Auto merge of rust-lang#57760 - dlrobertson:varargs1, r=alexreg

Support defining C compatible variadic functions

## Summary

Add support for defining C compatible variadic functions in unsafe rust with
`extern "C"` according to [RFC 2137].

## Details

### Parsing
When parsing a user defined function that is `unsafe` and `extern "C"` allow
variadic signatures and inject a "spoofed" `VaList` in the new functions
signature. This allows the user to interact with the variadic arguments via a
`VaList` instead of manually using `va_start` and `va_end` (See [RFC 2137] for
details).

### Codegen

When running codegen for a variadic function, remove the "spoofed" `VaList`
from the function signature and inject `va_start` when the arg local
references are created for the function and `va_end` on return.

## TODO

 - [x] Get feedback on injecting `va_start/va_end` in MIR vs codegen
 - [x] Properly inject `va_end` - It seems like it should be possible to inject
       `va_end` on the `TerminatorKind::Return`. I just need to figure out how
       to get the `LocalRef` here.
 - [x] Properly call Rust defined C variadic functions in Rust - The spoofed
       `VaList` causes problems here.

Related to: rust-lang#44930

r? @ghost

[RFC 2137]: https://github.com/rust-lang/rfcs/blob/master/text/2137-variadic.md

Author: bors

src/doc/unstable-book/src/language-features/c-variadic.md

@@ -0,0 +1,24 @@
+# `c_variadic`
+
+The tracking issue for this feature is: [#44930]
+
+[#44930]: https://github.com/rust-lang/rust/issues/44930
+
+------------------------
+
+The `c_variadic` language feature enables C-variadic functions to be
+defined in Rust. The may be called both from within Rust and via FFI.
+
+## Examples
+
+```rust
+#![feature(c_variadic)]
+
+pub unsafe extern "C" fn add(n: usize, mut args: ...) -> usize {
+    let mut sum = 0;
+    for _ in 0..n {
+        sum += args.arg::<usize>();
+    }
+    sum
+}
+```

src/doc/unstable-book/src/library-features/c-variadic.md

@@ -0,0 +1,26 @@
+# `c_variadic`
+
+The tracking issue for this feature is: [#44930]
+
+[#44930]: https://github.com/rust-lang/rust/issues/44930
+
+------------------------
+
+The `c_variadic` library feature exposes the `VaList` structure,
+Rust's analogue of C's `va_list` type.
+
+## Examples
+
+```rust
+#![feature(c_variadic)]
+
+use std::ffi::VaList;
+
+pub unsafe extern "C" fn vadd(n: usize, mut args: VaList) -> usize {
+    let mut sum = 0;
+    for _ in 0..n {
+        sum += args.arg::<usize>();
+    }
+    sum
+}
+```

src/librustc/hir/intravisit.rs

@@ -617,6 +617,9 @@ pub fn walk_ty<'v, V: Visitor<'v>>(visitor: &mut V, typ: &'v Ty) {
         TyKind::Typeof(ref expression) => {
             visitor.visit_anon_const(expression)
         }
+        TyKind::CVarArgs(ref lt) => {
+            visitor.visit_lifetime(lt)
+        }
         TyKind::Infer | TyKind::Err => {}
     }
 }

src/librustc/hir/lowering.rs

@@ -74,7 +74,7 @@ const HIR_ID_COUNTER_LOCKED: u32 = 0xFFFFFFFF;
 pub struct LoweringContext<'a> {
     crate_root: Option<&'static str>,
 
-    // Used to assign ids to HIR nodes that do not directly correspond to an AST node.
+    /// Used to assign ids to HIR nodes that do not directly correspond to an AST node.
     sess: &'a Session,
 
     cstore: &'a dyn CrateStore,
@@ -107,25 +107,25 @@ pub struct LoweringContext<'a> {
     /// written at all (e.g., `&T` or `std::cell::Ref<T>`).
     anonymous_lifetime_mode: AnonymousLifetimeMode,
 
