rustdoc-search: simplify rules for generics and type params

This commit is a response to feedback on the displayed type
signatures results, by making generics act stricter.

Generics are tightened by making order significant. This means
`Vec<Allocator>` now matches only with a true vector of allocators,
instead of matching the second type param. It also makes unboxing
within generics stricter, so `Result<A, B>` only matches if `B`
is in the error type and `A` is in the success type. The top level
of the function search is unaffected.

Find the discussion on:

* <https://rust-lang.zulipchat.com/#narrow/stream/393423-t-rustdoc.2Fmeetings/topic/meeting.202024-07-08/near/449965149>
* <https://github.com/rust-lang/rust/pull/124544#issuecomment-2204272265>
* <https://rust-lang.zulipchat.com/#narrow/channel/266220-t-rustdoc/topic/deciding.20on.20semantics.20of.20generics.20in.20rustdoc.20search/near/476841363>

1 files changed, 68 insertions, 0 deletions

diff --git a/tests/rustdoc-js/generics-match-ambiguity-no-unbox.js b/tests/rustdoc-js/generics-match-ambiguity-no-unbox.js
new file mode 100644
index 00000000000..ea4c26d311c
--- /dev/null
+++ b/tests/rustdoc-js/generics-match-ambiguity-no-unbox.js
@@ -0,0 +1,68 @@
+// ignore-order
+// exact-check
+
+// Make sure that results are order-agnostic, even when there's search items that only differ
+// by generics.
+
+const EXPECTED = [
+    {
+        'query': 'Wrap',
+        'in_args': [
+            { 'path': 'generics_match_ambiguity', 'name': 'bar' },
+            { 'path': 'generics_match_ambiguity', 'name': 'foo' },
+        ],
+    },
+    {
+        'query': 'Wrap<i32>',
+        'in_args': [
+            { 'path': 'generics_match_ambiguity', 'name': 'bar' },
+            { 'path': 'generics_match_ambiguity', 'name': 'foo' },
+        ],
+    },
+    {
+        'query': 'Wrap<i32>, Wrap<i32, u32>',
+        'others': [
+            { 'path': 'generics_match_ambiguity', 'name': 'bar' },
+            { 'path': 'generics_match_ambiguity', 'name': 'foo' },
+        ],
+    },
+    {
+        'query': 'Wrap<i32, u32>, Wrap<i32>',
+        'others': [
+            { 'path': 'generics_match_ambiguity', 'name': 'bar' },
+            { 'path': 'generics_match_ambiguity', 'name': 'foo' },
+        ],
+    },
+    {
+        'query': 'W3<i32>, W3<i32, u32>',
+        'others': [
+            { 'path': 'generics_match_ambiguity', 'name': 'baaa' },
+            { 'path': 'generics_match_ambiguity', 'name': 'baab' },
+        ],
+    },
+    {
+        'query': 'W3<i32, u32>, W3<i32>',
+        'others': [
+            { 'path': 'generics_match_ambiguity', 'name': 'baaa' },
+            { 'path': 'generics_match_ambiguity', 'name': 'baab' },
+        ],
+    },
+    {
+        // strict generics matching; W2<i32, u32> doesn't match W2<W3<i32, u32>>,
+        // even though W2<i32> works just fine (ignoring the W3)
+        'query': 'W2<i32>, W2<i32, u32>',
+        'others': [],
+    },
+    {
+        'query': 'W2<i32, u32>, W2<i32>',
+        'others': [],
+    },
+    {
+        'query': 'W2<i32>, W3<i32, u32>',
+        'others': [],
+    },
+    {
+        'query': 'W2<i32>, W2<i32>',
+        'others': [],
+    },
+];