Auto merge of #84217 - crlf0710:remove_main_attr_pure, r=petrochenkov

Remove #[main] attribute.

This removes the #[main] attribute support from the compiler according to the decisions within #29634. For existing use cases within test harness generation, replaced it with a newly-introduced internal attribute `#[rustc_main]`.

This is first part extracted from #84062 .

Closes #29634.

r? `@petrochenkov`

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -366,16 +366,6 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                          over time"
                     );
                 }
-                if self.sess.contains_name(&i.attrs[..], sym::main) {
-                    gate_feature_post!(
-                        &self,
-                        main,
-                        i.span,
-                        "declaration of a non-standard `#[main]` \
-                         function may change over time, for now \
-                         a top-level `fn main()` is required"
-                    );
-                }
             }
 
             ast::ItemKind::Struct(..) => {

compiler/rustc_builtin_macros/src/test_harness.rs

@@ -142,7 +142,7 @@ fn entry_point_type(sess: &Session, item: &ast::Item, depth: usize) -> EntryPoin
         ast::ItemKind::Fn(..) => {
             if sess.contains_name(&item.attrs, sym::start) {
                 EntryPointType::Start
-            } else if sess.contains_name(&item.attrs, sym::main) {
+            } else if sess.contains_name(&item.attrs, sym::rustc_main) {
                 EntryPointType::MainAttr
             } else if item.ident.name == sym::main {
                 if depth == 1 {
@@ -187,7 +187,7 @@ impl<'a> MutVisitor for EntryPointCleaner<'a> {
                     let attrs = attrs
                         .into_iter()
                         .filter(|attr| {
-                            !self.sess.check_name(attr, sym::main)
+                            !self.sess.check_name(attr, sym::rustc_main)
                                 && !self.sess.check_name(attr, sym::start)
                         })
                         .chain(iter::once(allow_dead_code))
@@ -220,7 +220,7 @@ fn generate_test_harness(
     let expn_id = ext_cx.resolver.expansion_for_ast_pass(
         DUMMY_SP,
         AstPass::TestHarness,
-        &[sym::main, sym::test, sym::rustc_attrs],
+        &[sym::test, sym::rustc_attrs],
         None,
     );
     let def_site = DUMMY_SP.with_def_site_ctxt(expn_id);
@@ -247,7 +247,7 @@ fn generate_test_harness(
 /// By default this expands to
 ///
 /// ```
-/// #[main]
+/// #[rustc_main]
 /// pub fn main() {
 ///     extern crate test;
 ///     test::test_main_static(&[
@@ -297,8 +297,8 @@ fn mk_main(cx: &mut TestCtxt<'_>) -> P<ast::Item> {
     let test_extern_stmt =
         ecx.stmt_item(sp, ecx.item(sp, test_id, vec![], ast::ItemKind::ExternCrate(None)));
 
-    // #[main]
-    let main_meta = ecx.meta_word(sp, sym::main);
+    // #[rustc_main]
+    let main_meta = ecx.meta_word(sp, sym::rustc_main);
     let main_attr = ecx.attribute(main_meta);
 
     // pub fn main() { ... }

compiler/rustc_error_codes/src/error_codes/E0137.md

@@ -1,8 +1,10 @@
+#### Note: this error code is no longer emitted by the compiler.
+
 More than one function was declared with the `#[main]` attribute.
 
 Erroneous code example:
 
-```compile_fail,E0137
+```compile_fail
 #![feature(main)]
 
 #[main]
@@ -16,7 +18,7 @@ This error indicates that the compiler found multiple functions with the
 `#[main]` attribute. This is an error because there must be a unique entry
 point into a Rust program. Example:
 
-```
+```compile_fail
 #![feature(main)]
 
 #[main]

compiler/rustc_feature/src/active.rs

@@ -134,9 +134,6 @@ declare_features! (
     /// Allows using the `box $expr` syntax.
     (active, box_syntax, "1.0.0", Some(49733), None),
 
-    /// Allows using `#[main]` to replace the entrypoint `#[lang = "start"]` calls.
-    (active, main, "1.0.0", Some(29634), None),
-
     /// Allows using `#[start]` on a function indicating that it is the program entrypoint.
     (active, start, "1.0.0", Some(29633), None),
 

compiler/rustc_feature/src/builtin_attrs.rs

@@ -536,6 +536,10 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         rustc_specialization_trait, Normal, template!(Word),
         "the `#[rustc_specialization_trait]` attribute is used to check specializations"
     ),
+    rustc_attr!(
+        rustc_main, Normal, template!(Word),
+        "the `#[rustc_main]` attribute is used internally to specify test entry point function",
+    ),
 
