syntax: improve parameter without type suggestions

This commit improves the suggestions provided when function parameters
do not have types:

- A new suggestion is added for arbitrary self types, which suggests
adding `self: ` before the type.

- Existing suggestions are now provided when a `<` is found where a `:`
was expected (previously only `,` and `)` or trait items), this gives
suggestions in the case where the unnamed parameter type is generic
in a free function.

- The suggestion that a type name be provided (e.g. `fn foo(HashMap<u32>)`
-> `fn foo(HashMap: TypeName<u32>)`) will no longer occur when a `<` was
found instead of `:`.

- The ident will not be used for recovery when a `<` was found instead
of `:`.

Signed-off-by: David Wood <david@davidtw.co>

src/libsyntax/parse/diagnostics.rs

@@ -1220,6 +1220,7 @@ impl<'a> Parser<'a> {
         err: &mut DiagnosticBuilder<'_>,
         pat: P<ast::Pat>,
         require_name: bool,
+        is_self_allowed: bool,
         is_trait_item: bool,
     ) -> Option<Ident> {
         // If we find a pattern followed by an identifier, it could be an (incorrect)
@@ -1241,14 +1242,27 @@ impl<'a> Parser<'a> {
             if require_name && (
                 is_trait_item ||
                 self.token == token::Comma ||
+                self.token == token::Lt ||
                 self.token == token::CloseDelim(token::Paren)
-            ) { // `fn foo(a, b) {}` or `fn foo(usize, usize) {}`
+            ) { // `fn foo(a, b) {}`, `fn foo(a<x>, b<y>) {}` or `fn foo(usize, usize) {}`
+                if is_self_allowed {
+                    err.span_suggestion(
+                        pat.span,
+                        "if this is a `self` type, give it a parameter name",
+                        format!("self: {}", ident),
+                        Applicability::MaybeIncorrect,
+                    );
+                }
+                // Avoid suggesting that `fn foo(HashMap<u32>)` is fixed with a change to
+                // `fn foo(HashMap: TypeName<u32>)`.
+                if self.token != token::Lt {
                     err.span_suggestion(
                         pat.span,
                         "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",
@@ -1256,7 +1270,9 @@ impl<'a> Parser<'a> {
                     Applicability::MachineApplicable,
                 );
                 err.note("anonymous parameters are removed in the 2018 edition (see RFC 1685)");
-                return Some(ident);
+
+                // Don't attempt to recover by using the `X` in `X<Y>` as the parameter name.
+                return if self.token == token::Lt { None } else { Some(ident) };
             }
         }
         None

src/libsyntax/parse/parser.rs

@@ -1212,6 +1212,7 @@ impl<'a> Parser<'a> {
                     &mut err,
                     pat,
                     is_name_required,
+                    is_self_allowed,
                     is_trait_item,
                 ) {
                     err.emit();

src/test/ui/anon-params-denied-2018.stderr

@@ -5,6 +5,10 @@ LL |     fn foo(i32);
    |               ^ expected one of `:`, `@`, or `|` here
    |
    = note: anonymous parameters are removed in the 2018 edition (see RFC 1685)
+help: if this is a `self` type, give it a parameter name
+   |
+LL |     fn foo(self: i32);
+   |            ^^^^^^^^^
 help: if this was a parameter name, give it a type
    |
 LL |     fn foo(i32: TypeName);
@@ -21,6 +25,10 @@ LL |     fn bar_with_default_impl(String, String) {}
    |                                    ^ expected one of `:`, `@`, or `|` here
    |
    = note: anonymous parameters are removed in the 2018 edition (see RFC 1685)
+help: if this is a `self` type, give it a parameter name
+   |
+LL |     fn bar_with_default_impl(self: String, String) {}
+   |                              ^^^^^^^^^^^^
 help: if this was a parameter name, give it a type
    |
 LL |     fn bar_with_default_impl(String: TypeName, String) {}

src/test/ui/parser/pat-lt-bracket-2.stderr

@@ -3,6 +3,12 @@ error: expected one of `:`, `@`, or `|`, found `<`
    |
 LL | fn a(B<) {}
    |       ^ expected one of `:`, `@`, or `|` here
+   |
+   = note: anonymous parameters are removed in the 2018 edition (see RFC 1685)
+help: if this is a type, explicitly ignore the parameter name
+   |
+LL | fn a(_: B<) {}
+   |      ^^^^
 
 error: aborting due to previous error
 

src/test/ui/rfc-2565-param-attrs/param-attrs-2018.stderr

@@ -5,6 +5,10 @@ LL | trait Trait2015 { fn foo(#[allow(C)] i32); }
    |                                         ^ expected one of `:`, `@`, or `|` here
    |
    = note: anonymous parameters are removed in the 2018 edition (see RFC 1685)
+help: if this is a `self` type, give it a parameter name
+   |
+LL | trait Trait2015 { fn foo(#[allow(C)] self: i32); }
+   |                                      ^^^^^^^^^
 help: if this was a parameter name, give it a type
    |
 LL | trait Trait2015 { fn foo(#[allow(C)] i32: TypeName); }

src/test/ui/span/issue-34264.stderr

@@ -3,6 +3,12 @@ error: expected one of `:`, `@`, or `|`, found `<`
    |
 LL | fn foo(Option<i32>, String) {}
    |              ^ expected one of `:`, `@`, or `|` here
+   |
+   = note: anonymous parameters are removed in the 2018 edition (see RFC 1685)
+help: if this is a type, explicitly ignore the parameter name
+   |
+LL | fn foo(_: Option<i32>, String) {}
+   |        ^^^^^^^^^
 
 error: expected one of `:`, `@`, or `|`, found `)`
   --> $DIR/issue-34264.rs:1:27

src/test/ui/suggestions/issue-64252-self-type.rs

@@ -0,0 +1,14 @@
+// This test checks that a suggestion to add a `self: ` parameter name is provided
+// to functions where this is applicable.
+
+pub fn foo(Box<Self>) { }
+//~^ ERROR expected one of `:`, `@`, or `|`, found `<`
+
+struct Bar;
+
+impl Bar {
+    fn bar(Box<Self>) { }
+    //~^ ERROR expected one of `:`, `@`, or `|`, found `<`
+}
+
+fn main() { }

src/test/ui/suggestions/issue-64252-self-type.stderr

@@ -0,0 +1,30 @@
+error: expected one of `:`, `@`, or `|`, found `<`
+  --> $DIR/issue-64252-self-type.rs:4:15
+   |
+LL | pub fn foo(Box<Self>) { }
+   |               ^ expected one of `:`, `@`, or `|` here
+   |
+   = note: anonymous parameters are removed in the 2018 edition (see RFC 1685)
+help: if this is a type, explicitly ignore the parameter name
+   |
+LL | pub fn foo(_: Box<Self>) { }
+   |            ^^^^^^
+
+error: expected one of `:`, `@`, or `|`, found `<`
+  --> $DIR/issue-64252-self-type.rs:10:15
+   |
+LL |     fn bar(Box<Self>) { }
+   |               ^ expected one of `:`, `@`, or `|` here
+   |
+   = note: anonymous parameters are removed in the 2018 edition (see RFC 1685)
+help: if this is a `self` type, give it a parameter name
+   |
+LL |     fn bar(self: Box<Self>) { }
+   |            ^^^^^^^^^
+help: if this is a type, explicitly ignore the parameter name
+   |
+LL |     fn bar(_: Box<Self>) { }
+   |            ^^^^^^
+
+error: aborting due to 2 previous errors
+