Introduce missing ABI lint on extern blocks

Cargo.lock

@@ -3896,6 +3896,7 @@ dependencies = [
  "rustc_macros",
  "rustc_serialize",
  "rustc_span",
+ "rustc_target",
  "tracing",
 ]
 

compiler/rustc_ast_lowering/src/item.rs

@@ -317,12 +317,17 @@ fn lower_item_kind(
                 })
             }
             ItemKind::Mod(ref m) => hir::ItemKind::Mod(self.lower_mod(m)),
-            ItemKind::ForeignMod(ref fm) => hir::ItemKind::ForeignMod {
-                abi: fm.abi.map_or(abi::Abi::C, |abi| self.lower_abi(abi)),
-                items: self
-                    .arena
-                    .alloc_from_iter(fm.items.iter().map(|x| self.lower_foreign_item_ref(x))),
-            },
+            ItemKind::ForeignMod(ref fm) => {
+                if fm.abi.is_none() {
+                    self.maybe_lint_missing_abi(span, id, abi::Abi::C);
+                }
+                hir::ItemKind::ForeignMod {
+                    abi: fm.abi.map_or(abi::Abi::C, |abi| self.lower_abi(abi)),
+                    items: self
+                        .arena
+                        .alloc_from_iter(fm.items.iter().map(|x| self.lower_foreign_item_ref(x))),
+                }
+            }
             ItemKind::GlobalAsm(ref ga) => hir::ItemKind::GlobalAsm(self.lower_global_asm(ga)),
             ItemKind::TyAlias(_, ref gen, _, Some(ref ty)) => {
                 // We lower

compiler/rustc_ast_lowering/src/lib.rs

@@ -53,13 +53,15 @@
 use rustc_hir::intravisit;
 use rustc_hir::{ConstArg, GenericArg, ParamName};
 use rustc_index::vec::{Idx, IndexVec};
-use rustc_session::lint::{builtin::BARE_TRAIT_OBJECTS, BuiltinLintDiagnostics, LintBuffer};
+use rustc_session::lint::builtin::{BARE_TRAIT_OBJECTS, MISSING_ABI};
+use rustc_session::lint::{BuiltinLintDiagnostics, LintBuffer};
 use rustc_session::parse::ParseSess;
 use rustc_session::Session;
 use rustc_span::hygiene::ExpnId;
 use rustc_span::source_map::{respan, DesugaringKind};
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::Span;
+use rustc_target::spec::abi::Abi;
 
 use smallvec::{smallvec, SmallVec};
 use std::collections::BTreeMap;
@@ -2777,6 +2779,26 @@ fn maybe_lint_bare_trait(&mut self, span: Span, id: NodeId, is_global: bool) {
             )
         }
     }
+
+    fn maybe_lint_missing_abi(&mut self, span: Span, id: NodeId, default: Abi) {
+        // FIXME(davidtwco): This is a hack to detect macros which produce spans of the
+        // call site which do not have a macro backtrace. See #61963.
+        let is_macro_callsite = self
+            .sess
+            .source_map()
+            .span_to_snippet(span)
+            .map(|snippet| snippet.starts_with("#["))
+            .unwrap_or(true);
+        if !is_macro_callsite {
+            self.resolver.lint_buffer().buffer_lint_with_diagnostic(
+                MISSING_ABI,
+                id,
+                span,
+                "extern declarations without an explicit ABI are deprecated",
+                BuiltinLintDiagnostics::MissingAbi(span, default),
+            )
+        }
+    }
 }
 
 fn body_ids(bodies: &BTreeMap<hir::BodyId, hir::Body<'_>>) -> Vec<hir::BodyId> {

compiler/rustc_lint/src/context.rs

@@ -600,6 +600,10 @@ fn lookup_with_diagnostics(
                 BuiltinLintDiagnostics::PatternsInFnsWithoutBody(span, ident) => {
                     db.span_suggestion(span, "remove `mut` from the parameter", ident.to_string(), Applicability::MachineApplicable);
                 }
+                BuiltinLintDiagnostics::MissingAbi(span, default_abi) => {
+                    db.span_label(span, "ABI should be specified here");
+                    db.help(&format!("the default ABI is {}", default_abi.name()));
+                }
             }
             // Rewrap `db`, and pass control to the user.
             decorate(LintDiagnosticBuilder::new(db));

compiler/rustc_lint_defs/Cargo.toml

@@ -11,3 +11,4 @@ rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_span = { path = "../rustc_span" }
 rustc_serialize = { path = "../rustc_serialize" }
 rustc_macros = { path = "../rustc_macros" }
+rustc_target = { path = "../rustc_target" }

compiler/rustc_lint_defs/src/builtin.rs

@@ -2917,6 +2917,7 @@
         FUNCTION_ITEM_REFERENCES,
         USELESS_DEPRECATED,
         UNSUPPORTED_NAKED_FUNCTIONS,
+        MISSING_ABI,
     ]
 }
 
@@ -2944,3 +2945,28 @@
 }
 
 declare_lint_pass!(UnusedDocComment => [UNUSED_DOC_COMMENTS]);
+
+declare_lint! {
+    /// The `missing_abi` lint detects cases where the ABI is omitted from
+    /// extern declarations.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// #![deny(missing_abi)]
+    ///
+    /// extern fn foo() {}
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Historically, Rust implicitly selected C as the ABI for extern
+    /// declarations. We expect to add new ABIs, like `C-unwind`, in the future,
+    /// though this has not yet happened, and especially with their addition
+    /// seeing the ABI easily will make code review easier.
+    pub MISSING_ABI,
+    Allow,
+    "No declared ABI for extern declaration"
+}

compiler/rustc_lint_defs/src/lib.rs

@@ -6,6 +6,7 @@
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher, ToStableHashKey};
 use rustc_span::edition::Edition;
 use rustc_span::{sym, symbol::Ident, MultiSpan, Span, Symbol};
+use rustc_target::spec::abi::Abi;
 
 pub mod builtin;
 
@@ -252,6 +253,7 @@ pub enum BuiltinLintDiagnostics {
     UnusedImports(String, Vec<(Span, String)>),
     RedundantImport(Vec<(Span, bool)>, Ident),
     DeprecatedMacro(Option<Symbol>, Span),
+    MissingAbi(Span, Abi),
     UnusedDocComment(Span),
     PatternsInFnsWithoutBody(Span, Ident),
 }