rustc_resolve: Test the order that preludes are resolved

tests/ui/resolve/auxiliary/macro_helpers.rs

@@ -0,0 +1,16 @@
+/* macro namespace. */
+
+extern crate proc_macro;
+use proc_macro::*;
+use std::str::FromStr;
+
+const ERROR: &str = "fn helper() { \"helper\" }";
+// https://doc.rust-lang.org/nightly/std/prelude/v1/index.html#attributes
+// NOTE: all the bang macros in std are currently unstable.
+#[proc_macro_attribute] pub fn test       // lang.
+    (_: TokenStream, _: TokenStream) -> TokenStream {
+        TokenStream::from_str("fn test_macro() { \"\" }").unwrap() }
+// https://doc.rust-lang.org/nightly/reference/attributes.html#built-in-attributes-index
+#[proc_macro_attribute] pub fn global_allocator // lang.
+    (_: TokenStream, _: TokenStream) -> TokenStream {
+        TokenStream::from_str("fn global_allocator_macro() { \"\" }").unwrap() }

tests/ui/resolve/prelude-order.rs

@@ -0,0 +1,89 @@
+//@ proc-macro:macro_helpers.rs
+//@ compile-flags: --crate-type=lib
+
+/* There are 5 preludes and 3 namespaces. Test the order in which they are resolved.
+ * See https://doc.rust-lang.org/nightly/reference/names/preludes.html.
+ *
+ * Macros cannot be in the type or value namespace.
+ * Tools and extern crates cannot be in the macro or value namespace.
+ *
+ * Test the following truth tables:
+
+Type:
+| ...... |  tool  | extern | macro  | lang   | libs |
+|   tool |    N/A |                   mirror
+| extern | extern |    N/A |             universe
+|  macro |    N/A |    N/A |    N/A |
+|   lang |   tool | extern |    N/A |   N/A  |
+|   libs |   tool | extern |    N/A |   X    |  N/A |
+
+Macro:
+| ...... |  tool  | extern | macro  | lang   | libs |
+|   tool |    N/A |                   mirror
+| extern |    N/A |    N/A |             universe
+|  macro |    N/A |    N/A |    N/A |
+|   lang |    N/A |    N/A |  macro |   N/A  |
+|   libs |    N/A |    N/A |  macro |   X    | N/A  |
+
+Value: N/A. Only libs has items in the value namespace.
+
+† ambiguous
+X don't care (controlled namespace with no overlap)
+
+* Types are tested with `#[name::inner]`. Macros are tested with `#[name]`.
+* WARNING: I have found in testing that attribute macros give ambiguity errors in some contexts
+* instead of choosing a prelude. Have not been able to replicate.
+*
+* There should be 7 total tests.
+* See `rustc_resolve::ident::visit_scopes` for more information,
+* and for a definition of "controlled namespace".
+*/
+
+#![feature(register_tool)]
+
+/* tool prelude */
+#![register_tool(type_ns)] // extern prelude. type.
+#![register_tool(i8)]      // lang   prelude. type.
+#![register_tool(Sync)]    // libs   prelude. type.
+
+/* extern prelude */
+extern crate macro_helpers as type_ns; // tool prelude. type.
+extern crate macro_helpers as usize;   // lang prelude. type.
+extern crate macro_helpers as Option;  // libs prelude. type.
+
+/* macro_use prelude */
+#[macro_use]
+extern crate macro_helpers as _;
+
+/* lang and libs implicitly in scope */
+
+// tool/extern -> extern
+#[type_ns::inner] //~ ERROR could not find `inner` in `type_ns`
+fn t1() {}
+
+// tool/lang -> tool
+#[i8::inner] // ok
+fn t2() {}
+
+// tool/libs -> tool
+#[Sync::not_real] // ok
+fn t3() {}
+
+// extern/lang -> extern
+#[usize::inner] //~ ERROR could not find `inner` in `usize`
+fn e1() {} // NOTE: testing with `-> usize` isn't valid, crates aren't considered in that scope
+           // (unless they have generic arguments, for some reason.)
+
+// extern/libs -> extern
+// https://github.com/rust-lang/rust/issues/139095
+fn e2() -> Option<i32> { None } //~ ERROR: expected type, found crate
+
+// macro/libs -> macro
+#[test] //~ ERROR mismatched types
+fn m1() {}
+
+// macro/lang -> macro
+#[global_allocator] //~ ERROR mismatched types
+fn m2() {}
+
+// lang/libs: no items that currently overlap, in either macro or type ns.

tests/ui/resolve/prelude-order.stderr

@@ -0,0 +1,47 @@
+error[E0433]: failed to resolve: could not find `inner` in `type_ns`
+  --> $DIR/prelude-order.rs:61:12
+   |
+LL | #[type_ns::inner]
+   |            ^^^^^ could not find `inner` in `type_ns`
+
+error[E0433]: failed to resolve: could not find `inner` in `usize`
+  --> $DIR/prelude-order.rs:73:10
+   |
+LL | #[usize::inner]
+   |          ^^^^^ could not find `inner` in `usize`
+
+error[E0573]: expected type, found crate `Option`
+  --> $DIR/prelude-order.rs:79:12
+   |
+LL | fn e2() -> Option<i32> { None }
+   |            ^^^^^^^^^^^ not a type
+   |
+help: consider importing this enum instead
+   |
+LL + use std::option::Option;
+   |
+
+error[E0308]: mismatched types
+  --> $DIR/prelude-order.rs:82:1
+   |
+LL | #[test]
+   | ^^^^^^^- help: try adding a return type: `-> &'static str`
+   | |
+   | expected `()`, found `&str`
+   |
+   = note: this error originates in the attribute macro `test` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0308]: mismatched types
+  --> $DIR/prelude-order.rs:86:1
+   |
+LL | #[global_allocator]
+   | ^^^^^^^^^^^^^^^^^^^- help: try adding a return type: `-> &'static str`
+   | |
+   | expected `()`, found `&str`
+   |
+   = note: this error originates in the attribute macro `global_allocator` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 5 previous errors
+
+Some errors have detailed explanations: E0308, E0433, E0573.
+For more information about an error, try `rustc --explain E0308`.