Rollup merge of #55630 - petrochenkov:noprelude, r=Centril

resolve: Filter away macro prelude in modules with `#[no_implicit_prelude]` on 2018 edition

This is a tiny thing.
For historical reasons macro prelude (macros from `#[macro_use] extern crate ...`, including `extern crate std`) is still available in modules with `#[no_implicit_prelude]`.
This PR provides proper isolation and removes those names from scope.

`#[no_implicit_prelude]` modules still have built-in types (`u8`), built-in attributes (`#[inline]`) and built-in macros (`env!("PATH")`) in scope. We can introduce some `#[no_implicit_prelude_at_all]` to remove those as well, but that's a separate issue.

The change is done only on 2018 edition for backward compatibility.
I'm pretty sure this can be done on 2015 as well because `#[no_implicit_prelude]` is rarely used, but I don't want to go through the crater/deprecation process right now, maybe later.

cc https://github.com/rust-lang/rust/issues/53977
r? @ghost

src/librustc/session/mod.rs

@@ -963,6 +963,10 @@ impl Session {
         self.opts.debugging_opts.teach && self.diagnostic().must_teach(code)
     }
 
+    pub fn rust_2015(&self) -> bool {
+        self.opts.edition == Edition::Edition2015
+    }
+
     /// Are we allowed to use features from the Rust 2018 edition?
     pub fn rust_2018(&self) -> bool {
         self.opts.edition >= Edition::Edition2018

src/librustc_resolve/macros.rs

@@ -663,10 +663,13 @@ impl<'a, 'cl> Resolver<'a, 'cl> {
                     binding.map(|binding| (binding, Flags::MODULE, Flags::empty()))
                 }
                 WhereToResolve::MacroUsePrelude => {
-                    match self.macro_use_prelude.get(&ident.name).cloned() {
-                        Some(binding) => Ok((binding, Flags::PRELUDE, Flags::empty())),
-                        None => Err(Determinacy::Determined),
+                    let mut result = Err(Determinacy::Determined);
+                    if use_prelude || self.session.rust_2015() {
+                        if let Some(binding) = self.macro_use_prelude.get(&ident.name).cloned() {
+                            result = Ok((binding, Flags::PRELUDE, Flags::empty()));
+                        }
                     }
+                    result
                 }
                 WhereToResolve::BuiltinMacros => {
                     match self.builtin_macros.get(&ident.name).cloned() {
@@ -685,7 +688,8 @@ impl<'a, 'cl> Resolver<'a, 'cl> {
                     }
                 }
                 WhereToResolve::LegacyPluginHelpers => {
-                    if self.session.plugin_attributes.borrow().iter()
+                    if (use_prelude || self.session.rust_2015()) &&
+                       self.session.plugin_attributes.borrow().iter()
                                                      .any(|(name, _)| ident.name == &**name) {
                         let binding = (Def::NonMacroAttr(NonMacroAttrKind::LegacyPluginHelper),
                                        ty::Visibility::Public, ident.span, Mark::root())

src/test/ui/hygiene/no_implicit_prelude-2018.rs

@@ -0,0 +1,11 @@
+// edition:2018
+
+#[no_implicit_prelude]
+mod bar {
+    fn f() {
+        ::std::print!(""); // OK
+        print!(); //~ ERROR cannot find macro `print!` in this scope
+    }
+}
+
+fn main() {}

src/test/ui/hygiene/no_implicit_prelude-2018.stderr

@@ -0,0 +1,10 @@
+error: cannot find macro `print!` in this scope
+  --> $DIR/no_implicit_prelude-2018.rs:7:9
+   |
+LL |         print!(); //~ ERROR cannot find macro `print!` in this scope
+   |         ^^^^^
+   |
+   = help: have you added the `#[macro_use]` on the module/import?
+
+error: aborting due to previous error
+

src/test/ui/hygiene/no_implicit_prelude.rs

@@ -21,7 +21,10 @@ mod bar {
         Vec::new(); //~ ERROR failed to resolve
         ().clone() //~ ERROR no method named `clone` found
     }
-    fn f() { ::foo::m!(); }
+    fn f() {
+        ::foo::m!();
+        println!(); // OK on 2015 edition (at least for now)
+    }
 }
 
 fn main() {}