Introduce `ArgSource` for diagnostics.

This commit introduces an `ArgSource` enum that is lowered into the HIR
so that diagnostics can correctly refer to the argument pattern's
original name rather than the generated pattern.

5 files changed, 35 insertions, 6 deletions

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index e6669e0d6ed..81d8cfd8622 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -1736,6 +1736,16 @@ pub struct Arg {
     pub ty: P<Ty>,
     pub pat: P<Pat>,
     pub id: NodeId,
+    pub source: ArgSource,
+}
+
+/// The source of an argument in a function header.
+#[derive(Clone, RustcEncodable, RustcDecodable, Debug)]
+pub enum ArgSource {
+    /// Argument as written by the user.
+    Normal,
+    /// Argument from `async fn` lowering, contains the original binding pattern.
+    AsyncFn(P<Pat>),
 }
 
 /// Alternative representation for `Arg`s describing `self` parameter of methods.
@@ -1795,6 +1805,7 @@ impl Arg {
             }),
             ty,
             id: DUMMY_NODE_ID,
+            source: ArgSource::Normal,
         };
         match eself.node {
             SelfKind::Explicit(ty, mutbl) => arg(mutbl, ty),
diff --git a/src/libsyntax/ext/build.rs b/src/libsyntax/ext/build.rs
index 029c45eaa72..40dd187ed28 100644
--- a/src/libsyntax/ext/build.rs
+++ b/src/libsyntax/ext/build.rs
@@ -979,7 +979,8 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
         ast::Arg {
             ty,
             pat: arg_pat,
-            id: ast::DUMMY_NODE_ID
+            id: ast::DUMMY_NODE_ID,
+            source: ast::ArgSource::Normal,
         }
     }
 
diff --git a/src/libsyntax/mut_visit.rs b/src/libsyntax/mut_visit.rs
index bb9116e678e..d3441a2039b 100644
--- a/src/libsyntax/mut_visit.rs
+++ b/src/libsyntax/mut_visit.rs
@@ -235,6 +235,10 @@ pub trait MutVisitor: Sized {
         noop_visit_arg(a, self);
     }
 
+    fn visit_arg_source(&mut self, a: &mut ArgSource) {
+        noop_visit_arg_source(a, self);
+    }
+
     fn visit_generics(&mut self, generics: &mut Generics) {
         noop_visit_generics(generics, self);
     }
@@ -564,10 +568,18 @@ pub fn noop_visit_meta_item<T: MutVisitor>(mi: &mut MetaItem, vis: &mut T) {
     vis.visit_span(span);
 }
 
-pub fn noop_visit_arg<T: MutVisitor>(Arg { id, pat, ty }: &mut Arg, vis: &mut T) {
+pub fn noop_visit_arg<T: MutVisitor>(Arg { id, pat, ty, source }: &mut Arg, vis: &mut T) {
     vis.visit_id(id);
     vis.visit_pat(pat);
     vis.visit_ty(ty);
+    vis.visit_arg_source(source);
+}
+
+pub fn noop_visit_arg_source<T: MutVisitor>(source: &mut ArgSource, vis: &mut T) {
+    match source {
+        ArgSource::Normal => {},
+        ArgSource::AsyncFn(pat) => vis.visit_pat(pat),
+    }
 }
 
 pub fn noop_visit_tt<T: MutVisitor>(tt: &mut TokenTree, vis: &mut T) {
diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 172d5c38c77..d7330ff55ad 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -1,7 +1,7 @@
 use crate::ast::{AngleBracketedArgs, AsyncArgument, ParenthesizedArgs, AttrStyle, BareFnTy};
 use crate::ast::{GenericBound, TraitBoundModifier};
 use crate::ast::Unsafety;
-use crate::ast::{Mod, AnonConst, Arg, Arm, Guard, Attribute, BindingMode, TraitItemKind};
+use crate::ast::{Mod, AnonConst, Arg, ArgSource, Arm, Guard, Attribute, BindingMode, TraitItemKind};
 use crate::ast::Block;
 use crate::ast::{BlockCheckMode, CaptureBy, Movability};
 use crate::ast::{Constness, Crate};
@@ -550,7 +550,7 @@ fn dummy_arg(span: Span) -> Arg {
         span,
         id: ast::DUMMY_NODE_ID
     };
-    Arg { ty: P(ty), pat: pat, id: ast::DUMMY_NODE_ID }
+    Arg { ty: P(ty), pat: pat, id: ast::DUMMY_NODE_ID, source: ast::ArgSource::Normal }
 }
 
 #[derive(Copy, Clone, Debug)]
@@ -2126,7 +2126,7 @@ impl<'a> Parser<'a> {
             }
         };
 
-        Ok(Arg { ty, pat, id: ast::DUMMY_NODE_ID })
+        Ok(Arg { ty, pat, id: ast::DUMMY_NODE_ID, source: ast::ArgSource::Normal })
     }
 
     /// Parses a single function argument.
@@ -2149,7 +2149,8 @@ impl<'a> Parser<'a> {
         Ok(Arg {
             ty: t,
             pat,
-            id: ast::DUMMY_NODE_ID
+            id: ast::DUMMY_NODE_ID,
+            source: ast::ArgSource::Normal,
         })
     }
 
@@ -8856,6 +8857,7 @@ impl<'a> Parser<'a> {
                         ),
                         span,
                     }),
+                    source: ArgSource::AsyncFn(input.pat.clone()),
                 };
 
                 // Construct a `let <pat> = __argN;` statement to insert at the top of the
diff --git a/src/libsyntax/visit.rs b/src/libsyntax/visit.rs
index fe74cbd6496..fc99d10b0b6 100644
--- a/src/libsyntax/visit.rs
+++ b/src/libsyntax/visit.rs
@@ -544,6 +544,9 @@ pub fn walk_fn_ret_ty<'a, V: Visitor<'a>>(visitor: &mut V, ret_ty: &'a FunctionR
 pub fn walk_fn_decl<'a, V: Visitor<'a>>(visitor: &mut V, function_declaration: &'a FnDecl) {
     for argument in &function_declaration.inputs {
         visitor.visit_pat(&argument.pat);
+        if let ArgSource::AsyncFn(pat) = &argument.source {
+            visitor.visit_pat(pat);
+        }
         visitor.visit_ty(&argument.ty)
     }
     visitor.visit_fn_ret_ty(&function_declaration.output)