Use cfg_attr in AST with a placeholder attribute for accurate suggestion

compiler/rustc_lint/messages.ftl

@@ -951,8 +951,9 @@ lint_unused_doc_comment = unused doc comment
     .label = rustdoc does not generate documentation for macro invocations
     .help = to document an item produced by a macro, the macro must produce the documentation as part of its expansion
 
-lint_unused_extern_crate = unused extern crate
-    .suggestion = remove it
+lint_unused_extern_crate = unused `extern crate`
+    .label = unused
+    .suggestion = remove the unused `extern crate`
 
 lint_unused_import_braces = braces around {$node} is unnecessary
 

compiler/rustc_lint/src/early/diagnostics.rs

@@ -292,8 +292,8 @@ pub(super) fn decorate_lint(
         BuiltinLintDiag::ByteSliceInPackedStructWithDerive { ty } => {
             lints::ByteSliceInPackedStructWithDerive { ty }.decorate_lint(diag);
         }
-        BuiltinLintDiag::UnusedExternCrate { removal_span } => {
-            lints::UnusedExternCrate { removal_span }.decorate_lint(diag);
+        BuiltinLintDiag::UnusedExternCrate { span, removal_span } => {
+            lints::UnusedExternCrate { span, removal_span }.decorate_lint(diag);
         }
         BuiltinLintDiag::ExternCrateNotIdiomatic { vis_span, ident_span } => {
             let suggestion_span = vis_span.between(ident_span);

compiler/rustc_lint/src/lints.rs

@@ -3041,7 +3041,9 @@ pub(crate) struct ByteSliceInPackedStructWithDerive {
 #[derive(LintDiagnostic)]
 #[diag(lint_unused_extern_crate)]
 pub(crate) struct UnusedExternCrate {
-    #[suggestion(code = "", applicability = "machine-applicable")]
+    #[label]
+    pub span: Span,
+    #[suggestion(code = "", applicability = "machine-applicable", style = "verbose")]
     pub removal_span: Span,
 }
 

compiler/rustc_lint_defs/src/lib.rs

@@ -736,6 +736,7 @@ pub enum BuiltinLintDiag {
         ty: String,
     },
     UnusedExternCrate {
+        span: Span,
         removal_span: Span,
     },
     ExternCrateNotIdiomatic {

compiler/rustc_resolve/src/check_unused.rs

@@ -154,6 +154,7 @@ impl<'a, 'ra, 'tcx> UnusedImportCheckVisitor<'a, 'ra, 'tcx> {
                         extern_crate.id,
                         span,
                         BuiltinLintDiag::UnusedExternCrate {
+                            span: extern_crate.span,
                             removal_span: extern_crate.span_with_attributes,
                         },
                     );

tests/ui/editions/edition-extern-crate-allowed.rs

@@ -5,6 +5,6 @@
 #![warn(rust_2018_idioms)]
 
 extern crate edition_extern_crate_allowed;
-//~^ WARNING unused extern crate
+//~^ WARNING unused `extern crate`
 
 fn main() {}

tests/ui/editions/edition-extern-crate-allowed.stderr

@@ -1,15 +1,19 @@
-warning: unused extern crate
+warning: unused `extern crate`
   --> $DIR/edition-extern-crate-allowed.rs:7:1
    |
 LL | extern crate edition_extern_crate_allowed;
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove it
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unused
    |
 note: the lint level is defined here
   --> $DIR/edition-extern-crate-allowed.rs:5:9
    |
 LL | #![warn(rust_2018_idioms)]
    |         ^^^^^^^^^^^^^^^^
    = note: `#[warn(unused_extern_crates)]` implied by `#[warn(rust_2018_idioms)]`
+help: remove the unused `extern crate`
+   |
+LL - extern crate edition_extern_crate_allowed;
+   |
 
 warning: 1 warning emitted
 

tests/ui/imports/extern-crate-used.rs

@@ -15,7 +15,7 @@ extern crate core as iso3;
 extern crate core as iso4;
 
 // Doesn't introduce its extern prelude entry, so it's still considered unused.
-extern crate core; //~ ERROR unused extern crate
+extern crate core; //~ ERROR unused `extern crate`
 
 mod m {
     use iso1::any as are_you_okay1;

tests/ui/imports/extern-crate-used.stderr

@@ -1,14 +1,19 @@
-error: unused extern crate
+error: unused `extern crate`
   --> $DIR/extern-crate-used.rs:18:1
    |
 LL | extern crate core;
-   | ^^^^^^^^^^^^^^^^^^ help: remove it
+   | ^^^^^^^^^^^^^^^^^^ unused
    |
 note: the lint level is defined here
   --> $DIR/extern-crate-used.rs:6:9
    |
 LL | #![deny(unused_extern_crates)]
    |         ^^^^^^^^^^^^^^^^^^^^
+help: remove the unused `extern crate`
+   |
+LL - extern crate core;
+LL +
+   |
 
