Port #![no_main] to the attribute parser.

compiler/rustc_attr_parsing/src/attributes/crate_level.rs

@@ -136,6 +136,15 @@ impl<S: Stage> NoArgsAttributeParser<S> for NoStdParser {
     const CREATE: fn(Span) -> AttributeKind = AttributeKind::NoStd;
 }
 
+pub(crate) struct NoMainParser;
+
+impl<S: Stage> NoArgsAttributeParser<S> for NoMainParser {
+    const PATH: &[Symbol] = &[sym::no_main];
+    const ON_DUPLICATE: OnDuplicate<S> = OnDuplicate::Warn;
+    const ALLOWED_TARGETS: AllowedTargets = AllowedTargets::AllowList(&[Allow(Target::Crate)]);
+    const CREATE: fn(Span) -> AttributeKind = |_| AttributeKind::NoMain;
+}
+
 pub(crate) struct RustcCoherenceIsCoreParser;
 
 impl<S: Stage> NoArgsAttributeParser<S> for RustcCoherenceIsCoreParser {

compiler/rustc_attr_parsing/src/context.rs

@@ -28,9 +28,9 @@ use crate::attributes::codegen_attrs::{
 };
 use crate::attributes::confusables::ConfusablesParser;
 use crate::attributes::crate_level::{
-    CrateNameParser, MoveSizeLimitParser, NoCoreParser, NoStdParser, PatternComplexityLimitParser,
-    RecursionLimitParser, RustcCoherenceIsCoreParser, TypeLengthLimitParser,
-    WindowsSubsystemParser,
+    CrateNameParser, MoveSizeLimitParser, NoCoreParser, NoMainParser, NoStdParser,
+    PatternComplexityLimitParser, RecursionLimitParser, RustcCoherenceIsCoreParser,
+    TypeLengthLimitParser, WindowsSubsystemParser,
 };
 use crate::attributes::debugger::DebuggerViualizerParser;
 use crate::attributes::deprecation::DeprecationParser;
@@ -263,6 +263,7 @@ attribute_parsers!(
         Single<WithoutArgs<NoCoreParser>>,
         Single<WithoutArgs<NoImplicitPreludeParser>>,
         Single<WithoutArgs<NoLinkParser>>,
+        Single<WithoutArgs<NoMainParser>>,
         Single<WithoutArgs<NoMangleParser>>,
         Single<WithoutArgs<NoStdParser>>,
         Single<WithoutArgs<NonExhaustiveParser>>,

compiler/rustc_hir/src/attrs/data_structures.rs

@@ -852,6 +852,9 @@ pub enum AttributeKind {
     /// Represents `#[no_link]`
     NoLink,
 
+    /// Represents `#[no_main]`
+    NoMain,
+
     /// Represents `#[no_mangle]`
     NoMangle(Span),
 

compiler/rustc_hir/src/attrs/encode_cross_crate.rs

@@ -79,6 +79,7 @@ impl AttributeKind {
             NoCore(..) => No,
             NoImplicitPrelude(..) => No,
             NoLink => No,
+            NoMain => No,
             NoMangle(..) => Yes, // Needed for rustdoc
             NoStd(..) => No,
             NonExhaustive(..) => Yes, // Needed for rustdoc

compiler/rustc_passes/src/check_attr.rs

@@ -297,6 +297,7 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                     | AttributeKind::PatternComplexityLimit { .. }
                     | AttributeKind::NoCore { .. }
                     | AttributeKind::NoStd { .. }
+                    | AttributeKind::NoMain
                     | AttributeKind::ObjcClass { .. }
                     | AttributeKind::ObjcSelector { .. }
                     | AttributeKind::RustcCoherenceIsCore(..)

compiler/rustc_passes/src/entry.rs

@@ -1,4 +1,3 @@
-use rustc_ast::attr;
 use rustc_ast::entry::EntryPointType;
 use rustc_errors::codes::*;
 use rustc_hir::attrs::AttributeKind;
@@ -7,7 +6,7 @@ use rustc_hir::{CRATE_HIR_ID, ItemId, Node, find_attr};
 use rustc_middle::query::Providers;
 use rustc_middle::ty::TyCtxt;
 use rustc_session::config::{CrateType, EntryFnType, sigpipe};
-use rustc_span::{RemapPathScopeComponents, Span, sym};
+use rustc_span::{RemapPathScopeComponents, Span};
 
 use crate::errors::{ExternMain, MultipleRustcMain, NoMainErr};
 