     // ==========================================================================
     // Internal attributes, Testing:

compiler/rustc_feature/src/removed.rs

@@ -132,6 +132,8 @@ declare_features! (
     (removed, link_args, "1.53.0", Some(29596), None,
      Some("removed in favor of using `-C link-arg=ARG` on command line, \
            which is available from cargo build scripts with `cargo:rustc-link-arg` now")),
+    /// Allows using `#[main]` to replace the entrypoint `#[lang = "start"]` calls.
+    (removed, main, "1.53.0", Some(29634), None, None),
 
     // -------------------------------------------------------------------------
     // feature-group-end: removed features

compiler/rustc_passes/src/check_attr.rs

@@ -1489,7 +1489,7 @@ fn check_invalid_crate_level_attr(tcx: TyCtxt<'_>, attrs: &[Attribute]) {
         sym::path,
         sym::automatically_derived,
         sym::start,
-        sym::main,
+        sym::rustc_main,
     ];
 
     for attr in attrs {

compiler/rustc_passes/src/entry.rs

@@ -84,7 +84,7 @@ fn entry_point_type(ctxt: &EntryContext<'_, '_>, item: &Item<'_>, at_root: bool)
     let attrs = ctxt.map.attrs(item.hir_id());
     if ctxt.session.contains_name(attrs, sym::start) {
         EntryPointType::Start
-    } else if ctxt.session.contains_name(attrs, sym::main) {
+    } else if ctxt.session.contains_name(attrs, sym::rustc_main) {
         EntryPointType::MainAttr
     } else if item.ident.name == sym::main {
         if at_root {
@@ -111,8 +111,8 @@ fn find_item(item: &Item<'_>, ctxt: &mut EntryContext<'_, '_>, at_root: bool) {
             if let Some(attr) = ctxt.session.find_by_name(attrs, sym::start) {
                 throw_attr_err(&ctxt.session, attr.span, "start");
             }
-            if let Some(attr) = ctxt.session.find_by_name(attrs, sym::main) {
-                throw_attr_err(&ctxt.session, attr.span, "main");
+            if let Some(attr) = ctxt.session.find_by_name(attrs, sym::rustc_main) {
+                throw_attr_err(&ctxt.session, attr.span, "rustc_main");
             }
         }
         EntryPointType::MainNamed => {
@@ -193,10 +193,7 @@ fn no_main_err(tcx: TyCtxt<'_>, visitor: &EntryContext<'_, '_>) {
             err.span_note(span, "here is a function named `main`");
         }
         err.note("you have one or more functions named `main` not defined at the crate level");
-        err.help(
-            "either move the `main` function definitions or attach the `#[main]` attribute \
-                  to one of them",
-        );
+        err.help("consider moving the `main` function definitions");
         // There were some functions named `main` though. Try to give the user a hint.
         format!(
             "the main function must be defined at the crate level{}",

compiler/rustc_span/src/symbol.rs

@@ -1008,6 +1008,7 @@ symbols! {
         rustc_layout_scalar_valid_range_start,
         rustc_legacy_const_generics,
         rustc_macro_transparency,
+        rustc_main,
         rustc_mir,
         rustc_nonnull_optimization_guaranteed,
         rustc_object_lifetime_default,

src/test/ui/feature-gates/issue-43106-gating-of-builtin-attrs-error.rs

@@ -11,8 +11,8 @@
 
 #![macro_export]
 //~^ ERROR: `macro_export` attribute cannot be used at crate level
-#![main]
-//~^ ERROR: `main` attribute cannot be used at crate level
+#![rustc_main] //~ ERROR: the `#[rustc_main]` attribute is used internally to specify
+//~^ ERROR: `rustc_main` attribute cannot be used at crate level
 #![start]
 //~^ ERROR: `start` attribute cannot be used at crate level
 #![repr()]
@@ -106,24 +106,6 @@ mod export_name {
     //~| NOTE not a function or static
 }
 
