Warn when relying on default musl target static linkage behaviour

This introduces a new lint, "musl_missing_crt_static" that warns when
compiling code for a musl target that defaults to static linkage without
explicitly specifying -Ctarget-feature=+crt-static.

The targets will be changed to link dynamically by default in the
future, after which this lint can be removed again.

Signed-off-by: Jens Reidel <adrian@travitia.xyz>

Author: Gelbpunkt

compiler/rustc_lint/messages.ftl

@@ -599,6 +599,10 @@ lint_mismatched_lifetime_syntaxes_suggestion_mixed =
 lint_mismatched_lifetime_syntaxes_suggestion_mixed_only_paths =
     use `'_` for type paths
 
+lint_missing_crt_static =
+    on musl targets, the implicit default for `crt-static` is going to change
+    .help = explicitly pass `-C target-feature=+crt-static` or `-C target-feature=-crt-static`
+
 lint_mixed_script_confusables =
     the usage of Script Group `{$set}` in this crate consists solely of mixed script confusables
     .includes_note = the usage includes {$includes}

compiler/rustc_lint/src/early/diagnostics.rs

@@ -300,6 +300,9 @@ pub fn decorate_builtin_lint(
             lints::UnusedVisibility { span }.decorate_lint(diag)
         }
         BuiltinLintDiag::AttributeLint(kind) => decorate_attribute_lint(sess, tcx, &kind, diag),
+        BuiltinLintDiag::MissingCrtStatic => {
+            lints::MissingCrtStatic.decorate_lint(diag);
+        }
     }
 }
 

compiler/rustc_lint/src/lints.rs

@@ -2800,6 +2800,18 @@ pub(crate) enum DeprecatedWhereClauseLocationSugg {
     },
 }
 
+#[derive(LintDiagnostic)]
+#[diag(lint_missing_crt_static)]
+#[help]
+pub(crate) struct MissingCrtStatic;
+
+#[derive(LintDiagnostic)]
+#[diag(lint_missing_unsafe_on_extern)]
+pub(crate) struct MissingUnsafeOnExtern {
+    #[suggestion(code = "unsafe ", applicability = "machine-applicable")]
+    pub suggestion: Span,
+}
+
 #[derive(LintDiagnostic)]
 #[diag(lint_single_use_lifetime)]
 pub(crate) struct SingleUseLifetime {

compiler/rustc_lint_defs/src/builtin.rs

@@ -68,6 +68,7 @@ declare_lint_pass! {
         MISPLACED_DIAGNOSTIC_ATTRIBUTES,
         MISSING_ABI,
         MISSING_UNSAFE_ON_EXTERN,
+        MUSL_MISSING_CRT_STATIC,
         MUST_NOT_SUSPEND,
         NAMED_ARGUMENTS_USED_POSITIONALLY,
         NEVER_TYPE_FALLBACK_FLOWING_INTO_UNSAFE,
@@ -5315,3 +5316,16 @@ declare_lint! {
         report_in_deps: false,
     };
 }
+
+declare_lint! {
+    /// The `musl_missing_crt_static` lint detects when code is compiled for a target using musl libc
+    /// without explicitly specifying `+crt-static` or `-crt-static`.
+    ///
+    /// ### Explanation
+    ///
+    /// The musl targets will change to be dynamically linked by default in the future, to avoid a
+    /// sudden change in behavior, the desired linkage should be specified in the interim period.
+    pub MUSL_MISSING_CRT_STATIC,
+    Warn,
+    "on musl targets, the default for `crt-static` will change; explicitly set `+crt-static` or `-crt-static`",
+}

compiler/rustc_lint_defs/src/lib.rs

@@ -748,6 +748,7 @@ pub enum BuiltinLintDiag {
     },
     UnusedVisibility(Span),
     AttributeLint(AttributeLintKind),
+    MissingCrtStatic,
 }
 
 #[derive(Debug, HashStable_Generic)]

compiler/rustc_session/src/session.rs

@@ -6,6 +6,7 @@ use std::sync::atomic::AtomicBool;
 use std::{env, io};
 
 use rand::{RngCore, rng};
+use rustc_ast::ast;
 use rustc_data_structures::base_n::{CASE_INSENSITIVE, ToBaseN};
 use rustc_data_structures::flock;
 use rustc_data_structures::fx::{FxHashMap, FxIndexSet};
@@ -18,18 +19,20 @@ use rustc_errors::json::JsonEmitter;
 use rustc_errors::timings::TimingSectionHandler;
 use rustc_errors::translation::Translator;
 use rustc_errors::{
-    Diag, DiagCtxt, DiagCtxtHandle, DiagMessage, Diagnostic, ErrorGuaranteed, FatalAbort,
-    TerminalUrl, fallback_fluent_bundle,
+    DecorateDiagCompat, Diag, DiagCtxt, DiagCtxtHandle, DiagMessage, Diagnostic, ErrorGuaranteed,
+    FatalAbort, TerminalUrl, fallback_fluent_bundle,
 };
 use rustc_hir::limit::Limit;
+use rustc_lint_defs::BuiltinLintDiag;
+use rustc_lint_defs::builtin::MUSL_MISSING_CRT_STATIC;
 use rustc_macros::HashStable_Generic;
 pub use rustc_span::def_id::StableCrateId;
 use rustc_span::edition::Edition;
 use rustc_span::source_map::{FilePathMapping, SourceMap};
 use rustc_span::{RealFileName, Span, Symbol};
 use rustc_target::asm::InlineAsmArch;
 use rustc_target::spec::{
-    Arch, CodeModel, DebuginfoKind, Os, PanicStrategy, RelocModel, RelroLevel, SanitizerSet,
+    Arch, CodeModel, DebuginfoKind, Env, Os, PanicStrategy, RelocModel, RelroLevel, SanitizerSet,
     SmallDataThresholdSupport, SplitDebuginfo, StackProtector, SymbolVisibility, Target,
     TargetTuple, TlsModel, apple,
 };
@@ -363,6 +366,15 @@ impl Session {
             // We can't check `#![crate_type = "proc-macro"]` here.
             false
         } else {
+            if self.target.env == Env::Musl && self.target.crt_static_default {
+                self.psess.opt_span_buffer_lint(
+                    MUSL_MISSING_CRT_STATIC,
+                    None,
+                    ast::CRATE_NODE_ID,
+                    DecorateDiagCompat::Builtin(BuiltinLintDiag::MissingCrtStatic),
+                );
+            }
+
             self.target.crt_static_default
         }
     }