Merge #9097

9097: feat: Implement per-edition preludes r=jonas-schievink a=jonas-schievink

Part of #9056

Our previous implementation was incorrect (presumably because of the misleading comment in libstd [here](https://github.com/rust-lang/rust/blob/a7890c7952bdc9445eb6c63dc671fa7a1ab0260d/library/std/src/lib.rs#L339-L343)). `#[prelude_import]` does not define the prelude, it can only override the implicit prelude for the current crate.

This PR fixes that, which also makes the prelude imports in `rustc_span` work. Closes #8815.

bors r+

Co-authored-by: Jonas Schievink <jonasschievink@gmail.com>

crates/hir_def/src/find_path.rs

@@ -682,9 +682,11 @@ pub struct S;
 //- /main.rs crate:main deps:std
 $0
 //- /std.rs crate:std
-pub mod prelude { pub struct S; }
-#[prelude_import]
-pub use prelude::*;
+pub mod prelude {
+    pub mod rust_2018 {
+        pub struct S;
+    }
+}
         "#,
             "S",
             "S",
@@ -700,11 +702,11 @@ pub use prelude::*;
 $0
 //- /std.rs crate:std
 pub mod prelude {
-    pub enum Option<T> { Some(T), None }
-    pub use Option::*;
+    pub mod rust_2018 {
+        pub enum Option<T> { Some(T), None }
+        pub use Option::*;
+    }
 }
-#[prelude_import]
-pub use prelude::*;
         "#;
         check_found_path(code, "None", "None", "None", "None");
         check_found_path(code, "Some", "Some", "Some", "Some");
@@ -1080,11 +1082,11 @@ fn f() {
 }
 //- /std.rs crate:std
 pub mod prelude {
-    pub enum Option { None }
-    pub use Option::*;
+    pub mod rust_2018 {
+        pub enum Option { None }
+        pub use Option::*;
+    }
 }
-#[prelude_import]
-pub use prelude::*;
         "#,
             "None",
             "None",

crates/hir_def/src/nameres/collector.rs

@@ -5,13 +5,13 @@
 
 use std::iter;
 
-use base_db::{CrateId, FileId, ProcMacroId};
+use base_db::{CrateId, Edition, FileId, ProcMacroId};
 use cfg::{CfgExpr, CfgOptions};
 use hir_expand::{
     ast_id_map::FileAstId,
     builtin_derive::find_builtin_derive,
     builtin_macro::find_builtin_macro,
-    name::{AsName, Name},
+    name::{name, AsName, Name},
     proc_macro::ProcMacroExpander,
     FragmentKind, HirFileId, MacroCallId, MacroCallKind, MacroDefId, MacroDefKind,
 };
@@ -67,14 +67,6 @@ pub(super) fn collect_defs(
             def_map
                 .extern_prelude
                 .insert(dep.as_name(), dep_def_map.module_id(dep_def_map.root).into());
-
-            // look for the prelude
-            // If the dependency defines a prelude, we overwrite an already defined
-            // prelude. This is necessary to import the "std" prelude if a crate
-            // depends on both "core" and "std".
-            if dep_def_map.prelude.is_some() {
-                def_map.prelude = dep_def_map.prelude;
-            }
         }
     }
 
@@ -283,6 +275,8 @@ impl DefCollector<'_> {
 
         let attrs = item_tree.top_level_attrs(self.db, self.def_map.krate);
         if attrs.cfg().map_or(true, |cfg| self.cfg_options.check(&cfg) != Some(false)) {
+            self.inject_prelude(&attrs);
+
             // Process other crate-level attributes.
             for attr in &*attrs {
                 let attr_name = match attr.path.as_ident() {
@@ -460,6 +454,58 @@ impl DefCollector<'_> {
         }
     }
 
