Implement support for DWARF version 5.

DWARF version 5 brings a number of improvements over version 4. Quoting from
the announcement [1]:

> Version 5 incorporates improvements in many areas: better data compression,
> separation of debugging data from executable files, improved description of
> macros and source files, faster searching for symbols, improved debugging
> optimized code, as well as numerous improvements in functionality and
> performance.

On platforms where DWARF version 5 is supported (Linux, primarily), this commit
adds support for it behind a new `-Z dwarf-version=5` flag.

[1]: https://dwarfstd.org/Public_Review.php

compiler/rustc_codegen_llvm/src/debuginfo/mod.rs

@@ -103,14 +103,14 @@ impl<'ll, 'tcx> CodegenUnitDebugContext<'ll, 'tcx> {
             // for macOS to understand. For more info see #11352
             // This can be overridden using --llvm-opts -dwarf-version,N.
             // Android has the same issue (#22398)
-            if let Some(version) = sess.target.dwarf_version {
-                llvm::LLVMRustAddModuleFlag(
-                    self.llmod,
-                    llvm::LLVMModFlagBehavior::Warning,
-                    "Dwarf Version\0".as_ptr().cast(),
-                    version,
-                )
-            }
+            let dwarf_version =
+                sess.opts.debugging_opts.dwarf_version.unwrap_or(sess.target.default_dwarf_version);
+            llvm::LLVMRustAddModuleFlag(
+                self.llmod,
+                llvm::LLVMModFlagBehavior::Warning,
+                "Dwarf Version\0".as_ptr().cast(),
+                dwarf_version,
+            );
 
             // Indicate that we want CodeView debug information on MSVC
             if sess.target.is_like_msvc {

compiler/rustc_interface/src/tests.rs

@@ -733,6 +733,7 @@ fn test_debugging_options_tracking_hash() {
     tracked!(dep_info_omit_d_target, true);
     tracked!(drop_tracking, true);
     tracked!(dual_proc_macros, true);
+    tracked!(dwarf_version, Some(5));
     tracked!(fewer_names, Some(true));
     tracked!(force_unstable_if_unmarked, true);
     tracked!(fuel, Some(("abc".to_string(), 99)));

compiler/rustc_session/src/options.rs

@@ -1269,6 +1269,8 @@ options! {
         computed `block` spans (one span encompassing a block's terminator and \
         all statements). If `-Z instrument-coverage` is also enabled, create \
         an additional `.html` file showing the computed coverage spans."),
+    dwarf_version: Option<u32> = (None, parse_opt_number, [TRACKED],
+        "version of DWARF debug information to emit (default: 2 or 4, depending on platform)"),
     emit_stack_sizes: bool = (false, parse_bool, [UNTRACKED],
         "emit a section containing stack size metadata (default: no)"),
     fewer_names: Option<bool> = (None, parse_opt_bool, [TRACKED],

compiler/rustc_session/src/session.rs

@@ -1466,6 +1466,12 @@ fn validate_commandline_args_with_session_available(sess: &Session) {
             ))
         }
     }
+
+    if let Some(dwarf_version) = sess.opts.debugging_opts.dwarf_version {
+        if dwarf_version > 5 {
+            sess.err(&format!("requested DWARF version {} is greater than 5", dwarf_version));
+        }
+    }
 }
 
 /// Holds data on the current incremental compilation session, if there is one.

compiler/rustc_target/src/spec/android_base.rs

@@ -3,7 +3,7 @@ use crate::spec::TargetOptions;
 pub fn opts() -> TargetOptions {
     let mut base = super::linux_base::opts();
     base.os = "android".into();
-    base.dwarf_version = Some(2);
+    base.default_dwarf_version = 2;
     base.position_independent_executables = true;
     base.has_thread_local = false;
     // This is for backward compatibility, see https://github.com/rust-lang/rust/issues/49867

compiler/rustc_target/src/spec/apple_base.rs

@@ -28,7 +28,7 @@ pub fn opts(os: &'static str) -> TargetOptions {
         executables: true,
         families: cvs!["unix"],
         is_like_osx: true,
-        dwarf_version: Some(2),
+        default_dwarf_version: 2,
         frame_pointer: FramePointer::Always,
         has_rpath: true,
         dll_suffix: ".dylib".into(),

compiler/rustc_target/src/spec/dragonfly_base.rs

@@ -9,7 +9,7 @@ pub fn opts() -> TargetOptions {
         has_rpath: true,
         position_independent_executables: true,
         relro_level: RelroLevel::Full,
-        dwarf_version: Some(2),
+        default_dwarf_version: 2,
         ..Default::default()
     }
 }

