Turn off debuginfo for build dependencies to improve compile times

src/cargo/core/compiler/custom_build.rs

@@ -399,7 +399,20 @@ fn build_work(cx: &mut Context<'_, '_>, unit: &Unit) -> CargoResult<Job> {
                 },
                 true,
             )
-            .with_context(|| format!("failed to run custom build command for `{}`", pkg_descr));
+            .with_context(|| {
+                let mut build_error_context = format!("failed to run custom build command for `{}`", pkg_descr);
+
+                // If we're opting into backtraces, mention that build dependencies' backtraces can
+                // be improved by setting a higher debuginfo level.
+                if let Ok(show_backtraces) = std::env::var("RUST_BACKTRACE") {
+                    if show_backtraces != "0" {
+                        build_error_context.push_str("\n\
+                            note: To improve backtraces for build dependencies, make sure full debug info is turned on. More details at https://doc.rust-lang.org/cargo/reference/profiles.html#build-dependencies");
+                    }
+                }
+
+                build_error_context
+            });
 
         if let Err(error) = output {
             insert_warnings_in_build_outputs(

src/cargo/core/profiles.rs

@@ -408,8 +408,11 @@ impl ProfileMaker {
             // basically turning down the optimization level and avoid limiting
             // codegen units. This ensures that we spend little time optimizing as
             // well as enabling parallelism by not constraining codegen units.
+            // Turning off debuginfo also allows the compiler to noticeably perform
+            // less work.
             profile.opt_level = InternedString::new("0");
             profile.codegen_units = None;
+            profile.debuginfo = None;
         }
         // ... and next comes any other sorts of overrides specified in
         // profiles, such as `[profile.release.build-override]` or

src/doc/src/reference/profiles.md

@@ -298,18 +298,27 @@ The `bench` profile inherits the settings from the [`release`](#release) profile
 
 #### Build Dependencies
 
-All profiles, by default, do not optimize build dependencies (build scripts,
-proc macros, and their dependencies). The default settings for build overrides
-are:
+To compile quickly, all profiles, by default, do not optimize build
+dependencies (build scripts, proc macros, and their dependencies), and avoid
+computing debug info. The default settings for build overrides are:
 
 ```toml
 [profile.dev.build-override]
 opt-level = 0
 codegen-units = 256
+debug = false
 
 [profile.release.build-override]
 opt-level = 0
 codegen-units = 256
+debug = false
+```
+
+However, if errors occur while running build dependencies, turning full debug
+info on will improve backtraces and debuggability when needed:
+
+```toml
+debug = true
 ```
 
 Build dependencies otherwise inherit settings from the active profile in use, as

tests/testsuite/build.rs

@@ -3777,7 +3777,7 @@ fn compiler_json_error_format() {
                 },
                 "profile": {
                     "debug_assertions": true,
-                    "debuginfo": 2,
+                    "debuginfo": null,
                     "opt_level": "0",
                     "overflow_checks": true,
                     "test": false

tests/testsuite/build_script.rs

@@ -44,6 +44,43 @@ Caused by:
         .run();
 }
 
+#[cargo_test]
+fn custom_build_script_failed_backtraces_message() {
+    // debuginfo is turned off by default in `dev.build-override`. However,
+    // if an error occurs running e.g. a build script, a message explaining
+    // how to improve backtraces is also displayed.
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [project]
+
+                name = "foo"
+                version = "0.5.0"
+                authors = ["[email protected]"]
+                build = "build.rs"
+            "#,
+        )
+        .file("src/main.rs", "fn main() {}")
+        .file("build.rs", "fn main() { std::process::exit(101); }")
+        .build();
+    p.cargo("build -v")
+        .env("RUST_BACKTRACE", "1")
+        .with_status(101)
+        .with_stderr(
+            "\
+[COMPILING] foo v0.5.0 ([CWD])
+[RUNNING] `rustc --crate-name build_script_build build.rs [..]--crate-type bin [..]`
+[RUNNING] `[..]/build-script-build`
+[ERROR] failed to run custom build command for `foo v0.5.0 ([CWD])`
+note: To improve backtraces for build dependencies[..]
+
+Caused by:
+  process didn't exit successfully: `[..]/build-script-build` (exit [..]: 101)",
+        )
+        .run();
+}
+
 #[cargo_test]
 fn custom_build_env_vars() {
     let p = project()
@@ -1629,14 +1666,14 @@ fn build_cmd_with_a_build_cmd() {
 [RUNNING] `rustc [..] a/build.rs [..] --extern b=[..]`
 [RUNNING] `[..]/a-[..]/build-script-build`
 [RUNNING] `rustc --crate-name a [..]lib.rs [..]--crate-type lib \
-    --emit=[..]link[..]-C debuginfo=2 \
+    --emit=[..]link[..] \
     -C metadata=[..] \
     --out-dir [..]target/debug/deps \
     -L [..]target/debug/deps`
 [COMPILING] foo v0.5.0 ([CWD])
 [RUNNING] `rustc --crate-name build_script_build build.rs [..]--crate-type bin \
     --emit=[..]link[..]\
-    -C debuginfo=2 -C metadata=[..] --out-dir [..] \
+    -C metadata=[..] --out-dir [..] \
     -L [..]target/debug/deps \
     --extern a=[..]liba[..].rlib`
 [RUNNING] `[..]/foo-[..]/build-script-build`
@@ -4379,6 +4416,10 @@ fn optional_build_script_dep() {
 
