Recover gracefully from argument with missing type or param name

src/libsyntax/parse/diagnostics.rs

@@ -1,7 +1,7 @@
 use crate::ast;
 use crate::ast::{
     BlockCheckMode, BinOpKind, Expr, ExprKind, Item, ItemKind, Pat, PatKind, PathSegment, QSelf,
-    Ty, TyKind, VariantData,
+    Ty, TyKind, VariantData, Ident,
 };
 use crate::parse::{SeqSep, token, PResult, Parser};
 use crate::parse::parser::{BlockMode, PathStyle, SemiColonMode, TokenType, TokenExpectType};
@@ -1092,12 +1092,12 @@ impl<'a> Parser<'a> {
         pat: P<ast::Pat>,
         require_name: bool,
         is_trait_item: bool,
-    ) {
+    ) -> Option<Ident> {
         // If we find a pattern followed by an identifier, it could be an (incorrect)
         // C-style parameter declaration.
         if self.check_ident() && self.look_ahead(1, |t| {
             *t == token::Comma || *t == token::CloseDelim(token::Paren)
-        }) {
+        }) { // `fn foo(String s) {}`
             let ident = self.parse_ident().unwrap();
             let span = pat.span.with_hi(ident.span.hi());
 
@@ -1107,18 +1107,30 @@ impl<'a> Parser<'a> {
                 String::from("<identifier>: <type>"),
                 Applicability::HasPlaceholders,
             );
-        } else if require_name && is_trait_item {
-            if let PatKind::Ident(_, ident, _) = pat.node {
+            return Some(ident);
+        } else if let PatKind::Ident(_, ident, _) = pat.node {
+            if require_name && (
+                is_trait_item ||
+                self.token == token::Comma ||
+                self.token == token::CloseDelim(token::Paren)
+            ) { // `fn foo(a, b) {}` or `fn foo(usize, usize) {}`
                 err.span_suggestion(
                     pat.span,
-                    "explicitly ignore parameter",
+                    "if this was a parameter name, give it a type",
+                    format!("{}: TypeName", ident),
+                    Applicability::HasPlaceholders,
+                );
+                err.span_suggestion(
+                    pat.span,
+                    "if this is a type, explicitly ignore the parameter name",
                     format!("_: {}", ident),
                     Applicability::MachineApplicable,
                 );
+                err.note("anonymous parameters are removed in the 2018 edition (see RFC 1685)");
+                return Some(ident);
             }
-
-            err.note("anonymous parameters are removed in the 2018 edition (see RFC 1685)");
         }
+        None
     }
 
     crate fn recover_arg_parse(&mut self) -> PResult<'a, (P<ast::Pat>, P<ast::Ty>)> {

src/libsyntax/parse/parser.rs

@@ -51,6 +51,7 @@ use crate::parse::diagnostics::Error;
 
 use errors::{Applicability, DiagnosticBuilder, DiagnosticId, FatalError};
 use rustc_target::spec::abi::{self, Abi};
+use rustc_data_structures::fx::FxHashSet;
 use syntax_pos::{Span, BytePos, DUMMY_SP, FileName, hygiene::CompilerDesugaringKind};
 use log::debug;
 
@@ -452,19 +453,18 @@ impl From<P<Expr>> for LhsExpr {
 }
 
 /// Creates a placeholder argument.
-fn dummy_arg(span: Span) -> Arg {
-    let ident = Ident::new(kw::Invalid, span);
+fn dummy_arg(ident: Ident) -> Arg {
     let pat = P(Pat {
         id: ast::DUMMY_NODE_ID,
         node: PatKind::Ident(BindingMode::ByValue(Mutability::Immutable), ident, None),
-        span,
+        span: ident.span,
     });
     let ty = Ty {
         node: TyKind::Err,
-        span,
+        span: ident.span,
         id: ast::DUMMY_NODE_ID
     };
-    Arg { ty: P(ty), pat: pat, id: ast::DUMMY_NODE_ID, source: ast::ArgSource::Normal }
+    Arg { ty: P(ty), pat: pat, id: ast::DUMMY_NODE_ID, source: ast::ArgSource::Recovery }
 }
 
 #[derive(Copy, Clone, Debug)]
@@ -1528,8 +1528,17 @@ impl<'a> Parser<'a> {
             let pat = self.parse_pat(Some("argument name"))?;
 
             if let Err(mut err) = self.expect(&token::Colon) {
-                self.argument_without_type(&mut err, pat, require_name, is_trait_item);
-                return Err(err);
+                if let Some(ident) = self.argument_without_type(
+                    &mut err,
+                    pat,
+                    require_name,
+                    is_trait_item,
+                ) {
+                    err.emit();
+                    return Ok(dummy_arg(ident));
+                } else {
+                    return Err(err);
+                }
             }
 
             self.eat_incorrect_doc_comment("a method argument's type");
@@ -5431,7 +5440,7 @@ impl<'a> Parser<'a> {
                             p.eat_to_tokens(&[&token::Comma, &token::CloseDelim(token::Paren)]);
                             // Create a placeholder argument for proper arg count (issue #34264).
                             let span = lo.to(p.prev_span);
-                            Ok(Some(dummy_arg(span)))
+                            Ok(Some(dummy_arg(Ident::new(kw::Invalid, span))))
                         }
                     }
                 }
@@ -5584,7 +5593,7 @@ impl<'a> Parser<'a> {
 
         // Parse the rest of the function parameter list.
         let sep = SeqSep::trailing_allowed(token::Comma);
-        let (fn_inputs, recovered) = if let Some(self_arg) = self_arg {
+        let (mut fn_inputs, recovered) = if let Some(self_arg) = self_arg {
             if self.check(&token::CloseDelim(token::Paren)) {
                 (vec![self_arg], false)
             } else if self.eat(&token::Comma) {
@@ -5607,6 +5616,24 @@ impl<'a> Parser<'a> {
             // Parse closing paren and return type.
             self.expect(&token::CloseDelim(token::Paren))?;
         }
+        // Replace duplicated recovered arguments with `_` pattern to avoid unecessary errors.
+        let mut seen_inputs = FxHashSet::default();
+        for input in fn_inputs.iter_mut() {
+            let opt_ident = if let (PatKind::Ident(_, ident, _), ast::ArgSource::Recovery) = (
+                &input.pat.node, &input.source,
+            ) {
+                Some(*ident)
+            } else {
+                None
+            };
+            if let Some(ident) = opt_ident {
+                if seen_inputs.contains(&ident) {
+                    input.pat.node = PatKind::Wild;
+                }
+                seen_inputs.insert(ident);
+            }
+        }
+
         Ok(P(FnDecl {
             inputs: fn_inputs,
             output: self.parse_ret_ty(true)?,