Auto merge of #87324 - asquared31415:named-asm-labels, r=Amanieu

Lint against named asm labels

This adds a deny-by-default lint to prevent the use of named labels in inline `asm!`.  Without a solution to #81088 about whether the compiler should rewrite named labels or a special syntax for labels, a lint against them should prevent users from writing assembly that could break for internal compiler reasons, such as inlining or anything else that could change the number of actual inline assembly blocks emitted.

This does **not** resolve the issue with rewriting labels, that still needs a decision if the compiler should do any more work to try to make them work.

compiler/rustc_lint_defs/src/builtin.rs

@@ -2468,6 +2468,38 @@ declare_lint! {
     "incorrect use of inline assembly",
 }
 
+declare_lint! {
+    /// The `named_asm_labels` lint detects the use of named labels in the
+    /// inline `asm!` macro.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// fn main() {
+    ///     unsafe {
+    ///         asm!("foo: bar");
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// LLVM is allowed to duplicate inline assembly blocks for any
+    /// reason, for example when it is in a function that gets inlined. Because
+    /// of this, GNU assembler [local labels] *must* be used instead of labels
+    /// with a name. Using named labels might cause assembler or linker errors.
+    ///
+    /// See the [unstable book] for more details.
+    ///
+    /// [local labels]: https://sourceware.org/binutils/docs/as/Symbol-Names.html#Local-Labels
+    /// [unstable book]: https://doc.rust-lang.org/nightly/unstable-book/library-features/asm.html#labels
+    pub NAMED_ASM_LABELS,
+    Deny,
+    "named labels in inline assembly",
+}
+
 declare_lint! {
     /// The `unsafe_op_in_unsafe_fn` lint detects unsafe operations in unsafe
     /// functions without an explicit unsafe block.
@@ -2988,6 +3020,7 @@ declare_lint_pass! {
         INLINE_NO_SANITIZE,
         BAD_ASM_STYLE,
         ASM_SUB_REGISTER,
+        NAMED_ASM_LABELS,
         UNSAFE_OP_IN_UNSAFE_FN,
         INCOMPLETE_INCLUDE,
         CENUM_IMPL_DROP_CAST,

compiler/rustc_lint_defs/src/lib.rs

@@ -305,6 +305,7 @@ pub enum BuiltinLintDiagnostics {
     ReservedPrefix(Span),
     TrailingMacro(bool, Ident),
     BreakWithLabelAndLoop(Span),
+    NamedAsmLabel(String),
 }
 
 /// Lints that are buffered up early on in the `Session` before the