Rollup merge of #95553 - jam1garner:naked-function-compile-error, r=tmiasko

Don't emit non-asm contents error for naked function composed of errors

## Motivation

For naked functions an error is emitted when they are composed of anything other than a single asm!() block. However, this error triggers in a couple situations in which it adds no additional information or is actively misleading.

One example is if you do have an asm!() block but simply one with a syntax error:
```rust
#[naked]
unsafe extern "C" fn compiler_errors() {
    asm!(invalid_syntax)
}
```

This results in two errors, one for the syntax error itself and another telling you that you need an asm block in your function:

```rust
error[E0787]: naked functions must contain a single asm block
 --> src/main.rs:6:1
  |
6 | / unsafe extern "C" fn naked_compile_error() {
7 | |     asm!(blah)
8 | | }
  | |_^
```

This issue also comes up when [utilizing `compile_error!()` for improving your diagnostics](https://twitter.com/steveklabnik/status/1509538243020218372), such as raising a compiler error when compiling for an unsupported target.

## Implementation

The rules this PR implements are as follows:

1. If any non-erroneous  non-asm statement is included, an error will still occur
2. If multiple asm statements are included, an error will still occur
3. If 0 or 1 asm statements are present, as well as any non-zero number of erroneous statements, then this error will *not* be raised as it is likely either redundant or incorrect

The rule of thumb is effectively "if an error is present and its correction could change things, don't raise an error".

compiler/rustc_passes/src/naked_functions.rs

@@ -147,7 +147,7 @@ impl<'tcx> Visitor<'tcx> for CheckParameters<'tcx> {
 fn check_asm<'tcx>(tcx: TyCtxt<'tcx>, body: &'tcx hir::Body<'tcx>, fn_span: Span) {
     let mut this = CheckInlineAssembly { tcx, items: Vec::new() };
     this.visit_body(body);
-    if let [(ItemKind::Asm, _)] = this.items[..] {
+    if let [(ItemKind::Asm | ItemKind::Err, _)] = this.items[..] {
         // Ok.
     } else {
         let mut diag = struct_span_err!(
@@ -156,19 +156,33 @@ fn check_asm<'tcx>(tcx: TyCtxt<'tcx>, body: &'tcx hir::Body<'tcx>, fn_span: Span
             E0787,
             "naked functions must contain a single asm block"
         );
+
+        let mut must_show_error = false;
         let mut has_asm = false;
+        let mut has_err = false;
         for &(kind, span) in &this.items {
             match kind {
                 ItemKind::Asm if has_asm => {
+                    must_show_error = true;
                     diag.span_label(span, "multiple asm blocks are unsupported in naked functions");
                 }
                 ItemKind::Asm => has_asm = true,
                 ItemKind::NonAsm => {
+                    must_show_error = true;
                     diag.span_label(span, "non-asm is unsupported in naked functions");
                 }
+                ItemKind::Err => has_err = true,
             }
         }
-        diag.emit();
+
+        // If the naked function only contains a single asm block and a non-zero number of
+        // errors, then don't show an additional error. This allows for appending/prepending
+        // `compile_error!("...")` statements and reduces error noise.
+        if must_show_error || !has_err {
+            diag.emit();
+        } else {
+            diag.cancel();
+        }
     }
 }
 
@@ -181,6 +195,7 @@ struct CheckInlineAssembly<'tcx> {
 enum ItemKind {
     Asm,
     NonAsm,
+    Err,
 }
 
 impl<'tcx> CheckInlineAssembly<'tcx> {
@@ -222,9 +237,13 @@ impl<'tcx> CheckInlineAssembly<'tcx> {
                 self.check_inline_asm(asm, span);
             }
 
-            ExprKind::DropTemps(..) | ExprKind::Block(..) | ExprKind::Err => {
+            ExprKind::DropTemps(..) | ExprKind::Block(..) => {
                 hir::intravisit::walk_expr(self, expr);
             }
+
+            ExprKind::Err => {
+                self.items.push((ItemKind::Err, span));
+            }
         }
     }
 

src/test/ui/asm/naked-functions.rs

@@ -196,3 +196,22 @@ pub unsafe extern "C" fn inline_never() {
 pub unsafe extern "C" fn inline_all() {
     asm!("", options(noreturn));
 }
+
+#[naked]
+pub unsafe extern "C" fn allow_compile_error(a: u32) -> u32 {
+    compile_error!("this is a user specified error")
+    //~^ ERROR this is a user specified error
+}
+
+#[naked]
+pub unsafe extern "C" fn allow_compile_error_and_asm(a: u32) -> u32 {
+    compile_error!("this is a user specified error");
+    //~^ ERROR this is a user specified error
+    asm!("", options(noreturn))
+}
+
+#[naked]
+pub unsafe extern "C" fn invalid_asm_syntax(a: u32) -> u32 {
+    asm!(invalid_syntax)
+    //~^ ERROR asm template must be a string literal
+}

src/test/ui/asm/naked-functions.stderr

@@ -4,6 +4,24 @@ error: asm with the `pure` option must have at least one output
 LL |     asm!("", options(readonly, nostack), options(pure));
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^  ^^^^^^^^^^^^^
 
+error: this is a user specified error
+  --> $DIR/naked-functions.rs:202:5
+   |
+LL |     compile_error!("this is a user specified error")
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: this is a user specified error
+  --> $DIR/naked-functions.rs:208:5
+   |
+LL |     compile_error!("this is a user specified error");
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: asm template must be a string literal
+  --> $DIR/naked-functions.rs:215:10
+   |
+LL |     asm!(invalid_syntax)
+   |          ^^^^^^^^^^^^^^
+
 error: patterns not allowed in naked function parameters
   --> $DIR/naked-functions.rs:20:5
    |
@@ -280,6 +298,6 @@ error: naked functions cannot be inlined
 LL | #[inline(never)]
    | ^^^^^^^^^^^^^^^^
 
-error: aborting due to 30 previous errors; 2 warnings emitted
+error: aborting due to 33 previous errors; 2 warnings emitted
 
 For more information about this error, try `rustc --explain E0787`.