Auto merge of #14746 - lowr:patch/associated-return-types, r=Veykril

Parse associated return type bounds

This PR implements parser support for associated return type bounds: `T: Foo<bar(): Send>`. This PR does not implement associated return types (`T::bar(): Send`) because it's not implemented even in rustc, and also removes `(..)`-style return type notation because it has been removed in rust-lang/rust#110203 (effectively reverting #14465).

I don't plan to proactively follow this unstable feature unless an RFC is accepted and my main motivation here is to remove no-longer-valid syntax `(..)` from our parser, nevertheless adding minimal parser support so anyone interested (as can be seen in #14465) can experiment it without rust-analyzer's syntax errors.

Author: bors

crates/hir-def/src/path/lower.rs

@@ -189,6 +189,10 @@ pub(super) fn lower_generic_args(
                 args.push(GenericArg::Type(type_ref));
             }
             ast::GenericArg::AssocTypeArg(assoc_type_arg) => {
+                if assoc_type_arg.param_list().is_some() {
+                    // We currently ignore associated return type bounds.
+                    continue;
+                }
                 if let Some(name_ref) = assoc_type_arg.name_ref() {
                     let name = name_ref.as_name();
                     let args = assoc_type_arg
@@ -216,9 +220,6 @@ pub(super) fn lower_generic_args(
                 let arg = ConstRefOrPath::from_expr_opt(arg.expr());
                 args.push(GenericArg::Const(arg))
             }
-            ast::GenericArg::ReturnTypeArg(_) => {
-                // FIXME: return type notation is experimental, we don't do anything with it yet.
-            }
         }
     }
 

crates/parser/src/grammar/generic_args.rs

@@ -76,7 +76,29 @@ fn generic_arg(p: &mut Parser<'_>) -> bool {
                 }
             }
         }
-        IDENT if p.nth(1) == T!['('] && p.nth_at(2, T![..]) => return_type_arg(p),
+        IDENT if p.nth_at(1, T!['(']) => {
+            let m = p.start();
+            name_ref(p);
+            params::param_list_fn_trait(p);
+            if p.at(T![:]) && !p.at(T![::]) {
+                // test associated_return_type_bounds
+                // fn foo<T: Foo<foo(): Send, bar(i32): Send, baz(i32, i32): Send>>() {}
+                generic_params::bounds(p);
+                m.complete(p, ASSOC_TYPE_ARG);
+            } else {
+                // test bare_dyn_types_with_paren_as_generic_args
+                // type A = S<Fn(i32)>;
+                // type A = S<Fn(i32) + Send>;
+                // type B = S<Fn(i32) -> i32>;
+                // type C = S<Fn(i32) -> i32 + Send>;
+                opt_ret_type(p);
+                let m = m.complete(p, PATH_SEGMENT).precede(p).complete(p, PATH);
+                let m = paths::type_path_for_qualifier(p, m);
+                let m = m.precede(p).complete(p, PATH_TYPE);
+                let m = types::opt_type_bounds_as_dyn_trait_type(p, m);
+                m.precede(p).complete(p, TYPE_ARG);
+            }
+        }
         _ if p.at_ts(types::TYPE_FIRST) => type_arg(p),
         _ => return false,
     }
@@ -140,20 +162,3 @@ fn type_arg(p: &mut Parser<'_>) {
     types::type_(p);
     m.complete(p, TYPE_ARG);
 }
-
-// test return_type_arg
-// type T = S<foo(..): Send>;
-pub(super) fn return_type_arg(p: &mut Parser<'_>) {
-    let m = p.start();
-    p.expect(IDENT);
-    p.expect(T!['(']);
-    p.expect(T![..]);
-    p.expect(T![')']);
-    if !p.at(T![:]) {
-        p.error("expected :");
-        m.abandon(p);
-        return;
-    }
-    generic_params::bounds(p);
-    m.complete(p, RETURN_TYPE_ARG);
-}

crates/parser/src/grammar/paths.rs

@@ -136,6 +136,7 @@ fn opt_path_type_args(p: &mut Parser<'_>, mode: Mode) {
         Mode::Type => {
             // test typepathfn_with_coloncolon
             // type F = Start::(Middle) -> (Middle)::End;
+            // type GenericArg = S<Start(Middle)::End>;
             if p.at(T![::]) && p.nth_at(2, T!['(']) {
                 p.bump(T![::]);
             }

crates/parser/src/grammar/types.rs

@@ -337,13 +337,16 @@ pub(super) fn path_type_(p: &mut Parser<'_>, allow_bounds: bool) {
 
 /// This turns a parsed PATH_TYPE or FOR_TYPE optionally into a DYN_TRAIT_TYPE
 /// with a TYPE_BOUND_LIST
-fn opt_type_bounds_as_dyn_trait_type(p: &mut Parser<'_>, type_marker: CompletedMarker) {
+pub(super) fn opt_type_bounds_as_dyn_trait_type(
+    p: &mut Parser<'_>,
+    type_marker: CompletedMarker,
+) -> CompletedMarker {
     assert!(matches!(
         type_marker.kind(),
         SyntaxKind::PATH_TYPE | SyntaxKind::FOR_TYPE | SyntaxKind::MACRO_TYPE
     ));
     if !p.at(T![+]) {
-        return;
+        return type_marker;
     }
 
