Format macro const literals with pretty printer

compiler/rustc_metadata/src/lib.rs

@@ -7,6 +7,7 @@
 #![feature(generators)]
 #![feature(iter_from_generator)]
 #![feature(let_chains)]
+#![feature(if_let_guard)]
 #![feature(proc_macro_internals)]
 #![feature(macro_metavar_expr)]
 #![feature(min_specialization)]

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -2375,31 +2375,32 @@ pub fn rendered_const<'tcx>(tcx: TyCtxt<'tcx>, body: hir::BodyId) -> String {
         }
     }
 
-    let classification = classify(value);
-
-    if classification == Literal
-        && !value.span.from_expansion()
-        && let Ok(snippet) = tcx.sess.source_map().span_to_snippet(value.span)
-    {
-        // For literals, we avoid invoking the pretty-printer and use the source snippet instead to
-        // preserve certain stylistic choices the user likely made for the sake legibility like
+    match classify(value) {
+        // For non-macro literals, we avoid invoking the pretty-printer and use the source snippet
+        // instead to preserve certain stylistic choices the user likely made for the sake of
+        // legibility, like:
         //
         // * hexadecimal notation
         // * underscores
         // * character escapes
         //
         // FIXME: This passes through `-/*spacer*/0` verbatim.
-        snippet
-    } else if classification == Simple {
+        Literal if !value.span.from_expansion()
+            && let Ok(snippet) = tcx.sess.source_map().span_to_snippet(value.span) => {
+            snippet
+        }
+
         // Otherwise we prefer pretty-printing to get rid of extraneous whitespace, comments and
         // other formatting artifacts.
-        id_to_string(&hir, body.hir_id)
-    } else if tcx.def_kind(hir.body_owner_def_id(body).to_def_id()) == DefKind::AnonConst {
+        Literal | Simple => id_to_string(&hir, body.hir_id),
+
         // FIXME: Omit the curly braces if the enclosing expression is an array literal
         //        with a repeated element (an `ExprKind::Repeat`) as in such case it
         //        would not actually need any disambiguation.
-        "{ _ }".to_owned()
-    } else {
-        "_".to_owned()
+        Complex => if tcx.def_kind(hir.body_owner_def_id(body).to_def_id()) == DefKind::AnonConst {
+            "{ _ }".to_owned()
+        } else {
+            "_".to_owned()
+        }
     }
 }

tests/rustdoc/issue-115295-macro-const-display.rs

@@ -0,0 +1,40 @@
+#![crate_name = "foo"]
+
+// @has foo/trait.Trait.html
+pub trait Trait<T> {}
+
+// @has foo/struct.WithConst.html
+pub struct WithConst<const N: usize>;
+
+macro_rules! spans_from_macro {
+    () => {
+        impl WithConst<42> {
+            pub fn new() -> Self {
+                Self
+            }
+        }
+        impl Trait<WithConst<42>> for WithConst<42> {}
+        impl Trait<WithConst<43>> for WithConst<{ 43 }> {}
+        impl Trait<WithConst<{ 44 }>> for WithConst<44> {}
+        pub struct Other {
+            pub field: WithConst<42>,
+        }
+    };
+}
+
+// @has - '//*[@class="impl"]//h3[@class="code-header"]' \
+//     "impl Trait<WithConst<41>> for WithConst<41>"
+impl Trait<WithConst<41>> for WithConst<41> {}
+
+// @has - '//*[@class="impl"]//h3[@class="code-header"]' \
+//     "impl WithConst<42>"
+// @has - '//*[@class="impl"]//h3[@class="code-header"]' \
+//     "impl Trait<WithConst<42>> for WithConst<42>"
+// @has - '//*[@class="impl"]//h3[@class="code-header"]' \
+//     "impl Trait<WithConst<43>> for WithConst<{ 43 }>"
+// @has - '//*[@class="impl"]//h3[@class="code-header"]' \
+//     "impl Trait<WithConst<44>> for WithConst<44>"
+
+// @has foo/struct.Other.html
+// @has - //pre "pub field: WithConst<42>"
+spans_from_macro!();