 error: aborting due to 1 previous error
 

tests/ui/lint/unnecessary-extern-crate.rs

@@ -4,10 +4,10 @@
 #![feature(test)]
 
 extern crate core;
-//~^ ERROR unused extern crate
+//~^ ERROR unused `extern crate`
 //~| HELP remove
 extern crate core as x;
-//~^ ERROR unused extern crate
+//~^ ERROR unused `extern crate`
 //~| HELP remove
 
 extern crate proc_macro;
@@ -29,11 +29,11 @@ mod foo {
     pub(super) extern crate alloc as d;
 
     extern crate core;
-    //~^ ERROR unused extern crate
+    //~^ ERROR unused `extern crate`
     //~| HELP remove
 
     extern crate core as x;
-    //~^ ERROR unused extern crate
+    //~^ ERROR unused `extern crate`
     //~| HELP remove
 
     pub extern crate test;
@@ -42,11 +42,11 @@ mod foo {
 
     mod bar {
         extern crate core;
-        //~^ ERROR unused extern crate
+        //~^ ERROR unused `extern crate`
         //~| HELP remove
 
         extern crate core as x;
-        //~^ ERROR unused extern crate
+        //~^ ERROR unused `extern crate`
         //~| HELP remove
 
         pub(in crate::foo::bar) extern crate alloc as e;

tests/ui/lint/unnecessary-extern-crate.stderr

@@ -1,44 +1,73 @@
-error: unused extern crate
+error: unused `extern crate`
   --> $DIR/unnecessary-extern-crate.rs:6:1
    |
 LL | extern crate core;
-   | ^^^^^^^^^^^^^^^^^^ help: remove it
+   | ^^^^^^^^^^^^^^^^^^ unused
    |
 note: the lint level is defined here
   --> $DIR/unnecessary-extern-crate.rs:3:9
    |
 LL | #![deny(unused_extern_crates)]
    |         ^^^^^^^^^^^^^^^^^^^^
+help: remove the unused `extern crate`
+   |
+LL - extern crate core;
+   |
 
-error: unused extern crate
+error: unused `extern crate`
   --> $DIR/unnecessary-extern-crate.rs:9:1
    |
 LL | extern crate core as x;
-   | ^^^^^^^^^^^^^^^^^^^^^^^ help: remove it
+   | ^^^^^^^^^^^^^^^^^^^^^^^ unused
+   |
+help: remove the unused `extern crate`
+   |
+LL - extern crate core as x;
+   |
 
-error: unused extern crate
+error: unused `extern crate`
   --> $DIR/unnecessary-extern-crate.rs:31:5
    |
 LL |     extern crate core;
-   |     ^^^^^^^^^^^^^^^^^^ help: remove it
+   |     ^^^^^^^^^^^^^^^^^^ unused
+   |
+help: remove the unused `extern crate`
+   |
+LL -     extern crate core;
+   |
 
-error: unused extern crate
+error: unused `extern crate`
   --> $DIR/unnecessary-extern-crate.rs:35:5
    |
 LL |     extern crate core as x;
-   |     ^^^^^^^^^^^^^^^^^^^^^^^ help: remove it
+   |     ^^^^^^^^^^^^^^^^^^^^^^^ unused
+   |
+help: remove the unused `extern crate`
+   |
+LL -     extern crate core as x;
+   |
 
-error: unused extern crate
+error: unused `extern crate`
   --> $DIR/unnecessary-extern-crate.rs:44:9
    |
 LL |         extern crate core;
-   |         ^^^^^^^^^^^^^^^^^^ help: remove it
+   |         ^^^^^^^^^^^^^^^^^^ unused
+   |
+help: remove the unused `extern crate`
+   |
+LL -         extern crate core;
+   |
 
-error: unused extern crate
+error: unused `extern crate`
   --> $DIR/unnecessary-extern-crate.rs:48:9
    |
 LL |         extern crate core as x;
-   |         ^^^^^^^^^^^^^^^^^^^^^^^ help: remove it
+   |         ^^^^^^^^^^^^^^^^^^^^^^^ unused
+   |
+help: remove the unused `extern crate`
+   |
+LL -         extern crate core as x;
+   |
 
 error: aborting due to 6 previous errors
 

tests/ui/lint/unused/lint-unused-extern-crate.rs

@@ -8,7 +8,7 @@
 #![allow(unused_variables)]
 #![allow(deprecated)]
 