compiler/rustc_target/src/spec/freebsd_base.rs

@@ -10,7 +10,7 @@ pub fn opts() -> TargetOptions {
         position_independent_executables: true,
         relro_level: RelroLevel::Full,
         abi_return_struct_as_int: true,
-        dwarf_version: Some(2),
+        default_dwarf_version: 2,
         ..Default::default()
     }
 }

compiler/rustc_target/src/spec/mod.rs

@@ -1281,9 +1281,9 @@ pub struct TargetOptions {
     pub is_like_fuchsia: bool,
     /// Whether a target toolchain is like WASM.
     pub is_like_wasm: bool,
-    /// Version of DWARF to use if not using the default.
+    /// Default supported version of DWARF on this platform.
     /// Useful because some platforms (osx, bsd) only want up to DWARF2.
-    pub dwarf_version: Option<u32>,
+    pub default_dwarf_version: u32,
     /// Whether the linker support GNU-like arguments such as -O. Defaults to true.
     pub linker_is_gnu: bool,
     /// The MinGW toolchain has a known issue that prevents it from correctly
@@ -1509,7 +1509,7 @@ impl Default for TargetOptions {
             is_like_msvc: false,
             is_like_fuchsia: false,
             is_like_wasm: false,
-            dwarf_version: None,
+            default_dwarf_version: 4,
             linker_is_gnu: true,
             allows_weak_linkage: true,
             has_rpath: false,
@@ -1748,13 +1748,13 @@ impl Target {
                     base.$key_name = s;
                 }
             } );
-            ($key_name:ident, Option<u32>) => ( {
+            ($key_name:ident, u32) => ( {
                 let name = (stringify!($key_name)).replace("_", "-");
                 if let Some(s) = obj.remove(&name).and_then(|b| b.as_u64()) {
                     if s < 1 || s > 5 {
                         return Err("Not a valid DWARF version number".into());
                     }
-                    base.$key_name = Some(s as u32);
+                    base.$key_name = s as u32;
                 }
             } );
             ($key_name:ident, Option<u64>) => ( {
@@ -2115,7 +2115,7 @@ impl Target {
         key!(is_like_emscripten, bool);
         key!(is_like_fuchsia, bool);
         key!(is_like_wasm, bool);
-        key!(dwarf_version, Option<u32>);
+        key!(default_dwarf_version, u32);
         key!(linker_is_gnu, bool);
         key!(allows_weak_linkage, bool);
         key!(has_rpath, bool);
@@ -2361,7 +2361,7 @@ impl ToJson for Target {
         target_option_val!(is_like_emscripten);
         target_option_val!(is_like_fuchsia);
         target_option_val!(is_like_wasm);
-        target_option_val!(dwarf_version);
+        target_option_val!(default_dwarf_version);
         target_option_val!(linker_is_gnu);
         target_option_val!(allows_weak_linkage);
         target_option_val!(has_rpath);

compiler/rustc_target/src/spec/netbsd_base.rs

@@ -11,7 +11,7 @@ pub fn opts() -> TargetOptions {
         position_independent_executables: true,
         relro_level: RelroLevel::Full,
         use_ctors_section: true,
-        dwarf_version: Some(2),
+        default_dwarf_version: 2,
         ..Default::default()
     }
 }

compiler/rustc_target/src/spec/openbsd_base.rs

@@ -11,7 +11,7 @@ pub fn opts() -> TargetOptions {
         position_independent_executables: true,
         frame_pointer: FramePointer::Always, // FIXME 43575: should be MayOmit...
         relro_level: RelroLevel::Full,
-        dwarf_version: Some(2),
+        default_dwarf_version: 2,
         ..Default::default()
     }
 }

src/doc/unstable-book/src/compiler-flags/dwarf-version.md

@@ -0,0 +1,9 @@
+## `dwarf-version`
+
+This option controls the version of DWARF that the compiler emits, on platforms
+that use DWARF to encode debug information. It takes one of the following
+values:
+
+* `2`: DWARF version 2 (the default on certain platforms, like macOS).
+* `4`: DWARF version 4 (the default on certain platforms, like Linux).
+* `5`: DWARF version 5.

src/test/assembly/dwarf5.rs

@@ -0,0 +1,20 @@
+// Makes sure that `-Z dwarf-version=5` causes `rustc` to emit DWARF version 5.
+// assembly-output: emit-asm
+// compile-flags: -g --target x86_64-unknown-linux-gnu -Z dwarf-version=5
+// needs-llvm-components: x86
+
+#![feature(no_core, lang_items)]
+#![crate_type = "rlib"]
+#![no_core]
+
+#[lang = "sized"]
+trait Sized {}
+#[lang = "copy"]
+trait Copy {}
+
+pub fn wibble() {}
+
+// CHECK: .section .debug_info
+// CHECK-NOT: .short 2
+// CHECK-NOT: .short 4
+// CHECK: .short 5