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.

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