Explain allow(unreachable_code).

Author: cjgillot

compiler/rustc_target/src/asm/mod.rs

@@ -326,6 +326,7 @@ impl InlineAsmReg {
             InlineAsmArch::RiscV32 | InlineAsmArch::RiscV64 => {
                 Self::RiscV(RiscVInlineAsmReg::parse(name)?)
             }
+            #[allow(unreachable_code)] // NvptxInlineAsmReg::parse does not return.
             InlineAsmArch::Nvptx64 => Self::Nvptx(NvptxInlineAsmReg::parse(name)?),
             InlineAsmArch::PowerPC | InlineAsmArch::PowerPC64 => {
                 Self::PowerPC(PowerPCInlineAsmReg::parse(name)?)
@@ -336,7 +337,9 @@ impl InlineAsmReg {
                 Self::Mips(MipsInlineAsmReg::parse(name)?)
             }
             InlineAsmArch::S390x => Self::S390x(S390xInlineAsmReg::parse(name)?),
+            #[allow(unreachable_code)] // SpirVInlineAsmReg::parse does not return.
             InlineAsmArch::SpirV => Self::SpirV(SpirVInlineAsmReg::parse(name)?),
+            #[allow(unreachable_code)] // WasmInlineAsmReg::parse does not return.
             InlineAsmArch::Wasm32 | InlineAsmArch::Wasm64 => {
                 Self::Wasm(WasmInlineAsmReg::parse(name)?)
             }

src/tools/miri/src/shims/unix/dlsym.rs

@@ -46,10 +46,10 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
         match dlsym {
             Dlsym::Android(dlsym) =>
                 android::EvalContextExt::call_dlsym(this, dlsym, args, dest, ret),
-            #[allow(unreachable_code)]
+            #[allow(unreachable_code)] // `dlsym` is uninhabited, and stage1 emits a lint.
             Dlsym::FreeBsd(dlsym) =>
                 freebsd::EvalContextExt::call_dlsym(this, dlsym, args, dest, ret),
-            #[allow(unreachable_code)]
+            #[allow(unreachable_code)] // `dlsym` is uninhabited, and stage1 emits a lint.
             Dlsym::Linux(dlsym) => linux::EvalContextExt::call_dlsym(this, dlsym, args, dest, ret),
             Dlsym::MacOs(dlsym) => macos::EvalContextExt::call_dlsym(this, dlsym, args, dest, ret),
         }

src/tools/rust-analyzer/crates/hir-ty/src/diagnostics/match_check/deconstruct_pat.rs

@@ -348,6 +348,7 @@ impl Constructor {
 
     fn as_slice(&self) -> Option<Slice> {
         match self {
+            #[allow(unreachable_code)] // `Slice` is uninhabilted.
             Slice(slice) => Some(*slice),
             _ => None,
         }
@@ -470,7 +471,7 @@ impl Constructor {
             (IntRange(self_range), IntRange(other_range)) => self_range.is_covered_by(other_range),
             (FloatRange(void), FloatRange(..)) => match *void {},
             (Str(void), Str(..)) => match *void {},
-            #[allow(unreachable_code)]
+            #[allow(unreachable_code)] // `Slice` is uninhabited, and stage1 emits a lint.
             (Slice(self_slice), Slice(other_slice)) => self_slice.is_covered_by(*other_slice),
 
             // We are trying to inspect an opaque constant. Thus we skip the row.
@@ -503,7 +504,7 @@ impl Constructor {
                 .iter()
                 .filter_map(|c| c.as_int_range())
                 .any(|other| range.is_covered_by(other)),
-            #[allow(unreachable_code)]
+            #[allow(unreachable_code)] // `Slice` is uninhabited, and stage1 emits a lint.
             Slice(slice) => used_ctors
                 .iter()
                 .filter_map(|c| c.as_slice())