+    fn inject_prelude(&mut self, crate_attrs: &Attrs) {
+        // See compiler/rustc_builtin_macros/src/standard_library_imports.rs
+
+        if crate_attrs.by_key("no_core").exists() {
+            // libcore does not get a prelude.
+            return;
+        }
+
+        let krate = if crate_attrs.by_key("no_std").exists() {
+            name![core]
+        } else {
+            let std = name![std];
+            if self.def_map.extern_prelude().any(|(name, _)| *name == std) {
+                std
+            } else {
+                // If `std` does not exist for some reason, fall back to core. This mostly helps
+                // keep r-a's own tests minimal.
+                name![core]
+            }
+        };
+
+        let edition = match self.def_map.edition {
+            Edition::Edition2015 => name![rust_2015],
+            Edition::Edition2018 => name![rust_2018],
+            Edition::Edition2021 => name![rust_2021],
+        };
+
+        let path_kind = if self.def_map.edition == Edition::Edition2015 {
+            PathKind::Plain
+        } else {
+            PathKind::Abs
+        };
+        let path =
+            ModPath::from_segments(path_kind, [krate, name![prelude], edition].iter().cloned());
+
+        let (per_ns, _) =
+            self.def_map.resolve_path(self.db, self.def_map.root, &path, BuiltinShadowMode::Other);
+
+        match &per_ns.types {
+            Some((ModuleDefId::ModuleId(m), _)) => {
+                self.def_map.prelude = Some(*m);
+            }
+            _ => {
+                log::error!(
+                    "could not resolve prelude path `{}` to module (resolved to {:?})",
+                    path,
+                    per_ns.types
+                );
+            }
+        }
+    }
+
     /// Adds a definition of procedural macro `name` to the root module.
     ///
     /// # Notes on procedural macro resolution
@@ -718,6 +764,8 @@ impl DefCollector<'_> {
             match def.take_types() {
                 Some(ModuleDefId::ModuleId(m)) => {
                     if import.is_prelude {
+                        // Note: This dodgily overrides the injected prelude. The rustc
+                        // implementation seems to work the same though.
                         cov_mark::hit!(std_prelude);
                         self.def_map.prelude = Some(m);
                     } else if m.krate != self.def_map.krate {

crates/hir_def/src/nameres/tests.rs

@@ -246,15 +246,16 @@ fn std_prelude() {
     check(
         r#"
 //- /main.rs crate:main deps:test_crate
+#[prelude_import]
+use ::test_crate::prelude::*;
+
 use Foo::*;
 
 //- /lib.rs crate:test_crate
-mod prelude;
-#[prelude_import]
-use prelude::*;
+pub mod prelude;
 
 //- /prelude.rs
-pub enum Foo { Bar, Baz };
+pub enum Foo { Bar, Baz }
 "#,
         expect![[r#"
             crate
@@ -466,6 +467,74 @@ pub struct Bar;
     );
 }
 
+#[test]
+fn no_std_prelude() {
+    check(
+        r#"
+        //- /main.rs crate:main deps:core,std
+        #![cfg_attr(not(never), no_std)]
+        use Rust;
+
+        //- /core.rs crate:core
+        pub mod prelude {
+            pud mod rust_2018 {
+                pub struct Rust;
+            }
+        }
+        //- /std.rs crate:std deps:core
+        pub mod prelude {
+            pud mod rust_2018 {
+            }
+        }
+    "#,
+        expect![[r#"
+        crate
+        Rust: t v
+    "#]],
+    );
+}
+
+#[test]
+fn edition_specific_preludes() {
+    // We can't test the 2015 prelude here since you can't reexport its contents with 2015's
+    // absolute paths.
+
+    check(
+        r#"
+        //- /main.rs edition:2018 crate:main deps:std
+        use Rust2018;
+
+        //- /std.rs crate:std
+        pub mod prelude {
+            pud mod rust_2018 {
+                pub struct Rust2018;
+            }
+        }
+    "#,
+        expect![[r#"
+        crate
+        Rust2018: t v
+    "#]],
+    );
+    check(
+        r#"
+        //- /main.rs edition:2021 crate:main deps:std
+        use Rust2021;
+
+        //- /std.rs crate:std
+        pub mod prelude {
+            pud mod rust_2021 {
+                pub struct Rust2021;
+            }
+        }
+    "#,
+        expect![[r#"
+        crate
+        Rust2021: t v
+    "#]],
+    );
+}
+
 #[test]
 fn std_prelude_takes_precedence_above_core_prelude() {
     check(
@@ -474,18 +543,18 @@ fn std_prelude_takes_precedence_above_core_prelude() {
 use {Foo, Bar};
 
