Emit diagnostics for incorrect deployment targets

compiler/rustc_codegen_ssa/messages.ftl

@@ -2,6 +2,12 @@ codegen_ssa_L4Bender_exporting_symbols_unimplemented = exporting symbols not imp
 
 codegen_ssa_add_native_library = failed to add native library {$library_path}: {$error}
 
+codegen_ssa_apple_deployment_target_invalid =
+    failed to parse deployment target specified in {$env_var}: {$error}
+
+codegen_ssa_apple_deployment_target_too_low =
+    deployment target in {$env_var} was set to {$version}, but the minimum supported by `rustc` is {$os_min}
+
 codegen_ssa_apple_sdk_error_sdk_path = failed to get {$sdk_name} SDK path: {$error}
 
 codegen_ssa_archive_build_failure = failed to build archive at `{$path}`: {$error}

compiler/rustc_codegen_ssa/src/apple.rs

@@ -1,10 +1,13 @@
 use std::borrow::Cow;
 use std::env;
+use std::fmt::{Display, from_fn};
 use std::num::ParseIntError;
 
 use rustc_middle::bug;
 use rustc_session::Session;
 
+use crate::errors::AppleDeploymentTarget;
+
 #[cfg(test)]
 mod tests;
 
@@ -13,6 +16,17 @@ mod tests;
 /// The size of the numbers in here are limited by Mach-O's `LC_BUILD_VERSION`.
 pub type OsVersion = (u16, u8, u8);
 
+pub fn pretty_version(version: OsVersion) -> impl Display {
+    let (major, minor, patch) = version;
+    from_fn(move |f| {
+        write!(f, "{major}.{minor}")?;
+        if patch != 0 {
+            write!(f, ".{patch}")?;
+        }
+        Ok(())
+    })
+}
+
 /// Parse an OS version triple (SDK version or deployment target).
 fn parse_version(version: &str) -> Result<OsVersion, ParseIntError> {
     if let Some((major, minor)) = version.split_once('.') {
@@ -74,14 +88,26 @@ pub fn deployment_target(sess: &Session) -> OsVersion {
 
     if let Ok(deployment_target) = env::var(env_var) {
         match parse_version(&deployment_target) {
-            // It is common that the deployment target is set too low, e.g. on macOS Aarch64 to also
-            // target older x86_64, the user may set a lower deployment target than supported.
-            //
-            // To avoid such issues, we silently raise the deployment target here.
-            // FIXME: We want to show a warning when `version < os_min`.
-            Ok(version) => version.max(min),
-            // FIXME: Report erroneous environment variable to user.
-            Err(_) => min,
+            Ok(version) => {
+                // It is common that the deployment target is set a bit too low, for example on
+                // macOS Aarch64 to also target older x86_64. So we only want to warn when variable
+                // is lower than the minimum OS supported by rustc, not when the variable is lower
+                // than the minimum for a specific target.
+                if version < os_min {
+                    sess.dcx().emit_warn(AppleDeploymentTarget::TooLow {
+                        env_var,
+                        version: pretty_version(version).to_string(),
+                        os_min: pretty_version(os_min).to_string(),
+                    });
+                }
+
+                // Raise the deployment target to the minimum supported.
+                version.max(min)
+            }
+            Err(error) => {
+                sess.dcx().emit_err(AppleDeploymentTarget::Invalid { env_var, error });
+                min
+            }
         }
     } else {
         // If no deployment target variable is set, default to the minimum found above.

compiler/rustc_codegen_ssa/src/errors.rs

@@ -2,6 +2,7 @@
 
 use std::borrow::Cow;
 use std::io::Error;
+use std::num::ParseIntError;
 use std::path::{Path, PathBuf};
 use std::process::ExitStatus;
 
@@ -539,6 +540,14 @@ pub(crate) struct UnsupportedArch<'a> {
     pub os: &'a str,
 }
 
+#[derive(Diagnostic)]
+pub(crate) enum AppleDeploymentTarget {
+    #[diag(codegen_ssa_apple_deployment_target_invalid)]
+    Invalid { env_var: &'static str, error: ParseIntError },
+    #[diag(codegen_ssa_apple_deployment_target_too_low)]
+    TooLow { env_var: &'static str, version: String, os_min: String },
+}
+
 #[derive(Diagnostic)]
 pub(crate) enum AppleSdkRootError<'a> {
     #[diag(codegen_ssa_apple_sdk_error_sdk_path)]

compiler/rustc_codegen_ssa/src/lib.rs

@@ -6,6 +6,7 @@
 #![doc(rust_logo)]
 #![feature(assert_matches)]
 #![feature(box_patterns)]
+#![feature(debug_closure_helpers)]
 #![feature(file_buffered)]
 #![feature(if_let_guard)]
 #![feature(let_chains)]

compiler/rustc_driver_impl/src/lib.rs

@@ -856,10 +856,10 @@ fn print_crate_info(
                 }
             }
             DeploymentTarget => {
+                use rustc_codegen_ssa::apple::{deployment_target, pretty_version};
+
                 if sess.target.is_like_osx {
-                    let (major, minor, patch) = rustc_codegen_ssa::apple::deployment_target(sess);
-                    let patch = if patch != 0 { format!(".{patch}") } else { String::new() };
-                    println_info!("deployment_target={major}.{minor}{patch}")
+                    println_info!("deployment_target={}", pretty_version(deployment_target(sess)))
                 } else {
                     #[allow(rustc::diagnostic_outside_of_impl)]
                     sess.dcx().fatal("only Apple targets currently support deployment version info")