Rollup merge of #72825 - Amanieu:asm-warning, r=davidtwco

Clarify errors and warnings about the transition to the new asm!

Hopefully addresses the concerns from https://github.com/rust-lang/rust/pull/71007#issuecomment-636412905.

src/libcore/macros/mod.rs

@@ -1315,7 +1315,7 @@ pub(crate) mod builtin {
     #[unstable(
         feature = "llvm_asm",
         issue = "70173",
-        reason = "LLVM-style inline assembly will never be stabilized, prefer using asm! instead"
+        reason = "prefer using the new asm! syntax instead"
     )]
     #[rustc_builtin_macro]
     #[macro_export]

src/librustc_builtin_macros/asm.rs

@@ -33,7 +33,10 @@ fn parse_args<'a>(
 
     // Detect use of the legacy llvm_asm! syntax (which used to be called asm!)
     if p.look_ahead(1, |t| *t == token::Colon || *t == token::ModSep) {
-        let mut err = ecx.struct_span_err(sp, "legacy asm! syntax is no longer supported");
+        let mut err =
+            ecx.struct_span_err(sp, "the legacy LLVM-style asm! syntax is no longer supported");
+        err.note("consider migrating to the new asm! syntax specified in RFC 2873");
+        err.note("alternatively, switch to llvm_asm! to keep your code working as it is");
 
         // Find the span of the "asm!" so that we can offer an automatic suggestion
         let asm_span = sp.from_inner(InnerSpan::new(0, 4));

src/test/ui/asm/rustfix-asm.fixed

@@ -8,9 +8,9 @@ fn main() {
         let x = 1;
         let y: i32;
         llvm_asm!("" :: "r" (x));
-        //~^ ERROR legacy asm! syntax is no longer supported
+        //~^ ERROR the legacy LLVM-style asm! syntax is no longer supported
         llvm_asm!("" : "=r" (y));
-        //~^ ERROR legacy asm! syntax is no longer supported
+        //~^ ERROR the legacy LLVM-style asm! syntax is no longer supported
         let _ = y;
     }
 }

src/test/ui/asm/rustfix-asm.rs

@@ -8,9 +8,9 @@ fn main() {
         let x = 1;
         let y: i32;
         asm!("" :: "r" (x));
-        //~^ ERROR legacy asm! syntax is no longer supported
+        //~^ ERROR the legacy LLVM-style asm! syntax is no longer supported
         asm!("" : "=r" (y));
-        //~^ ERROR legacy asm! syntax is no longer supported
+        //~^ ERROR the legacy LLVM-style asm! syntax is no longer supported
         let _ = y;
     }
 }

src/test/ui/asm/rustfix-asm.stderr

@@ -1,18 +1,24 @@
-error: legacy asm! syntax is no longer supported
+error: the legacy LLVM-style asm! syntax is no longer supported
   --> $DIR/rustfix-asm.rs:10:9
    |
 LL |         asm!("" :: "r" (x));
    |         ----^^^^^^^^^^^^^^^^
    |         |
    |         help: replace with: `llvm_asm!`
+   |
+   = note: consider migrating to the new asm! syntax specified in RFC 2873
+   = note: alternatively, switch to llvm_asm! to keep your code working as it is
 
-error: legacy asm! syntax is no longer supported
+error: the legacy LLVM-style asm! syntax is no longer supported
   --> $DIR/rustfix-asm.rs:12:9
    |
 LL |         asm!("" : "=r" (y));
    |         ----^^^^^^^^^^^^^^^^
    |         |
    |         help: replace with: `llvm_asm!`
+   |
+   = note: consider migrating to the new asm! syntax specified in RFC 2873
+   = note: alternatively, switch to llvm_asm! to keep your code working as it is
 
 error: aborting due to 2 previous errors
 

src/test/ui/feature-gates/feature-gate-asm.rs

@@ -5,6 +5,6 @@ fn main() {
         asm!("");
         //~^ ERROR inline assembly is not stable enough
         llvm_asm!("");
-        //~^ ERROR LLVM-style inline assembly will never be stabilized
+        //~^ ERROR prefer using the new asm! syntax instead
     }
 }

src/test/ui/feature-gates/feature-gate-asm.stderr

@@ -7,7 +7,7 @@ LL |         asm!("");
    = note: see issue #72016 <https://github.com/rust-lang/rust/issues/72016> for more information
    = help: add `#![feature(asm)]` to the crate attributes to enable
 
-error[E0658]: use of unstable library feature 'llvm_asm': LLVM-style inline assembly will never be stabilized, prefer using asm! instead
+error[E0658]: use of unstable library feature 'llvm_asm': prefer using the new asm! syntax instead
   --> $DIR/feature-gate-asm.rs:7:9
    |
 LL |         llvm_asm!("");

src/test/ui/feature-gates/feature-gate-asm2.rs

@@ -5,6 +5,6 @@ fn main() {
         println!("{:?}", asm!(""));
         //~^ ERROR inline assembly is not stable enough
         println!("{:?}", llvm_asm!(""));
-        //~^ ERROR LLVM-style inline assembly will never be stabilized
+        //~^ ERROR prefer using the new asm! syntax instead
     }
 }

src/test/ui/feature-gates/feature-gate-asm2.stderr

@@ -7,7 +7,7 @@ LL |         println!("{:?}", asm!(""));
    = note: see issue #72016 <https://github.com/rust-lang/rust/issues/72016> for more information
    = help: add `#![feature(asm)]` to the crate attributes to enable
 
-error[E0658]: use of unstable library feature 'llvm_asm': LLVM-style inline assembly will never be stabilized, prefer using asm! instead
+error[E0658]: use of unstable library feature 'llvm_asm': prefer using the new asm! syntax instead
   --> $DIR/feature-gate-asm2.rs:7:26
    |
 LL |         println!("{:?}", llvm_asm!(""));