Add cfg_os_version_min feature

Based on in-progress RFC: rust-lang/rfcs#3750.

Only implemented for Apple platforms for now, but written in a way that
should be easily expandable to include other platforms.

Author: madsmtm

Diff for: Cargo.lock

@@ -3348,6 +3348,7 @@ dependencies = [
  "rustc_macros",
  "rustc_session",
  "rustc_span",
+ "rustc_target",
  "thin-vec",
 ]
 

Diff for: compiler/rustc_attr_parsing/Cargo.toml

@@ -17,5 +17,6 @@ rustc_lexer = { path = "../rustc_lexer" }
 rustc_macros = { path = "../rustc_macros" }
 rustc_session = { path = "../rustc_session" }
 rustc_span = { path = "../rustc_span" }
+rustc_target = { path = "../rustc_target" }
 thin-vec = "0.2.12"
 # tidy-alphabetical-end

Diff for: compiler/rustc_attr_parsing/messages.ftl

@@ -1,3 +1,9 @@
+attr_parsing_apple_version_invalid =
+    failed parsing version: {$error}
+
+attr_parsing_apple_version_unnecessarily_low =
+    version is set unnecessarily low, the minimum supported by Rust on this platform is {$os_min}
+
 attr_parsing_cfg_predicate_identifier =
     `cfg` predicate key must be an identifier
 
@@ -11,6 +17,12 @@ attr_parsing_empty_confusables =
 attr_parsing_expected_one_cfg_pattern =
     expected 1 cfg-pattern
 
+attr_parsing_expected_platform_and_version_literals =
+    expected two literals, a platform and a version
+
+attr_parsing_expected_platform_literal =
+    expected a platform literal
+
 attr_parsing_expected_single_version_literal =
     expected single version literal
 
@@ -108,6 +120,9 @@ attr_parsing_unknown_meta_item =
     unknown meta item '{$item}'
     .label = expected one of {$expected}
 
+attr_parsing_unknown_platform_literal =
+    unknown platform literal, expected values are: {$possibilities}
+
 attr_parsing_unknown_version_literal =
     unknown version literal format, assuming it refers to a future version
 

Diff for: compiler/rustc_attr_parsing/src/attributes/cfg.rs

@@ -1,6 +1,7 @@
 use rustc_ast::{LitKind, MetaItem, MetaItemInner, MetaItemKind, MetaItemLit, NodeId};
 use rustc_ast_pretty::pprust;
 use rustc_attr_data_structures::RustcVersion;
+use rustc_errors::Applicability;
 use rustc_feature::{Features, GatedCfg, find_gated_cfg};
 use rustc_session::Session;
 use rustc_session::config::ExpectedValues;
@@ -9,6 +10,7 @@ use rustc_session::lint::builtin::UNEXPECTED_CFGS;
 use rustc_session::parse::feature_err;
 use rustc_span::symbol::kw;
 use rustc_span::{Span, Symbol, sym};
+use rustc_target::spec::apple;
 