 #[cargo_test]
 fn optional_build_dep_and_required_normal_dep() {
+    // Note: `dev` and `dev.build-override` have different defaults. `bar` would
+    // be built twice in the general case: once without debuginfo and once with
+    // debuginfo = 2. Setting `debug = 2` in `dev.build-override` ensures it's
+    // only built once in this test.
     let p = project()
         .file(
             "Cargo.toml",
@@ -4393,6 +4434,9 @@ fn optional_build_dep_and_required_normal_dep() {
 
             [build-dependencies]
             bar = { path = "./bar" }
+
+            [profile.dev.build-override]
+            debug = 2 # see note above
             "#,
         )
         .file("build.rs", "extern crate bar; fn main() { bar::bar(); }")

tests/testsuite/dep_info.rs

@@ -238,6 +238,11 @@ fn relative_depinfo_paths_ws() {
 
     // Test relative dep-info paths in a workspace with --target with
     // proc-macros and other dependency kinds.
+    //
+    // Note: `dev` and `dev.build-override` have different defaults. `pmdep`
+    // would be built twice in the general case: once without debuginfo and once
+    // with debuginfo = 2. Setting `debug = 2` in `dev.build-override` ensures
+    // it will only create a single dep-info file.
     Package::new("regdep", "0.1.0")
         .file("src/lib.rs", "pub fn f() {}")
         .publish();
@@ -255,6 +260,9 @@ fn relative_depinfo_paths_ws() {
             r#"
             [workspace]
             members = ["foo"]
+
+            [profile.dev.build-override]
+            debug = 2 # see note above
             "#,
         )
         /*********** Main Project ***********/

tests/testsuite/features2.rs

@@ -1050,13 +1050,21 @@ fn decouple_proc_macro() {
 #[cargo_test]
 fn proc_macro_ws() {
     // Checks for bug with proc-macro in a workspace with dependency (shouldn't panic).
+    //
+    // Note, `dev` and `dev.build-override` have different defaults. `pm` would
+    // be built twice in the general case: once without debuginfo and once with
+    // debuginfo = 2. Setting `debug = 2` in `dev.build-override` ensures it's
+    // only built once and `foo` remains fresh during the second check build.
     let p = project()
         .file(
             "Cargo.toml",
             r#"
             [workspace]
             members = ["foo", "pm"]
             resolver = "2"
+
+            [profile.dev.build-override]
+            debug = 2 # see note above
             "#,
         )
         .file(
@@ -2161,13 +2169,21 @@ fn pm_with_int_shared() {
     // with `--workspace`, see https://github.com/rust-lang/cargo/issues/8312.
     //
     // This uses a const-eval hack to do compile-time feature checking.
+    //
+    // Note, `dev` and `dev.build-override` have different defaults. `pm` would
+    // be built twice in the general case: once without debuginfo and once with
+    // debuginfo = 2. Setting `debug = 2` in `dev.build-override` ensures it's
+    // only built once in this test.
     let p = project()
         .file(
             "Cargo.toml",
             r#"
                 [workspace]
                 members = ["foo", "pm", "shared"]
                 resolver = "2"
+
+                [profile.dev.build-override]
+                debug = 2 # see note above
             "#,
         )
         .file(