@@ -30,7 +29,7 @@ fn entry_fn(tcx: TyCtxt<'_>, (): ()) -> Option<(DefId, EntryFnType)> {
     }
 
     // If the user wants no main function at all, then stop here.
-    if attr::contains_name(tcx.hir_attrs(CRATE_HIR_ID), sym::no_main) {
+    if find_attr!(tcx.hir_attrs(CRATE_HIR_ID), AttributeKind::NoMain) {
         return None;
     }
 

tests/ui/feature-gates/issue-43106-gating-of-builtin-attrs.rs

@@ -884,26 +884,26 @@ mod feature {
 
 #[no_main]
 //~^ WARN crate-level attribute should be an inner attribute
-//~| HELP add a `!`
 mod no_main_1 {
+    //~^ NOTE: This attribute does not have an `!`, which means it is applied to this module
     mod inner { #![no_main] }
-//~^ WARN crate-level attribute should be in the root module
+    //~^ WARN the `#![no_main]` attribute can only be used at the crate root
 
     #[no_main] fn f() { }
     //~^ WARN crate-level attribute should be an inner attribute
-    //~| HELP add a `!`
+    //~| NOTE This attribute does not have an `!`, which means it is applied to this function
 
     #[no_main] struct S;
     //~^ WARN crate-level attribute should be an inner attribute
-    //~| HELP add a `!`
+    //~| NOTE This attribute does not have an `!`, which means it is applied to this struct
 
     #[no_main] type T = S;
     //~^ WARN crate-level attribute should be an inner attribute
-    //~| HELP add a `!`
+    //~| NOTE This attribute does not have an `!`, which means it is applied to this type alias
 
     #[no_main] impl S { }
     //~^ WARN crate-level attribute should be an inner attribute
-    //~| HELP add a `!`
+    //~| NOTE This attribute does not have an `!`, which means it is applied to this implementation
 }
 
 #[no_builtins]

tests/ui/feature-gates/issue-43106-gating-of-builtin-attrs.stderr

@@ -240,17 +240,6 @@ help: add a `!`
 LL | #![feature(x0600)]
    |  +
 
-warning: crate-level attribute should be an inner attribute
-  --> $DIR/issue-43106-gating-of-builtin-attrs.rs:885:1
-   |
-LL | #[no_main]
-   | ^^^^^^^^^^
-   |
-help: add a `!`
-   |
-LL | #![no_main]
-   |  +
-
 warning: crate-level attribute should be an inner attribute
   --> $DIR/issue-43106-gating-of-builtin-attrs.rs:909:1
    |
@@ -476,56 +465,6 @@ help: add a `!`
 LL |     #![feature(x0600)] impl S { }
    |      +
 
-warning: crate-level attribute should be in the root module
-  --> $DIR/issue-43106-gating-of-builtin-attrs.rs:889:17
-   |
-LL |     mod inner { #![no_main] }
-   |                 ^^^^^^^^^^^
-
-warning: crate-level attribute should be an inner attribute
-  --> $DIR/issue-43106-gating-of-builtin-attrs.rs:892:5
-   |
-LL |     #[no_main] fn f() { }
-   |     ^^^^^^^^^^
-   |
-help: add a `!`
-   |
-LL |     #![no_main] fn f() { }
-   |      +
-
-warning: crate-level attribute should be an inner attribute
-  --> $DIR/issue-43106-gating-of-builtin-attrs.rs:896:5
-   |
-LL |     #[no_main] struct S;
-   |     ^^^^^^^^^^
-   |
-help: add a `!`
-   |
-LL |     #![no_main] struct S;
-   |      +
-
-warning: crate-level attribute should be an inner attribute
-  --> $DIR/issue-43106-gating-of-builtin-attrs.rs:900:5
-   |
-LL |     #[no_main] type T = S;
-   |     ^^^^^^^^^^
-   |
-help: add a `!`
-   |
-LL |     #![no_main] type T = S;
-   |      +
-
-warning: crate-level attribute should be an inner attribute
-  --> $DIR/issue-43106-gating-of-builtin-attrs.rs:904:5
-   |
-LL |     #[no_main] impl S { }
-   |     ^^^^^^^^^^
-   |
-help: add a `!`
-   |
-LL |     #![no_main] impl S { }
-   |      +
-
 warning: crate-level attribute should be in the root module
   --> $DIR/issue-43106-gating-of-builtin-attrs.rs:913:17
    |
@@ -1407,6 +1346,76 @@ note: This attribute does not have an `!`, which means it is applied to this imp
 LL |     #[crate_name = "0900"] impl S { }
    |                            ^^^^^^^^^^
 
