modified
-        let modified = Timestamp::from_path(uv_fs::canonicalize_executable(executable)?)?;
+        let modified =
+            Timestamp::from_path(uv_fs::canonicalize_executable(executable).map_err(|err| {
+                if err.kind() == io::ErrorKind::NotFound {
+                    Error::NotFound(executable.to_path_buf())
+                } else {
+                    err.into()
+                }
+            })?)?;
 
         // Read from the cache.
         if cache

crates/uv/tests/pip_compile.rs

@@ -129,33 +129,19 @@ fn missing_venv() -> Result<()> {
     context.temp_dir.child("requirements.in").touch()?;
     fs_err::remove_dir_all(context.temp_dir.child(".venv").path())?;
 
-    if cfg!(windows) {
-        uv_snapshot!(context.filters(), context.compile()
-                .arg("requirements.in"), @r###"
-        success: false
-        exit_code: 2
-        ----- stdout -----
+    uv_snapshot!(context.filters(), context.compile()
+            .arg("requirements.in"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    # This file was autogenerated by uv via the following command:
+    #    uv pip compile --cache-dir [CACHE_DIR] --exclude-newer 2024-03-25T00:00:00Z requirements.in
 
-        ----- stderr -----
-        warning: Requirements file requirements.in does not contain any dependencies
-        error: failed to canonicalize path `[VENV]/Scripts/python.exe`
-          Caused by: The system cannot find the path specified. (os error 3)
-        "###
-        );
-    } else {
-        uv_snapshot!(context.filters(), context.compile()
-                .arg("requirements.in"), @r###"
-        success: false
-        exit_code: 2
-        ----- stdout -----
-
-        ----- stderr -----
-        warning: Requirements file requirements.in does not contain any dependencies
-        error: failed to canonicalize path `[VENV]/bin/python3`
-          Caused by: No such file or directory (os error 2)
-        "###
-        );
-    }
+    ----- stderr -----
+    warning: Requirements file requirements.in does not contain any dependencies
+    Resolved 0 packages in [TIME]
+    "###
+    );
 
     context
         .temp_dir

crates/uv/tests/pip_sync.rs

@@ -110,27 +110,14 @@ fn missing_venv() -> Result<()> {
     requirements.write_str("anyio")?;
     fs::remove_dir_all(&context.venv)?;
 
-    if cfg!(windows) {
-        uv_snapshot!(context.filters(), sync_without_exclude_newer(&context).arg("requirements.txt"), @r###"
-        success: false
-        exit_code: 2
-        ----- stdout -----
+    uv_snapshot!(context.filters(), sync_without_exclude_newer(&context).arg("requirements.txt"), @r###"
+    success: false
+    exit_code: 2
+    ----- stdout -----
 
-        ----- stderr -----
-        error: failed to canonicalize path `[VENV]/Scripts/python.exe`
-          Caused by: The system cannot find the path specified. (os error 3)
-        "###);
-    } else {
-        uv_snapshot!(context.filters(), sync_without_exclude_newer(&context).arg("requirements.txt"), @r###"
-        success: false
-        exit_code: 2
-        ----- stdout -----
-
-        ----- stderr -----
-        error: failed to canonicalize path `[VENV]/bin/python3`
-          Caused by: No such file or directory (os error 2)
-        "###);
-    }
+    ----- stderr -----
+    error: No Python interpreters found in virtual environments
+    "###);
 
     assert!(predicates::path::missing().eval(&context.venv));