 //- /std.rs crate:std deps:core
-#[prelude_import]
-pub use self::prelude::*;
-mod prelude {
-    pub struct Foo;
-    pub use core::prelude::Bar;
+pub mod prelude {
+    pub mod rust_2018 {
+        pub struct Foo;
+        pub use core::prelude::rust_2018::Bar;
+    }
 }
 
 //- /core.rs crate:core
-#[prelude_import]
-pub use self::prelude::*;
-mod prelude {
-    pub struct Bar;
+pub mod prelude {
+    pub mod rust_2018 {
+        pub struct Bar;
+    }
 }
 "#,
         expect![[r#"
@@ -504,15 +573,15 @@ fn cfg_not_test() {
 use {Foo, Bar, Baz};
 
 //- /lib.rs crate:std
-#[prelude_import]
-pub use self::prelude::*;
-mod prelude {
-    #[cfg(test)]
-    pub struct Foo;
-    #[cfg(not(test))]
-    pub struct Bar;
-    #[cfg(all(not(any()), feature = "foo", feature = "bar", opt = "42"))]
-    pub struct Baz;
+pub mod prelude {
+    pub mod rust_2018 {
+        #[cfg(test)]
+        pub struct Foo;
+        #[cfg(not(test))]
+        pub struct Bar;
+        #[cfg(all(not(any()), feature = "foo", feature = "bar", opt = "42"))]
+        pub struct Baz;
+    }
 }
 "#,
         expect![[r#"
@@ -532,15 +601,15 @@ fn cfg_test() {
 use {Foo, Bar, Baz};
 
 //- /lib.rs crate:std cfg:test,feature=foo,feature=bar,opt=42
-#[prelude_import]
-pub use self::prelude::*;
-mod prelude {
-    #[cfg(test)]
-    pub struct Foo;
-    #[cfg(not(test))]
-    pub struct Bar;
-    #[cfg(all(not(any()), feature = "foo", feature = "bar", opt = "42"))]
-    pub struct Baz;
+pub mod prelude {
+    pub mod rust_2018 {
+        #[cfg(test)]
+        pub struct Foo;
+        #[cfg(not(test))]
+        pub struct Bar;
+        #[cfg(all(not(any()), feature = "foo", feature = "bar", opt = "42"))]
+        pub struct Baz;
+    }
 }
 "#,
         expect![[r#"

crates/hir_def/src/nameres/tests/macros.rs

@@ -264,7 +264,7 @@ fn prelude_is_macro_use() {
     cov_mark::check!(prelude_is_macro_use);
     check(
         r#"
-//- /main.rs crate:main deps:foo
+//- /main.rs crate:main deps:std
 structs!(Foo);
 structs_priv!(Bar);
 structs_outside!(Out);
@@ -276,21 +276,20 @@ mod bar;
 structs!(Baz);
 crate::structs!(MacroNotResolved3);
 
-//- /lib.rs crate:foo
-#[prelude_import]
-use self::prelude::*;
-
-mod prelude {
-    #[macro_export]
-    macro_rules! structs {
-        ($i:ident) => { struct $i; }
-    }
-
-    mod priv_mod {
+//- /lib.rs crate:std
+pub mod prelude {
+    pub mod rust_2018 {
         #[macro_export]
-        macro_rules! structs_priv {
+        macro_rules! structs {
             ($i:ident) => { struct $i; }
         }
+
+        mod priv_mod {
+            #[macro_export]
+            macro_rules! structs_priv {
+                ($i:ident) => { struct $i; }
+            }
+        }
     }
 }
 
@@ -617,12 +616,11 @@ fn macro_dollar_crate_is_correct_in_indirect_deps() {
 foo!();
 
 //- /std.rs crate:std deps:core
-#[prelude_import]
-use self::prelude::*;
-
 pub use core::foo;
 
-mod prelude {}
+pub mod prelude {
+    pub mod rust_2018 {}
+}
 
 #[macro_use]
 mod std_macros;

crates/hir_expand/src/name.rs

@@ -176,6 +176,10 @@ pub mod known {
         result,
         boxed,
         option,
+        prelude,
+        rust_2015,
+        rust_2018,
+        rust_2021,
         // Components of known path (type name)
         Iterator,
         IntoIterator,