rustdoc: fix inserting source code spans for constant values

This will go wrong when the constants partially result from macro expansion.
Instead, use the expressions and pretty-print them as Rust code.

Fixes: #33302

src/test/rustdoc/issue-33302.rs

@@ -0,0 +1,46 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Ensure constant and array length values are not taken from source
+// code, which wreaks havoc with macros.
+
+#![feature(associated_consts)]
+
+macro_rules! make {
+    ($n:expr) => {
+        pub struct S;
+
+        // @has issue_33302/constant.CST.html \
+        //        '//pre[@class="rust const"]' 'pub const CST: i32 = 4 * 4'
+        pub const CST: i32 = ($n * $n);
+        // @has issue_33302/static.ST.html \
+        //        '//pre[@class="rust static"]' 'pub static ST: i32 = 4 * 4'
+        pub static ST: i32 = ($n * $n);
+
+        pub trait T<X> {
+            fn ignore(_: &X) {}
+            const C: X;
+            // @has issue_33302/trait.T.html \
+            //        '//*[@class="rust trait"]' 'const D: i32 = 4 * 4;'
+            // @has - '//*[@id="associatedconstant.D"]' 'const D: i32 = 4 * 4'
+            const D: i32 = ($n * $n);
+        }
+
+        // @has issue_33302/struct.S.html \
+        //        '//h3[@class="impl"]' 'impl T<[i32; 4 * 4]> for S'
+        // @has - '//*[@id="associatedconstant.C"]' 'const C: [i32; 4 * 4] = [0; 4 * 4]'
+        // @has - '//*[@id="associatedconstant.D"]' 'const D: i32 = 4 * 4'
+        impl T<[i32; ($n * $n)]> for S {
+            const C: [i32; ($n * $n)] = [0; ($n * $n)];
+        }
+    }
+}
+
+make!(4);