Record span of const kw in GenericParamKind

Context: this is needed to fix rust-lang/rustfmt#4263,
which currently records the span of a const generic param incorrectly
because the location of the `const` kw is not known.

I am not sure how to add tests for this; any guidance in how to do so
would be appreciated 🙂

src/librustc_ast/ast.rs

@@ -335,6 +335,8 @@ pub enum GenericParamKind {
     },
     Const {
         ty: P<Ty>,
+        /// Span of the `const` keyword.
+        kw_span: Span,
     },
 }
 

src/librustc_ast/mut_visit.rs

@@ -762,7 +762,7 @@ pub fn noop_flat_map_generic_param<T: MutVisitor>(
         GenericParamKind::Type { default } => {
             visit_opt(default, |default| vis.visit_ty(default));
         }
-        GenericParamKind::Const { ty } => {
+        GenericParamKind::Const { ty, kw_span: _ } => {
             vis.visit_ty(ty);
         }
     }

src/librustc_ast_lowering/lib.rs

@@ -2126,7 +2126,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
 
                 (hir::ParamName::Plain(param.ident), kind)
             }
-            GenericParamKind::Const { ref ty } => {
+            GenericParamKind::Const { ref ty, kw_span: _ } => {
                 let ty = self
                     .with_anonymous_lifetime_mode(AnonymousLifetimeMode::ReportError, |this| {
                         this.lower_ty(&ty, ImplTraitContext::disallowed())

src/librustc_ast_passes/ast_validation.rs

@@ -1135,9 +1135,9 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
             generics.params.iter().map(|param| {
                 let ident = Some(param.ident.to_string());
                 let (kind, ident) = match &param.kind {
-                    GenericParamKind::Lifetime { .. } => (ParamKindOrd::Lifetime, ident),
-                    GenericParamKind::Type { .. } => (ParamKindOrd::Type, ident),
-                    GenericParamKind::Const { ref ty } => {
+                    GenericParamKind::Lifetime => (ParamKindOrd::Lifetime, ident),
+                    GenericParamKind::Type { default: _ } => (ParamKindOrd::Type, ident),
+                    GenericParamKind::Const { ref ty, kw_span: _ } => {
                         let ty = pprust::ty_to_string(ty);
                         (ParamKindOrd::Const, Some(format!("const {}: {}", param.ident, ty)))
                     }

src/librustc_ast_pretty/pprust.rs

@@ -2578,7 +2578,7 @@ impl<'a> State<'a> {
                         s.print_type(default)
                     }
                 }
-                ast::GenericParamKind::Const { ref ty } => {
+                ast::GenericParamKind::Const { ref ty, kw_span: _ } => {
                     s.word_space("const");
                     s.print_ident(param.ident);
                     s.s.space();

src/librustc_builtin_macros/deriving/mod.rs

@@ -123,7 +123,7 @@ fn inject_impl_of_structural_trait(
                 *default = None;
                 ast::GenericArg::Type(cx.ty_ident(span, param.ident))
             }
-            ast::GenericParamKind::Const { ty: _ } => {
+            ast::GenericParamKind::Const { ty: _, kw_span: _ } => {
                 ast::GenericArg::Const(cx.const_ident(span, param.ident))
             }
         })

src/librustc_parse/parser/generics.rs

@@ -47,21 +47,21 @@ impl<'a> Parser<'a> {
     }
 
     fn parse_const_param(&mut self, preceding_attrs: Vec<Attribute>) -> PResult<'a, GenericParam> {
-        let lo = self.token.span;
+        let const_span = self.token.span;
 
         self.expect_keyword(kw::Const)?;
         let ident = self.parse_ident()?;
         self.expect(&token::Colon)?;
         let ty = self.parse_ty()?;
 
-        self.sess.gated_spans.gate(sym::const_generics, lo.to(self.prev_token.span));
+        self.sess.gated_spans.gate(sym::const_generics, const_span.to(self.prev_token.span));
 
         Ok(GenericParam {
             ident,
             id: ast::DUMMY_NODE_ID,
             attrs: preceding_attrs.into(),
             bounds: Vec::new(),
-            kind: GenericParamKind::Const { ty },
+            kind: GenericParamKind::Const { ty, kw_span: const_span },
             is_placeholder: false,
         })
     }

src/librustc_resolve/late.rs

@@ -534,8 +534,8 @@ impl<'a, 'ast> Visitor<'ast> for LateResolutionVisitor<'a, '_, 'ast> {
 
         for param in &generics.params {
             match param.kind {
-                GenericParamKind::Lifetime { .. } => self.visit_generic_param(param),
-                GenericParamKind::Type { ref default, .. } => {
+                GenericParamKind::Lifetime => self.visit_generic_param(param),
+                GenericParamKind::Type { ref default } => {
                     for bound in &param.bounds {
                         self.visit_param_bound(bound);
                     }
@@ -549,7 +549,7 @@ impl<'a, 'ast> Visitor<'ast> for LateResolutionVisitor<'a, '_, 'ast> {
                     // Allow all following defaults to refer to this type parameter.
                     default_ban_rib.bindings.remove(&Ident::with_dummy_span(param.ident.name));
                 }
-                GenericParamKind::Const { ref ty } => {
+                GenericParamKind::Const { ref ty, kw_span: _ } => {
                     for bound in &param.bounds {
                         self.visit_param_bound(bound);
                     }

src/tools/clippy/clippy_lints/src/utils/ast_utils.rs

@@ -476,7 +476,7 @@ pub fn eq_generic_param(l: &GenericParam, r: &GenericParam) -> bool {
         && match (&l.kind, &r.kind) {
             (Lifetime, Lifetime) => true,
             (Type { default: l }, Type { default: r }) => both(l, r, |l, r| eq_ty(l, r)),
-            (Const { ty: l }, Const { ty: r }) => eq_ty(l, r),
+            (Const { ty: l, kw_span: _ }, Const { ty: r, kw_span: _ }) => eq_ty(l, r),
             _ => false,
         }
         && over(&l.attrs, &r.attrs, |l, r| eq_attr(l, r))