-extern crate lint_unused_extern_crate5; //~ ERROR: unused extern crate
+extern crate lint_unused_extern_crate5; //~ ERROR: unused `extern crate`
 
 pub extern crate lint_unused_extern_crate4; // no error, it is re-exported
 
@@ -26,7 +26,7 @@ use other::*;
 
 mod foo {
     // Test that this is unused even though an earlier `extern crate` is used.
-    extern crate lint_unused_extern_crate2; //~ ERROR unused extern crate
+    extern crate lint_unused_extern_crate2; //~ ERROR unused `extern crate`
 }
 
 fn main() {

tests/ui/lint/unused/lint-unused-extern-crate.stderr

@@ -1,20 +1,31 @@
-error: unused extern crate
+error: unused `extern crate`
   --> $DIR/lint-unused-extern-crate.rs:11:1
    |
 LL | extern crate lint_unused_extern_crate5;
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove it
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unused
    |
 note: the lint level is defined here
   --> $DIR/lint-unused-extern-crate.rs:7:9
    |
 LL | #![deny(unused_extern_crates)]
    |         ^^^^^^^^^^^^^^^^^^^^
+help: remove the unused `extern crate`
+   |
+LL - extern crate lint_unused_extern_crate5;
+LL +
+   |
 
-error: unused extern crate
+error: unused `extern crate`
   --> $DIR/lint-unused-extern-crate.rs:29:5
    |
 LL |     extern crate lint_unused_extern_crate2;
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove it
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unused
+   |
+help: remove the unused `extern crate`
+   |
+LL -     extern crate lint_unused_extern_crate2;
+LL +
+   |
 
 error: aborting due to 2 previous errors
 

tests/ui/proc-macro/auxiliary/cfg-placeholder.rs

@@ -0,0 +1,31 @@
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+
+#[proc_macro_attribute]
+pub fn my_proc_macro(_: TokenStream, input: TokenStream) -> TokenStream {
+    if format!("{input:#?}").contains("my_attr1") {
+        panic!("found gated attribute my_attr1");
+    }
+    if format!("{input:#?}").contains("placeholder") {
+        panic!("found placeholder attribute");
+    }
+    if !format!("{input:#?}").contains("my_attr2") {
+        panic!("didn't if gated my_attr2");
+    }
+    input
+}
+
+#[proc_macro_attribute]
+pub fn my_attr1(_: TokenStream, input: TokenStream) -> TokenStream {
+    panic!("my_attr1 was called");
+    input
+}
+
+#[proc_macro_attribute]
+pub fn my_attr2(_: TokenStream, input: TokenStream) -> TokenStream {
+    if format!("{input:#?}").contains("my_attr1") {
+        panic!("found gated attribute my_attr1");
+    }
+    input
+}

tests/ui/proc-macro/cfg-attr-placeholder-invisible-to-proc-macro.rs

@@ -0,0 +1,13 @@
+// Ensure that `rustc-cfg-placeholder` isn't visible to proc-macros.
+//@proc-macro: cfg-placeholder.rs
+//@check-pass
+#![feature(cfg_eval)]
+#[macro_use] extern crate cfg_placeholder;
+
+#[cfg_eval]
+#[my_proc_macro]
+#[cfg_attr(FALSE, my_attr1)]
+#[cfg_attr(all(), my_attr2)]
+struct S {}
+
+fn main() {}

tests/ui/proc-macro/no-macro-use-attr.rs

@@ -4,6 +4,6 @@
 #![warn(unused_extern_crates)]
 
 extern crate test_macros;
-//~^ WARN unused extern crate
+//~^ WARN unused `extern crate`
 
 fn main() {}

tests/ui/proc-macro/no-macro-use-attr.stderr

@@ -1,14 +1,18 @@
-warning: unused extern crate
+warning: unused `extern crate`
   --> $DIR/no-macro-use-attr.rs:6:1
    |
 LL | extern crate test_macros;
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove it
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^ unused
    |
 note: the lint level is defined here
   --> $DIR/no-macro-use-attr.rs:4:9
    |
 LL | #![warn(unused_extern_crates)]
    |         ^^^^^^^^^^^^^^^^^^^^
+help: remove the unused `extern crate`
+   |
+LL - extern crate test_macros;
+   |
 
 warning: 1 warning emitted
 

tests/ui/rust-2018/extern-crate-idiomatic-in-2018.fixed

@@ -9,7 +9,7 @@
 #![deny(rust_2018_idioms)]
 #![allow(dead_code)]
 
-//~^ ERROR unused extern crate
+//~^ ERROR unused `extern crate`
 
 // Shouldn't suggest changing to `use`, as `bar`
 // would no longer be added to the prelude which could cause

tests/ui/rust-2018/extern-crate-idiomatic-in-2018.rs

@@ -10,7 +10,7 @@
 #![allow(dead_code)]
 
 extern crate edition_lint_paths;
-//~^ ERROR unused extern crate
+//~^ ERROR unused `extern crate`
 
 // Shouldn't suggest changing to `use`, as `bar`
 // would no longer be added to the prelude which could cause