+warning: crate-level attribute should be an inner attribute: add an exclamation mark: `#![no_main]`
+  --> $DIR/issue-43106-gating-of-builtin-attrs.rs:885:1
+   |
+LL | #[no_main]
+   | ^^^^^^^^^^
+   |
+note: This attribute does not have an `!`, which means it is applied to this module
+  --> $DIR/issue-43106-gating-of-builtin-attrs.rs:887:1
+   |
+LL | / mod no_main_1 {
+LL | |
+LL | |     mod inner { #![no_main] }
+...  |
+LL | | }
+   | |_^
+
+warning: the `#![no_main]` attribute can only be used at the crate root
+  --> $DIR/issue-43106-gating-of-builtin-attrs.rs:889:17
+   |
+LL |     mod inner { #![no_main] }
+   |                 ^^^^^^^^^^^
+
+warning: crate-level attribute should be an inner attribute: add an exclamation mark: `#![no_main]`
+  --> $DIR/issue-43106-gating-of-builtin-attrs.rs:892:5
+   |
+LL |     #[no_main] fn f() { }
+   |     ^^^^^^^^^^
+   |
+note: This attribute does not have an `!`, which means it is applied to this function
+  --> $DIR/issue-43106-gating-of-builtin-attrs.rs:892:16
+   |
+LL |     #[no_main] fn f() { }
+   |                ^^^^^^^^^^
+
+warning: crate-level attribute should be an inner attribute: add an exclamation mark: `#![no_main]`
+  --> $DIR/issue-43106-gating-of-builtin-attrs.rs:896:5
+   |
+LL |     #[no_main] struct S;
+   |     ^^^^^^^^^^
+   |
+note: This attribute does not have an `!`, which means it is applied to this struct
+  --> $DIR/issue-43106-gating-of-builtin-attrs.rs:896:16
+   |
+LL |     #[no_main] struct S;
+   |                ^^^^^^^^^
+
+warning: crate-level attribute should be an inner attribute: add an exclamation mark: `#![no_main]`
+  --> $DIR/issue-43106-gating-of-builtin-attrs.rs:900:5
+   |
+LL |     #[no_main] type T = S;
+   |     ^^^^^^^^^^
+   |
+note: This attribute does not have an `!`, which means it is applied to this type alias
+  --> $DIR/issue-43106-gating-of-builtin-attrs.rs:900:16
+   |
+LL |     #[no_main] type T = S;
+   |                ^^^^^^^^^^^
+
+warning: crate-level attribute should be an inner attribute: add an exclamation mark: `#![no_main]`
+  --> $DIR/issue-43106-gating-of-builtin-attrs.rs:904:5
+   |
+LL |     #[no_main] impl S { }
+   |     ^^^^^^^^^^
+   |
+note: This attribute does not have an `!`, which means it is applied to this implementation block
+  --> $DIR/issue-43106-gating-of-builtin-attrs.rs:904:16
+   |
+LL |     #[no_main] impl S { }
+   |                ^^^^^^^^^^
+
 warning: crate-level attribute should be an inner attribute: add an exclamation mark: `#![recursion_limit]`
   --> $DIR/issue-43106-gating-of-builtin-attrs.rs:933:1
    |