Do not duplicate const keyword on parameters

Author: topecongiro

src/formatting/spanned.rs

@@ -117,7 +117,9 @@ impl Spanned for ast::Param {
 
 impl Spanned for ast::GenericParam {
     fn span(&self) -> Span {
-        let lo = if self.attrs.is_empty() {
+        let lo = if let ast::GenericParamKind::Const { ty: _, kw_span } = self.kind {
+            kw_span.lo()
+        } else if self.attrs.is_empty() {
             self.ident.span.lo()
         } else {
             self.attrs[0].span.lo()

src/formatting/types.rs

@@ -559,7 +559,7 @@ impl Rewrite for ast::GenericParam {
             _ => {}
         }
 
-        if let rustc_ast::ast::GenericParamKind::Const { ref ty } = &self.kind {
+        if let ast::GenericParamKind::Const { ref ty, kw_span: _ } = &self.kind {
             result.push_str("const ");
             result.push_str(rewrite_ident(context, self.ident));
             result.push_str(": ");

tests/source/const_generics.rs

@@ -34,3 +34,11 @@ type Foo<const N: usize> = [i32; N + 1];
 pub trait Foo: Bar<{Baz::COUNT}> {
 	const ASD: usize;
 }
+
+// #4263
+fn const_generics_on_params<
+    // AAAA
+        const BBBB: usize,
+    /* CCCC */
+    const DDDD: usize,
+    >() {}

tests/target/const_generics.rs

@@ -26,3 +26,12 @@ type Foo<const N: usize> = [i32; N + 1];
 pub trait Foo: Bar<{ Baz::COUNT }> {
     const ASD: usize;
 }
+
+// #4263
+fn const_generics_on_params<
+    // AAAA
+    const BBBB: usize,
+    /* CCCC */
+    const DDDD: usize,
+>() {
+}