 use crate::session_diagnostics::{self, UnsupportedLiteralReason};
 use crate::{fluent_generated, parse_version};
@@ -149,6 +151,129 @@ pub fn eval_condition(
                 RustcVersion::CURRENT >= min_version
             }
         }
+        MetaItemKind::List(mis) if cfg.name_or_empty() == sym::os_version_min => {
+            try_gate_cfg(sym::os_version_min, cfg.span, sess, features);
+
+            let (platform, version) = match &mis[..] {
+                [platform, version] => (platform, version),
+                [..] => {
+                    dcx.emit_err(session_diagnostics::ExpectedPlatformAndVersionLiterals {
+                        span: cfg.span,
+                    });
+                    return false;
+                }
+            };
+
+            let (platform_sym, platform_span) = match platform {
+                MetaItemInner::Lit(MetaItemLit {
+                    kind: LitKind::Str(platform_sym, ..),
+                    span: platform_span,
+                    ..
+                }) => (platform_sym, platform_span),
+                MetaItemInner::Lit(MetaItemLit { span, .. })
+                | MetaItemInner::MetaItem(MetaItem { span, .. }) => {
+                    dcx.emit_err(session_diagnostics::ExpectedPlatformLiteral { span: *span });
+                    return false;
+                }
+            };
+
+            let (version_sym, version_span) = match version {
+                MetaItemInner::Lit(MetaItemLit {
+                    kind: LitKind::Str(version_sym, ..),
+                    span: version_span,
+                    ..
+                }) => (version_sym, version_span),
+                MetaItemInner::Lit(MetaItemLit { span, .. })
+                | MetaItemInner::MetaItem(MetaItem { span, .. }) => {
+                    dcx.emit_err(session_diagnostics::ExpectedVersionLiteral { span: *span });
+                    return false;
+                }
+            };
+
+            // Always parse version, regardless of current target platform.
+            let version = match *platform_sym {
+                // Apple platforms follow the same versioning schema.
+                sym::macos | sym::ios | sym::tvos | sym::watchos | sym::visionos => {
+                    match version_sym.as_str().parse() {
+                        Ok(version) => {
+                            let os_min = apple::OSVersion::os_minimum_deployment_target(
+                                &platform_sym.as_str(),
+                            );
+
+                            // It's unnecessary to specify `cfg_target_os(...)` for a platform
+                            // version that is lower than the minimum targetted by `rustc` (instead,
+                            // make the item always available).
+                            //
+                            // This is correct _now_, but once we bump versions next time, we should
+                            // maybe make this a lint so that users can opt-in to supporting older
+                            // `rustc` versions? Or perhaps only fire the warning when Cargo's
+                            // `rust-version` field is above the version where the bump happened? Or
+                            // perhaps keep the version we check against low for a sufficiently long
+                            // time?
+                            if version <= os_min {
+                                sess.dcx()
+                                    .create_warn(
+                                        session_diagnostics::AppleVersionUnnecessarilyLow {
+                                            span: *version_span,
+                                            os_min: os_min.fmt_pretty().to_string(),
+                                        },
+                                    )
+                                    .with_span_suggestion(
+                                        cfg.span,
+                                        "use `target_os` instead",
+                                        format!("target_os = \"{platform_sym}\""),
+                                        Applicability::MachineApplicable,
+                                    )
+                                    .emit();
+                            }
+
+                            PlatformVersion::Apple { os: *platform_sym, version }
+                        }
+                        Err(error) => {
+                            sess.dcx().emit_err(session_diagnostics::AppleVersionInvalid {
+                                span: *version_span,
+                                error,
+                            });
+                            return false;
+                        }
+                    }
+                }
+                // FIXME(madsmtm): Handle further platforms as specified in the RFC.
+                sym::windows | sym::libc => {
+                    #[allow(rustc::untranslatable_diagnostic)] // Temporary
+                    dcx.span_err(*platform_span, "unimplemented platform");
+                    return false;
+                }
+                _ => {
+                    // Unknown platform. This is intentionally a warning (and not an error) to be
+                    // future-compatible with later additions.
+                    let known_platforms = [
+                        sym::macos,
+                        sym::ios,
+                        sym::tvos,
+                        sym::watchos,
+                        sym::visionos,
+                        // sym::windows,
+                        // sym::libc,
+                    ];
+                    dcx.emit_warn(session_diagnostics::UnknownPlatformLiteral {
+                        span: *platform_span,
+                        possibilities: known_platforms.into_iter().collect(),
+                    });
+                    return false;
+                }
+            };
+
+            // Figure out actual cfg-status based on current platform.
+            match version {
+                PlatformVersion::Apple { os, version } if os.as_str() == sess.target.os => {
+                    let deployment_target = sess.apple_deployment_target();
+                    version <= deployment_target
+                }
+                // If a `cfg`-value does not apply to a specific platform, assume
+                _ => false,
+            }
+        }
         MetaItemKind::List(mis) => {
             for mi in mis.iter() {
                 if mi.meta_item_or_bool().is_none() {
@@ -245,3 +370,8 @@ pub fn eval_condition(
         }
     }
 }
+
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+enum PlatformVersion {
+    Apple { os: Symbol, version: apple::OSVersion },
+}

Diff for: compiler/rustc_attr_parsing/src/session_diagnostics.rs

@@ -1,8 +1,10 @@
-use std::num::IntErrorKind;
+use std::num::{IntErrorKind, ParseIntError};
 
 use rustc_ast as ast;
 use rustc_errors::codes::*;
-use rustc_errors::{Applicability, Diag, DiagCtxtHandle, Diagnostic, EmissionGuarantee, Level};
+use rustc_errors::{
+    Applicability, Diag, DiagCtxtHandle, DiagSymbolList, Diagnostic, EmissionGuarantee, Level,
+};
 use rustc_macros::{Diagnostic, Subdiagnostic};
 use rustc_span::{Span, Symbol};
 
@@ -16,6 +18,22 @@ pub(crate) enum UnsupportedLiteralReason {
     DeprecatedKvPair,
 }
 