-    // Used to create lifetime definitions from in-band lifetime usages.
-    // e.g., `fn foo(x: &'x u8) -> &'x u8` to `fn foo<'x>(x: &'x u8) -> &'x u8`
-    // When a named lifetime is encountered in a function or impl header and
-    // has not been defined
-    // (i.e., it doesn't appear in the in_scope_lifetimes list), it is added
-    // to this list. The results of this list are then added to the list of
-    // lifetime definitions in the corresponding impl or function generics.
+    /// Used to create lifetime definitions from in-band lifetime usages.
+    /// e.g., `fn foo(x: &'x u8) -> &'x u8` to `fn foo<'x>(x: &'x u8) -> &'x u8`
+    /// When a named lifetime is encountered in a function or impl header and
+    /// has not been defined
+    /// (i.e., it doesn't appear in the in_scope_lifetimes list), it is added
+    /// to this list. The results of this list are then added to the list of
+    /// lifetime definitions in the corresponding impl or function generics.
     lifetimes_to_define: Vec<(Span, ParamName)>,
 
-    // Whether or not in-band lifetimes are being collected. This is used to
-    // indicate whether or not we're in a place where new lifetimes will result
-    // in in-band lifetime definitions, such a function or an impl header,
-    // including implicit lifetimes from `impl_header_lifetime_elision`.
+    /// Whether or not in-band lifetimes are being collected. This is used to
+    /// indicate whether or not we're in a place where new lifetimes will result
+    /// in in-band lifetime definitions, such a function or an impl header,
+    /// including implicit lifetimes from `impl_header_lifetime_elision`.
     is_collecting_in_band_lifetimes: bool,
 
-    // Currently in-scope lifetimes defined in impl headers, fn headers, or HRTB.
-    // When `is_collectin_in_band_lifetimes` is true, each lifetime is checked
-    // against this list to see if it is already in-scope, or if a definition
-    // needs to be created for it.
+    /// Currently in-scope lifetimes defined in impl headers, fn headers, or HRTB.
+    /// When `is_collectin_in_band_lifetimes` is true, each lifetime is checked
+    /// against this list to see if it is already in-scope, or if a definition
+    /// needs to be created for it.
     in_scope_lifetimes: Vec<Ident>,
 
     current_module: NodeId,
@@ -954,7 +954,7 @@ impl<'a> LoweringContext<'a> {
         let decl = FnDecl {
             inputs: vec![],
             output,
-            variadic: false
+            c_variadic: false
         };
         let body_id = self.record_body(body_expr, Some(&decl));
         self.is_generator = prev_is_generator;
@@ -1345,6 +1345,12 @@ impl<'a> LoweringContext<'a> {
                 }
             }
             TyKind::Mac(_) => panic!("TyMac should have been expanded by now."),
+            TyKind::CVarArgs => {
+                // Create the implicit lifetime of the "spoofed" `VaList`.
+                let span = self.sess.source_map().next_point(t.span.shrink_to_lo());
+                let lt = self.new_implicit_lifetime(span);
+                hir::TyKind::CVarArgs(lt)
+            },
         };
 
         let LoweredNodeId { node_id: _, hir_id } = self.lower_node_id(t.id);
@@ -2112,7 +2118,7 @@ impl<'a> LoweringContext<'a> {
         P(hir::FnDecl {
             inputs,
             output,
-            variadic: decl.variadic,
+            c_variadic: decl.c_variadic,
             implicit_self: decl.inputs.get(0).map_or(
                 hir::ImplicitSelfKind::None,
                 |arg| {
@@ -3967,7 +3973,7 @@ impl<'a> LoweringContext<'a> {
                     let outer_decl = FnDecl {
                         inputs: decl.inputs.clone(),
                         output: FunctionRetTy::Default(fn_decl_span),
-                        variadic: false,
+                        c_variadic: false,
                     };
                     // We need to lower the declaration outside the new scope, because we
                     // have to conserve the state of being inside a loop condition for the

src/librustc/hir/mod.rs

@@ -1829,6 +1829,9 @@ pub enum TyKind {
     Infer,
     /// Placeholder for a type that has failed to be defined.
     Err,
+    /// Placeholder for C-variadic arguments. We "spoof" the `VaList` created
+    /// from the variadic arguments. This type is only valid up to typeck.
+    CVarArgs(Lifetime),
 }
 
