Differentiate between implicit vs explicit architecture requests (#13723)

In https://github.com/astral-sh/uv/pull/13721#issuecomment-2920530601 I
presumed that all the installation problems in
https://github.com/astral-sh/uv/pull/13722 were solved by
https://github.com/astral-sh/uv/pull/13709 but they were not because we
don't differentiate between implicit and explicit architecture requests
so a request for `aarch64` is considered satisfied by an existing
`x86-64` installation even if the user explicitly requested that
architecture.

Now, we track if it was explicit or implicit, requiring an exact match
in the former case, and a `supports` in the latter.

We considered doing this for other items in the request, like the
operating system but we don't have a `supports()` concept there. It
might make sense for libc in the future.

crates/uv-python/src/discovery.rs

@@ -2723,7 +2723,7 @@ mod tests {
 
     use crate::{
         discovery::{PythonRequest, VersionRequest},
-        downloads::PythonDownloadRequest,
+        downloads::{ArchRequest, PythonDownloadRequest},
         implementation::ImplementationName,
         platform::{Arch, Libc, Os},
     };
@@ -2813,10 +2813,10 @@ mod tests {
                     PythonVariant::Default
                 )),
                 implementation: Some(ImplementationName::CPython),
-                arch: Some(Arch {
+                arch: Some(ArchRequest::Explicit(Arch {
                     family: Architecture::Aarch64(Aarch64Architecture::Aarch64),
                     variant: None
-                }),
+                })),
                 os: Some(Os(target_lexicon::OperatingSystem::Darwin(None))),
                 libc: Some(Libc::None),
                 prereleases: None
@@ -2848,10 +2848,10 @@ mod tests {
                     PythonVariant::Default
                 )),
                 implementation: None,
-                arch: Some(Arch {
+                arch: Some(ArchRequest::Explicit(Arch {
                     family: Architecture::Aarch64(Aarch64Architecture::Aarch64),
                     variant: None
-                }),
+                })),
                 os: None,
                 libc: None,
                 prereleases: None

crates/uv-python/src/downloads.rs

@@ -116,7 +116,7 @@ pub struct ManagedPythonDownload {
 pub struct PythonDownloadRequest {
     pub(crate) version: Option<VersionRequest>,
     pub(crate) implementation: Option<ImplementationName>,
-    pub(crate) arch: Option<Arch>,
+    pub(crate) arch: Option<ArchRequest>,
     pub(crate) os: Option<Os>,
     pub(crate) libc: Option<Libc>,
 
@@ -125,11 +125,40 @@ pub struct PythonDownloadRequest {
     pub(crate) prereleases: Option<bool>,
 }
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum ArchRequest {
+    Explicit(Arch),
+    Environment(Arch),
+}
+
+impl Display for ArchRequest {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::Explicit(arch) | Self::Environment(arch) => write!(f, "{arch}"),
+        }
+    }
+}
+
+impl ArchRequest {
+    pub(crate) fn satisfied_by(self, arch: Arch) -> bool {
+        match self {
+            Self::Explicit(request) => request == arch,
+            Self::Environment(env) => env.supports(arch),
+        }
+    }
+
+    pub fn inner(&self) -> Arch {
+        match self {
+            Self::Explicit(arch) | Self::Environment(arch) => *arch,
+        }
+    }
+}
+
 impl PythonDownloadRequest {
     pub fn new(
         version: Option<VersionRequest>,
         implementation: Option<ImplementationName>,
-        arch: Option<Arch>,
+        arch: Option<ArchRequest>,
         os: Option<Os>,
         libc: Option<Libc>,
         prereleases: Option<bool>,
@@ -158,7 +187,7 @@ impl PythonDownloadRequest {
 
     #[must_use]
     pub fn with_arch(mut self, arch: Arch) -> Self {
-        self.arch = Some(arch);
+        self.arch = Some(ArchRequest::Explicit(arch));
         self
     }
 
@@ -219,7 +248,7 @@ impl PythonDownloadRequest {
     /// Platform information is pulled from the environment.
     pub fn fill_platform(mut self) -> Result<Self, Error> {
         if self.arch.is_none() {
-            self.arch = Some(Arch::from_env());
+            self.arch = Some(ArchRequest::Environment(Arch::from_env()));
         }
         if self.os.is_none() {
             self.os = Some(Os::from_env());
@@ -238,18 +267,6 @@ impl PythonDownloadRequest {
         Ok(self)
     }
 
-    /// Construct a new [`PythonDownloadRequest`] with platform information from the environment.
-    pub fn from_env() -> Result<Self, Error> {
-        Ok(Self::new(
-            None,
-            None,
-            Some(Arch::from_env()),
-            Some(Os::from_env()),
-            Some(Libc::from_env()?),
-            None,
-        ))
-    }
-
     pub fn implementation(&self) -> Option<&ImplementationName> {
         self.implementation.as_ref()
     }
@@ -258,7 +275,7 @@ impl PythonDownloadRequest {
         self.version.as_ref()
     }
 
-    pub fn arch(&self) -> Option<&Arch> {
+    pub fn arch(&self) -> Option<&ArchRequest> {
         self.arch.as_ref()
     }
 
@@ -288,7 +305,7 @@ impl PythonDownloadRequest {
         }
 
         if let Some(arch) = &self.arch {
-            if !arch.supports(key.arch) {
+            if !arch.satisfied_by(key.arch) {
                 return false;
             }
         }
@@ -366,7 +383,7 @@ impl PythonDownloadRequest {
         }
         if let Some(arch) = self.arch() {
             let interpreter_arch = Arch::from(&interpreter.platform().arch());
-            if &interpreter_arch != arch {
+            if !arch.satisfied_by(interpreter_arch) {
                 debug!(
                     "Skipping interpreter at `{executable}`: architecture `{interpreter_arch}` does not match request `{arch}`"
                 );
@@ -408,7 +425,7 @@ impl From<&ManagedPythonInstallation> for PythonDownloadRequest {
                     "Managed Python installations are expected to always have known implementation names, found {name}"
                 ),
             },
-            Some(key.arch),
+            Some(ArchRequest::Explicit(key.arch)),
             Some(key.os),
             Some(key.libc),
             Some(key.prerelease.is_some()),
@@ -478,7 +495,7 @@ impl FromStr for PythonDownloadRequest {
                     );
                 }
                 3 => os = Some(Os::from_str(part)?),
-                4 => arch = Some(Arch::from_str(part)?),
+                4 => arch = Some(ArchRequest::Explicit(Arch::from_str(part)?)),
                 5 => libc = Some(Libc::from_str(part)?),
                 _ => return Err(Error::TooManyParts(s.to_string())),
             }