Rollup merge of #91002 - petrochenkov:nosynth, r=davidtwco

rustc: Remove `#[rustc_synthetic]`

This function parameter attribute was introduced in https://github.com/rust-lang/rust/pull/44866 as an intermediate step in implementing `impl Trait`, it's not necessary or used anywhere by itself.

Noticed while reviewing https://github.com/rust-lang/rust/pull/90947.

src/librustdoc/clean/mod.rs

@@ -459,9 +459,7 @@ impl Clean<Generics> for hir::Generics<'_> {
         // scans them first.
         fn is_impl_trait(param: &hir::GenericParam<'_>) -> bool {
             match param.kind {
-                hir::GenericParamKind::Type { synthetic, .. } => {
-                    synthetic == Some(hir::SyntheticTyParamKind::ImplTrait)
-                }
+                hir::GenericParamKind::Type { synthetic, .. } => synthetic,
                 _ => false,
             }
         }
@@ -560,7 +558,7 @@ impl<'a, 'tcx> Clean<Generics> for (&'a ty::Generics, ty::GenericPredicates<'tcx
                         assert_eq!(param.index, 0);
                         return None;
                     }
-                    if synthetic == Some(hir::SyntheticTyParamKind::ImplTrait) {
+                    if synthetic {
                         impl_trait.insert(param.index.into(), vec![]);
                         return None;
                     }

src/librustdoc/clean/types.rs

@@ -1238,20 +1238,9 @@ impl WherePredicate {
 
 #[derive(Clone, PartialEq, Eq, Debug, Hash)]
 crate enum GenericParamDefKind {
-    Lifetime {
-        outlives: Vec<Lifetime>,
-    },
-    Type {
-        did: DefId,
-        bounds: Vec<GenericBound>,
-        default: Option<Box<Type>>,
-        synthetic: Option<hir::SyntheticTyParamKind>,
-    },
-    Const {
-        did: DefId,
-        ty: Box<Type>,
-        default: Option<Box<String>>,
-    },
+    Lifetime { outlives: Vec<Lifetime> },
+    Type { did: DefId, bounds: Vec<GenericBound>, default: Option<Box<Type>>, synthetic: bool },
+    Const { did: DefId, ty: Box<Type>, default: Option<Box<String>> },
 }
 
 impl GenericParamDefKind {
@@ -1285,7 +1274,7 @@ impl GenericParamDef {
     crate fn is_synthetic_type_param(&self) -> bool {
         match self.kind {
             GenericParamDefKind::Lifetime { .. } | GenericParamDefKind::Const { .. } => false,
-            GenericParamDefKind::Type { ref synthetic, .. } => synthetic.is_some(),
+            GenericParamDefKind::Type { synthetic, .. } => synthetic,
         }
     }
 

src/test/ui/synthetic-param.rs

@@ -1,28 +0,0 @@
-#![feature(rustc_attrs)]
-
-fn func<#[rustc_synthetic] T>(_: T) {}
-
-struct Foo;
-
-impl Foo {
-    pub fn func<#[rustc_synthetic] T>(_: T) {}
-}
-
-struct Bar<S> {
-    t: S
-}
-
-impl<S> Bar<S> {
-    pub fn func<#[rustc_synthetic] T>(_: T) {}
-}
-
-fn main() {
-    func::<u8>(42); //~ ERROR cannot provide explicit generic arguments
-    func(42); // Ok
-
-    Foo::func::<u8>(42); //~ ERROR cannot provide explicit generic arguments
-    Foo::func(42); // Ok
-
-    Bar::<i8>::func::<u8>(42); //~ ERROR cannot provide explicit generic arguments
-    Bar::<i8>::func(42); // Ok
-}

src/test/ui/synthetic-param.stderr

@@ -1,30 +0,0 @@
-error[E0632]: cannot provide explicit generic arguments when `impl Trait` is used in argument position
-  --> $DIR/synthetic-param.rs:20:12
-   |
-LL |     func::<u8>(42);
-   |            ^^ explicit generic argument not allowed
-   |
-   = note: see issue #83701 <https://github.com/rust-lang/rust/issues/83701> for more information
-   = help: add `#![feature(explicit_generic_args_with_impl_trait)]` to the crate attributes to enable
-
-error[E0632]: cannot provide explicit generic arguments when `impl Trait` is used in argument position
-  --> $DIR/synthetic-param.rs:23:17
-   |
-LL |     Foo::func::<u8>(42);
-   |                 ^^ explicit generic argument not allowed
-   |
-   = note: see issue #83701 <https://github.com/rust-lang/rust/issues/83701> for more information
-   = help: add `#![feature(explicit_generic_args_with_impl_trait)]` to the crate attributes to enable
-
-error[E0632]: cannot provide explicit generic arguments when `impl Trait` is used in argument position
-  --> $DIR/synthetic-param.rs:26:23
-   |
-LL |     Bar::<i8>::func::<u8>(42);
-   |                       ^^ explicit generic argument not allowed
-   |
-   = note: see issue #83701 <https://github.com/rust-lang/rust/issues/83701> for more information
-   = help: add `#![feature(explicit_generic_args_with_impl_trait)]` to the crate attributes to enable
-
-error: aborting due to 3 previous errors
-
-For more information about this error, try `rustc --explain E0632`.

src/tools/clippy/clippy_lints/src/types/borrowed_box.rs

@@ -3,10 +3,8 @@ use clippy_utils::source::snippet;
 use clippy_utils::{match_def_path, paths};
 use if_chain::if_chain;
 use rustc_errors::Applicability;
-use rustc_hir::{
-    self as hir, GenericArg, GenericBounds, GenericParamKind, HirId, Lifetime, MutTy, Mutability, Node, QPath,
-    SyntheticTyParamKind, TyKind,
-};
+use rustc_hir::{self as hir, GenericArg, GenericBounds, GenericParamKind};
+use rustc_hir::{HirId, Lifetime, MutTy, Mutability, Node, QPath, TyKind};
 use rustc_lint::LateContext;
 
 use super::BORROWED_BOX;
@@ -105,7 +103,7 @@ fn get_bounds_if_impl_trait<'tcx>(cx: &LateContext<'tcx>, qpath: &QPath<'_>, id:
         if let Some(did) = cx.qpath_res(qpath, id).opt_def_id();
         if let Some(Node::GenericParam(generic_param)) = cx.tcx.hir().get_if_local(did);
         if let GenericParamKind::Type { synthetic, .. } = generic_param.kind;
-        if synthetic == Some(SyntheticTyParamKind::ImplTrait);
+        if synthetic;
         then {
             Some(generic_param.bounds)
         } else {