     // First create a TYPE_BOUND from the completed PATH_TYPE
@@ -360,5 +363,5 @@ fn opt_type_bounds_as_dyn_trait_type(p: &mut Parser<'_>, type_marker: CompletedM
     let m = generic_params::bounds_without_colon_m(p, m);
 
     // Finally precede everything with DYN_TRAIT_TYPE
-    m.precede(p).complete(p, DYN_TRAIT_TYPE);
+    m.precede(p).complete(p, DYN_TRAIT_TYPE)
 }

crates/parser/test_data/parser/inline/ok/0202_typepathfn_with_coloncolon.rast

@@ -41,3 +41,41 @@ SOURCE_FILE
             IDENT "End"
     SEMICOLON ";"
   WHITESPACE "\n"
+  TYPE_ALIAS
+    TYPE_KW "type"
+    WHITESPACE " "
+    NAME
+      IDENT "GenericArg"
+    WHITESPACE " "
+    EQ "="
+    WHITESPACE " "
+    PATH_TYPE
+      PATH
+        PATH_SEGMENT
+          NAME_REF
+            IDENT "S"
+          GENERIC_ARG_LIST
+            L_ANGLE "<"
+            TYPE_ARG
+              PATH_TYPE
+                PATH
+                  PATH
+                    PATH_SEGMENT
+                      NAME_REF
+                        IDENT "Start"
+                      PARAM_LIST
+                        L_PAREN "("
+                        PARAM
+                          PATH_TYPE
+                            PATH
+                              PATH_SEGMENT
+                                NAME_REF
+                                  IDENT "Middle"
+                        R_PAREN ")"
+                  COLON2 "::"
+                  PATH_SEGMENT
+                    NAME_REF
+                      IDENT "End"
+            R_ANGLE ">"
+    SEMICOLON ";"
+  WHITESPACE "\n"

crates/parser/test_data/parser/inline/ok/0202_typepathfn_with_coloncolon.rs

@@ -1 +1,2 @@
 type F = Start::(Middle) -> (Middle)::End;
+type GenericArg = S<Start(Middle)::End>;

crates/parser/test_data/parser/inline/ok/0206_return_type_arg.rast

@@ -0,0 +1,102 @@
+SOURCE_FILE
+  FN
+    FN_KW "fn"
+    WHITESPACE " "
+    NAME
+      IDENT "foo"
+    GENERIC_PARAM_LIST
+      L_ANGLE "<"
+      TYPE_PARAM
+        NAME
+          IDENT "T"
+        COLON ":"
+        WHITESPACE " "
+        TYPE_BOUND_LIST
+          TYPE_BOUND
+            PATH_TYPE
+              PATH
+                PATH_SEGMENT
+                  NAME_REF
+                    IDENT "Foo"
+                  GENERIC_ARG_LIST
+                    L_ANGLE "<"
+                    ASSOC_TYPE_ARG
+                      NAME_REF
+                        IDENT "foo"
+                      PARAM_LIST
+                        L_PAREN "("
+                        R_PAREN ")"
+                      COLON ":"
+                      WHITESPACE " "
+                      TYPE_BOUND_LIST
+                        TYPE_BOUND
+                          PATH_TYPE
+                            PATH
+                              PATH_SEGMENT
+                                NAME_REF
+                                  IDENT "Send"
+                    COMMA ","
+                    WHITESPACE " "
+                    ASSOC_TYPE_ARG
+                      NAME_REF
+                        IDENT "bar"
+                      PARAM_LIST
+                        L_PAREN "("
+                        PARAM
+                          PATH_TYPE
+                            PATH
+                              PATH_SEGMENT
+                                NAME_REF
+                                  IDENT "i32"
+                        R_PAREN ")"
+                      COLON ":"
+                      WHITESPACE " "
+                      TYPE_BOUND_LIST
+                        TYPE_BOUND
+                          PATH_TYPE
+                            PATH
+                              PATH_SEGMENT
+                                NAME_REF
+                                  IDENT "Send"
+                    COMMA ","
+                    WHITESPACE " "
+                    ASSOC_TYPE_ARG
+                      NAME_REF
+                        IDENT "baz"
+                      PARAM_LIST
+                        L_PAREN "("
+                        PARAM
+                          PATH_TYPE
+                            PATH
+                              PATH_SEGMENT
+                                NAME_REF
+                                  IDENT "i32"
+                        COMMA ","
+                        WHITESPACE " "
+                        PARAM
+                          PATH_TYPE
+                            PATH
+                              PATH_SEGMENT
+                                NAME_REF
+                                  IDENT "i32"
+                        R_PAREN ")"
+                      COLON ":"
+                      WHITESPACE " "
+                      TYPE_BOUND_LIST
+                        TYPE_BOUND
+                          PATH_TYPE
+                            PATH
+                              PATH_SEGMENT
+                                NAME_REF
+                                  IDENT "Send"
+                    R_ANGLE ">"
+      R_ANGLE ">"
+    PARAM_LIST
+      L_PAREN "("
+      R_PAREN ")"
+    WHITESPACE " "
+    BLOCK_EXPR
+      STMT_LIST
+        L_CURLY "{"
+        R_CURLY "}"
+  WHITESPACE "\n"

crates/parser/test_data/parser/inline/ok/0206_return_type_arg.rs

@@ -0,0 +1 @@
+fn foo<T: Foo<foo(): Send, bar(i32): Send, baz(i32, i32): Send>>() {}