-#[main]
-//~^ ERROR: `main` attribute can only be used on functions
-mod main {
-    mod inner { #![main] }
-    //~^ ERROR: `main` attribute can only be used on functions
-
-    // for `fn f()` case, see feature-gate-main.rs
-
-    #[main] struct S;
-    //~^ ERROR: `main` attribute can only be used on functions
-
-    #[main] type T = S;
-    //~^ ERROR: `main` attribute can only be used on functions
-
-    #[main] impl S { }
-    //~^ ERROR: `main` attribute can only be used on functions
-}
-
 #[start]
 //~^ ERROR: `start` attribute can only be used on functions
 mod start {

src/test/ui/feature-gates/issue-43106-gating-of-builtin-attrs-error.stderr

@@ -1,3 +1,11 @@
+error[E0658]: the `#[rustc_main]` attribute is used internally to specify test entry point function
+  --> $DIR/issue-43106-gating-of-builtin-attrs-error.rs:14:1
+   |
+LL | #![rustc_main]
+   | ^^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(rustc_attrs)]` to the crate attributes to enable
+
 error: attribute must be of the form `#[inline]` or `#[inline(always|never)]`
   --> $DIR/issue-43106-gating-of-builtin-attrs-error.rs:40:5
    |
@@ -8,62 +16,32 @@ LL |     #[inline = "2100"] fn f() { }
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #57571 <https://github.com/rust-lang/rust/issues/57571>
 
-error: `main` attribute can only be used on functions
-  --> $DIR/issue-43106-gating-of-builtin-attrs-error.rs:109:1
-   |
-LL | #[main]
-   | ^^^^^^^
-
-error: `main` attribute can only be used on functions
-  --> $DIR/issue-43106-gating-of-builtin-attrs-error.rs:112:17
-   |
-LL |     mod inner { #![main] }
-   |                 ^^^^^^^^
-
-error: `main` attribute can only be used on functions
-  --> $DIR/issue-43106-gating-of-builtin-attrs-error.rs:117:5
-   |
-LL |     #[main] struct S;
-   |     ^^^^^^^
-
-error: `main` attribute can only be used on functions
-  --> $DIR/issue-43106-gating-of-builtin-attrs-error.rs:120:5
-   |
-LL |     #[main] type T = S;
-   |     ^^^^^^^
-
-error: `main` attribute can only be used on functions
-  --> $DIR/issue-43106-gating-of-builtin-attrs-error.rs:123:5
-   |
-LL |     #[main] impl S { }
-   |     ^^^^^^^
-
 error: `start` attribute can only be used on functions
-  --> $DIR/issue-43106-gating-of-builtin-attrs-error.rs:127:1
+  --> $DIR/issue-43106-gating-of-builtin-attrs-error.rs:109:1
    |
 LL | #[start]
    | ^^^^^^^^
 
 error: `start` attribute can only be used on functions
-  --> $DIR/issue-43106-gating-of-builtin-attrs-error.rs:130:17
+  --> $DIR/issue-43106-gating-of-builtin-attrs-error.rs:112:17
    |
 LL |     mod inner { #![start] }
    |                 ^^^^^^^^^
 
 error: `start` attribute can only be used on functions
-  --> $DIR/issue-43106-gating-of-builtin-attrs-error.rs:135:5
+  --> $DIR/issue-43106-gating-of-builtin-attrs-error.rs:117:5
    |
 LL |     #[start] struct S;
    |     ^^^^^^^^
 
 error: `start` attribute can only be used on functions
-  --> $DIR/issue-43106-gating-of-builtin-attrs-error.rs:138:5
+  --> $DIR/issue-43106-gating-of-builtin-attrs-error.rs:120:5
    |
 LL |     #[start] type T = S;
    |     ^^^^^^^^
 
 error: `start` attribute can only be used on functions
-  --> $DIR/issue-43106-gating-of-builtin-attrs-error.rs:141:5
+  --> $DIR/issue-43106-gating-of-builtin-attrs-error.rs:123:5
    |
 LL |     #[start] impl S { }
    |     ^^^^^^^^
@@ -137,11 +115,11 @@ error: `macro_export` attribute cannot be used at crate level
 LL | #![macro_export]
    | ^^^^^^^^^^^^^^^^
 
-error: `main` attribute cannot be used at crate level
+error: `rustc_main` attribute cannot be used at crate level
   --> $DIR/issue-43106-gating-of-builtin-attrs-error.rs:14:1
    |
-LL | #![main]
-   | ^^^^^^^^
+LL | #![rustc_main]
+   | ^^^^^^^^^^^^^^
 
 error: `start` attribute cannot be used at crate level
   --> $DIR/issue-43106-gating-of-builtin-attrs-error.rs:16:1
@@ -245,6 +223,7 @@ error: attribute should be applied to a function or static
 LL |     #[export_name = "2200"] impl S { }
    |     ^^^^^^^^^^^^^^^^^^^^^^^ ---------- not a function or static
 
-error: aborting due to 36 previous errors
+error: aborting due to 32 previous errors
 
-For more information about this error, try `rustc --explain E0518`.
+Some errors have detailed explanations: E0518, E0658.
+For more information about an error, try `rustc --explain E0518`.