 #[derive(Clone, RustcEncodable, RustcDecodable, Debug)]
@@ -1865,7 +1868,7 @@ pub struct Arg {
 pub struct FnDecl {
     pub inputs: HirVec<Ty>,
     pub output: FunctionRetTy,
-    pub variadic: bool,
+    pub c_variadic: bool,
     /// Does the function have an implicit self?
     pub implicit_self: ImplicitSelfKind,
 }

src/librustc/hir/print.rs

@@ -434,6 +434,9 @@ impl<'a> State<'a> {
                 self.s.word("/*ERROR*/")?;
                 self.pclose()?;
             }
+            hir::TyKind::CVarArgs(_) => {
+                self.s.word("...")?;
+            }
         }
         self.end()
     }
@@ -2004,7 +2007,7 @@ impl<'a> State<'a> {
             s.print_type(ty)?;
             s.end()
         })?;
-        if decl.variadic {
+        if decl.c_variadic {
             self.s.word(", ...")?;
         }
         self.pclose()?;

src/librustc/ich/impls_hir.rs

@@ -361,13 +361,14 @@ impl_stable_hash_for!(enum hir::TyKind {
     TraitObject(trait_refs, lifetime),
     Typeof(body_id),
     Err,
-    Infer
+    Infer,
+    CVarArgs(lt),
 });
 
 impl_stable_hash_for!(struct hir::FnDecl {
     inputs,
     output,
-    variadic,
+    c_variadic,
     implicit_self
 });
 

src/librustc/ich/impls_ty.rs

@@ -232,7 +232,7 @@ impl_stable_hash_for!(struct ty::GenSig<'tcx> {
 
 impl_stable_hash_for!(struct ty::FnSig<'tcx> {
     inputs_and_output,
-    variadic,
+    c_variadic,
     unsafety,
     abi
 });

src/librustc/middle/resolve_lifetime.rs

@@ -764,6 +764,13 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
                     });
                 }
             }
+            hir::TyKind::CVarArgs(ref lt) => {
+                // Resolve the generated lifetime for the C-variadic arguments.
+                // The lifetime is generated in AST -> HIR lowering.
+                if lt.name.is_elided() {
+                    self.resolve_elided_lifetimes(vec![lt])
+                }
+            }
             _ => intravisit::walk_ty(self, ty),
         }
     }
@@ -2225,18 +2232,22 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
                 if let hir::TyKind::BareFn(_) = ty.node {
                     self.outer_index.shift_in(1);
                 }
-                if let hir::TyKind::TraitObject(ref bounds, ref lifetime) = ty.node {
-                    for bound in bounds {
-                        self.visit_poly_trait_ref(bound, hir::TraitBoundModifier::None);
-                    }
+                match ty.node {
+                    hir::TyKind::TraitObject(ref bounds, ref lifetime) => {
+                        for bound in bounds {
+                            self.visit_poly_trait_ref(bound, hir::TraitBoundModifier::None);
+                        }
 
-                    // Stay on the safe side and don't include the object
-                    // lifetime default (which may not end up being used).
-                    if !lifetime.is_elided() {
-                        self.visit_lifetime(lifetime);
+                        // Stay on the safe side and don't include the object
+                        // lifetime default (which may not end up being used).
+                        if !lifetime.is_elided() {
+                            self.visit_lifetime(lifetime);
+                        }
+                    }
+                    hir::TyKind::CVarArgs(_) => {}
+                    _ => {
+                        intravisit::walk_ty(self, ty);
                     }
-                } else {
-                    intravisit::walk_ty(self, ty);
                 }
                 if let hir::TyKind::BareFn(_) = ty.node {
                     self.outer_index.shift_out(1);

src/librustc/traits/select.rs

@@ -1944,7 +1944,7 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
                 if let ty::FnSig {
                     unsafety: hir::Unsafety::Normal,
                     abi: Abi::Rust,
-                    variadic: false,
+                    c_variadic: false,
                     ..
                 } = self_ty.fn_sig(self.tcx()).skip_binder()
                 {