Respect MACOSX_DEPLOYMENT_TARGET in --python-platform (#3470)

## Summary

This is universal environment variable used to determine the mac OS
deployment target. We now respect it in `--python-platform` -- so we
default to 12.0, but users can override it as needed.

Cargo.lock

@@ -4736,6 +4736,7 @@ dependencies = [
  "schemars",
  "serde",
  "serde_json",
+ "tracing",
  "uv-auth",
  "uv-cache",
  "uv-normalize",

README.md

@@ -584,6 +584,9 @@ In addition, uv respects the following environment variables:
 - `ZSH_VERSION`: Used to detect the use of the Zsh shell.
 - `RAYON_NUM_THREADS`: Used to control the number of threads used when unzipping and installing
   packages. See the [rayon documentation](https://docs.rs/rayon/latest/rayon/) for more.
+- `MACOSX_DEPLOYMENT_TARGET`: Used with `--python-platform macos` and related variants to set the
+  deployment target (i.e., the minimum supported macOS version). Defaults to `12.0`, the
+  least-recent non-EOL macOS version at time of writing.
 
 ## Versioning
 

crates/uv-configuration/Cargo.toml

@@ -27,6 +27,7 @@ rustc-hash = { workspace = true }
 schemars = { workspace = true, optional = true }
 serde = { workspace = true }
 serde_json = { workspace = true }
+tracing = { workspace = true }
 
 [features]
 default = []

crates/uv-configuration/src/target_triple.rs

@@ -1,3 +1,5 @@
+use tracing::debug;
+
 use pep508_rs::MarkerEnvironment;
 use platform_tags::{Arch, Os, Platform};
 
@@ -29,13 +31,18 @@ pub enum TargetTriple {
     #[cfg_attr(feature = "schemars", schemars(rename = "x86_64-unknown-linux-gnu"))]
     X8664UnknownLinuxGnu,
 
-    /// An ARM-based macOS target, as seen on Apple Silicon devices, with support for the
-    /// least-recent, non-EOL macOS version (12.0).
+    /// An ARM-based macOS target, as seen on Apple Silicon devices
+    ///
+    /// By default, assumes the least-recent, non-EOL macOS version (12.0), but respects
+    /// the `MACOSX_DEPLOYMENT_TARGET` environment variable if set.
     #[cfg_attr(feature = "clap", value(name = "aarch64-apple-darwin"))]
     #[cfg_attr(feature = "schemars", schemars(rename = "aarch64-apple-darwin"))]
     Aarch64AppleDarwin,
 
-    /// An x86 macOS target, with support for the least-recent, non-EOL macOS version (12.0).
+    /// An x86 macOS target.
+    ///
+    /// By default, assumes the least-recent, non-EOL macOS version (12.0), but respects
+    /// the `MACOSX_DEPLOYMENT_TARGET` environment variable if set.
     #[cfg_attr(feature = "clap", value(name = "x86_64-apple-darwin"))]
     #[cfg_attr(feature = "schemars", schemars(rename = "x86_64-apple-darwin"))]
     X8664AppleDarwin,
@@ -88,20 +95,20 @@ impl TargetTriple {
                 },
                 Arch::X86_64,
             ),
-            Self::Macos | Self::Aarch64AppleDarwin => Platform::new(
-                Os::Macos {
-                    major: 12,
-                    minor: 0,
-                },
-                Arch::Aarch64,
-            ),
-            Self::X8664AppleDarwin => Platform::new(
-                Os::Macos {
-                    major: 12,
-                    minor: 0,
-                },
-                Arch::X86_64,
-            ),
+            Self::Macos | Self::Aarch64AppleDarwin => {
+                let (major, minor) = macos_deployment_target().map_or((12, 0), |(major, minor)| {
+                    debug!("Found macOS deployment target: {}.{}", major, minor);
+                    (major, minor)
+                });
+                Platform::new(Os::Macos { major, minor }, Arch::Aarch64)
+            }
+            Self::X8664AppleDarwin => {
+                let (major, minor) = macos_deployment_target().map_or((12, 0), |(major, minor)| {
+                    debug!("Found macOS deployment target: {}.{}", major, minor);
+                    (major, minor)
+                });
+                Platform::new(Os::Macos { major, minor }, Arch::X86_64)
+            }
             Self::Aarch64UnknownLinuxGnu => Platform::new(
                 Os::Manylinux {
                     major: 2,
@@ -271,3 +278,17 @@ impl TargetTriple {
         }
     }
 }
+
+/// Return the macOS deployment target as parsed from the environment.
+fn macos_deployment_target() -> Option<(u16, u16)> {
+    let version = std::env::var("MACOSX_DEPLOYMENT_TARGET").ok()?;
+    let mut parts = version.split('.');
+
+    // Parse the major version (e.g., `12` in `12.0`).
+    let major = parts.next()?.parse::<u16>().ok()?;
+
+    // Parse the minor version (e.g., `0` in `12.0`), with a default of `0`.
+    let minor = parts.next().unwrap_or("0").parse::<u16>().ok()?;
+
+    Some((major, minor))
+}

uv.schema.json

@@ -815,14 +815,14 @@
           ]
         },
         {
-          "description": "An ARM-based macOS target, as seen on Apple Silicon devices, with support for the least-recent, non-EOL macOS version (12.0).",
+          "description": "An ARM-based macOS target, as seen on Apple Silicon devices\n\nBy default, assumes the least-recent, non-EOL macOS version (12.0), but respects the `MACOSX_DEPLOYMENT_TARGET` environment variable if set.",
           "type": "string",
           "enum": [
             "aarch64-apple-darwin"
           ]
         },
         {
-          "description": "An x86 macOS target, with support for the least-recent, non-EOL macOS version (12.0).",
+          "description": "An x86 macOS target.\n\nBy default, assumes the least-recent, non-EOL macOS version (12.0), but respects the `MACOSX_DEPLOYMENT_TARGET` environment variable if set.",
           "type": "string",
           "enum": [
             "x86_64-apple-darwin"