+#[derive(Diagnostic)]
+#[diag(attr_parsing_apple_version_invalid)]
+pub(crate) struct AppleVersionInvalid {
+    #[primary_span]
+    pub span: Span,
+    pub error: ParseIntError,
+}
+
+#[derive(Diagnostic)]
+#[diag(attr_parsing_apple_version_unnecessarily_low)]
+pub(crate) struct AppleVersionUnnecessarilyLow {
+    #[primary_span]
+    pub span: Span,
+    pub os_min: String,
+}
+
 #[derive(Diagnostic)]
 #[diag(attr_parsing_expected_one_cfg_pattern, code = E0536)]
 pub(crate) struct ExpectedOneCfgPattern {
@@ -386,6 +404,20 @@ pub(crate) struct DeprecatedItemSuggestion {
     pub details: (),
 }
 
+#[derive(Diagnostic)]
+#[diag(attr_parsing_expected_platform_and_version_literals)]
+pub(crate) struct ExpectedPlatformAndVersionLiterals {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(attr_parsing_expected_platform_literal)]
+pub(crate) struct ExpectedPlatformLiteral {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(attr_parsing_expected_single_version_literal)]
 pub(crate) struct ExpectedSingleVersionLiteral {
@@ -432,6 +464,14 @@ pub(crate) struct SoftNoArgs {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(attr_parsing_unknown_platform_literal)]
+pub(crate) struct UnknownPlatformLiteral {
+    #[primary_span]
+    pub span: Span,
+    pub possibilities: DiagSymbolList,
+}
+
 #[derive(Diagnostic)]
 #[diag(attr_parsing_unknown_version_literal)]
 pub(crate) struct UnknownVersionLiteral {

Diff for: compiler/rustc_feature/src/builtin_attrs.rs

@@ -33,6 +33,7 @@ const GATED_CFGS: &[GatedCfg] = &[
     ),
     (sym::sanitize, sym::cfg_sanitize, Features::cfg_sanitize),
     (sym::version, sym::cfg_version, Features::cfg_version),
+    (sym::os_version_min, sym::cfg_os_version_min, Features::cfg_os_version_min),
     (sym::relocation_model, sym::cfg_relocation_model, Features::cfg_relocation_model),
     (sym::sanitizer_cfi_generalize_pointers, sym::cfg_sanitizer_cfi, Features::cfg_sanitizer_cfi),
     (sym::sanitizer_cfi_normalize_integers, sym::cfg_sanitizer_cfi, Features::cfg_sanitizer_cfi),

Diff for: compiler/rustc_feature/src/unstable.rs

@@ -395,6 +395,8 @@ declare_features! (
     (unstable, cfg_boolean_literals, "1.83.0", Some(131204)),
     /// Allows the use of `#[cfg(contract_checks)` to check if contract checks are enabled.
     (unstable, cfg_contract_checks, "1.86.0", Some(128044)),
+    /// Allow conditional compilation depending on target platform version.
+    (unstable, cfg_os_version_min, "CURRENT_RUSTC_VERSION", Some(136866)),
     /// Allows the use of `#[cfg(overflow_checks)` to check if integer overflow behaviour.
     (unstable, cfg_overflow_checks, "1.71.0", Some(111466)),
     /// Provides the relocation model information as cfg entry

Diff for: compiler/rustc_span/src/symbol.rs

@@ -611,6 +611,7 @@ symbols! {
         cfg_eval,
         cfg_fmt_debug,
         cfg_hide,
+        cfg_os_version_min,
         cfg_overflow_checks,
         cfg_panic,
         cfg_relocation_model,
@@ -1194,6 +1195,7 @@ symbols! {
         intrinsics_unaligned_volatile_store,
         io_stderr,
         io_stdout,
+        ios,
         irrefutable_let_patterns,
         is,
         is_val_statically_known,
@@ -1279,6 +1281,7 @@ symbols! {
         loop_break_value,
         lt,
         m68k_target_feature,
+        macos,
         macro_at_most_once_rep,
         macro_attributes_in_derive_output,
         macro_escape,
@@ -1502,6 +1505,7 @@ symbols! {
         ord_cmp_method,
         os_str_to_os_string,
         os_string_as_os_str,
+        os_version_min,
         other,
         out,
         overflow_checks,
@@ -2124,6 +2128,7 @@ symbols! {
         tuple,
         tuple_indexing,
         tuple_trait,
+        tvos,
         two_phase,
         ty,
         type_alias_enum_variants,
@@ -2270,6 +2275,7 @@ symbols! {
         vfp2,
         vis,
         visible_private_types,
+        visionos,
         volatile,
         volatile_copy_memory,
         volatile_copy_nonoverlapping_memory,
@@ -2286,6 +2292,7 @@ symbols! {
         wasm_abi,
         wasm_import_module,
         wasm_target_feature,
+        watchos,
         where_clause_attrs,
         while_let,
         width,