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

@@ -557,6 +557,10 @@ lint_mismatched_lifetime_syntaxes_suggestion_mixed_only_paths =
 lint_missing_unsafe_on_extern = extern blocks should be unsafe
     .suggestion = needs `unsafe` before the extern keyword
 
+lint_missing_crt_static =
+    on musl targets, the implicit default for `crt-static` is going to change
+    .suggestion = 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

@@ -467,6 +467,9 @@ pub fn decorate_builtin_lint(
         }
         BuiltinLintDiag::UnexpectedBuiltinCfg { cfg, cfg_name, controlled_by } => {
             lints::UnexpectedBuiltinCfg { cfg, cfg_name, controlled_by }.decorate_lint(diag)
+        },
+        BuiltinLintDiag::MissingCrtStatic => {
+            lints::MissingCrtStatic.decorate_lint(diag);
         }
     }
 }

compiler/rustc_lint/src/lints.rs

@@ -2977,6 +2977,10 @@ pub(crate) enum DeprecatedWhereClauseLocationSugg {
     },
 }
 
+#[derive(LintDiagnostic)]
+#[diag(lint_missing_crt_static)]
+pub(crate) struct MissingCrtStatic;
+
 #[derive(LintDiagnostic)]
 #[diag(lint_missing_unsafe_on_extern)]
 pub(crate) struct MissingUnsafeOnExtern {

compiler/rustc_lint_defs/src/builtin.rs

@@ -69,6 +69,7 @@ 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,
@@ -5099,3 +5100,16 @@ declare_lint! {
         report_in_deps: true,
     };
 }
+
+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

@@ -807,6 +807,7 @@ pub enum BuiltinLintDiag {
         cfg_name: Symbol,
         controlled_by: &'static str,
     },
+    MissingCrtStatic,
 }
 
 /// Lints that are buffered up early on in the `Session` before the

compiler/rustc_session/src/session.rs

@@ -7,6 +7,7 @@ use std::sync::atomic::AtomicBool;
 use std::{env, fmt, 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};
@@ -24,6 +25,8 @@ use rustc_errors::{
     Diag, DiagCtxt, DiagCtxtHandle, DiagMessage, Diagnostic, ErrorGuaranteed, FatalAbort,
     TerminalUrl, fallback_fluent_bundle,
 };
+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;
@@ -423,6 +426,15 @@ impl Session {
             // We can't check `#![crate_type = "proc-macro"]` here.
             false
         } else {
+            if self.target.env == "musl" && self.target.crt_static_default {
+                self.psess.opt_span_buffer_lint(
+                    MUSL_MISSING_CRT_STATIC,
+                    None,
+                    ast::CRATE_NODE_ID,
+                    BuiltinLintDiag::MissingCrtStatic,
+                );
+            }
+
             self.target.crt_static_default
         }
     }