tests/testsuite/freshness.rs

@@ -1344,6 +1344,10 @@ fn fingerprint_cleaner_does_not_rebuild() {
 
 #[cargo_test]
 fn reuse_panic_build_dep_test() {
+    // Note: `dev` and `dev.build-override` have different defaults. `bar` would
+    // be built twice in the general case: once without debuginfo and once with
+    // debuginfo = 2. Setting `debug = 2` in `dev.build-override` ensures it's
+    // only built once in this test.
     let p = project()
         .file(
             "Cargo.toml",
@@ -1360,6 +1364,9 @@ fn reuse_panic_build_dep_test() {
 
                 [profile.dev]
                 panic = "abort"
+
+                [profile.dev.build-override]
+                debug = 2 # see note above
             "#,
         )
         .file("src/lib.rs", "")
@@ -1431,7 +1438,7 @@ fn reuse_panic_pm() {
         .with_stderr_unordered(
             "\
 [COMPILING] bar [..]
-[RUNNING] `rustc --crate-name bar bar/src/lib.rs [..]--crate-type lib --emit=[..]link[..]-C debuginfo=2 [..]
+[RUNNING] `rustc --crate-name bar bar/src/lib.rs [..]--crate-type lib --emit=[..]link[..]
 [RUNNING] `rustc --crate-name bar bar/src/lib.rs [..]--crate-type lib --emit=[..]link -C panic=abort[..]-C debuginfo=2 [..]
 [COMPILING] somepm [..]
 [RUNNING] `rustc --crate-name somepm [..]

tests/testsuite/proc_macro.rs

@@ -498,13 +498,20 @@ fn proc_macro_extern_prelude() {
 
 #[cargo_test]
 fn proc_macro_built_once() {
+    // Note: `dev` and `dev.build-override` have different defaults. `the-macro`
+    // would be built twice in the general case: once without debuginfo and once
+    // with debuginfo = 2. Setting `debug = 2` in `dev.build-override` ensures
+    // it's only built once in this test.
     let p = project()
         .file(
             "Cargo.toml",
             r#"
                 [workspace]
                 members = ['a', 'b']
                 resolver = "2"
+
+                [profile.dev.build-override]
+                debug = 2 # see note above
             "#,
         )
         .file(

tests/testsuite/profile_config.rs

@@ -384,7 +384,7 @@ fn named_config_profile() {
     assert_eq!(bo.name, "foo");
     assert_eq!(bo.codegen_units, Some(6)); // "foo" build override from config
     assert_eq!(bo.opt_level, "0"); // default to zero
-    assert_eq!(bo.debuginfo, Some(1)); // SAME as normal
+    assert_eq!(bo.debuginfo, None); // default to none
     assert_eq!(bo.debug_assertions, false); // "foo" build override from manifest
     assert_eq!(bo.overflow_checks, true); // SAME as normal
 

tests/testsuite/profile_overrides.rs

@@ -135,6 +135,9 @@ fn profile_override_bad_settings() {
 #[cargo_test]
 fn profile_override_hierarchy() {
     // Test that the precedence rules are correct for different types.
+    // Note, `dev` and `dev.build-override` have different defaults. `dep` would be built twice in
+    // the general case: once without debuginfo and once with debuginfo = 2. Setting `debug = 2` in
+    // `dev.build-override` ensures it's only built once in this test.
     let p = project()
         .file(
             "Cargo.toml",
@@ -153,6 +156,7 @@ fn profile_override_hierarchy() {
 
             [profile.dev.build-override]
             codegen-units = 4
+            debug = 2 # see note above
             "#,
         )
         // m1

tests/testsuite/profile_targets.rs

@@ -85,11 +85,11 @@ fn profile_selection_build() {
     p.cargo("build -vv").with_stderr_unordered("\
 [COMPILING] bar [..]
 [RUNNING] `[..] rustc --crate-name bar bar/src/lib.rs [..]--crate-type lib --emit=[..]link -C panic=abort[..]-C codegen-units=1 -C debuginfo=2 [..]
-[RUNNING] `[..] rustc --crate-name bar bar/src/lib.rs [..]--crate-type lib --emit=[..]link[..]-C codegen-units=5 -C debuginfo=2 [..]
+[RUNNING] `[..] rustc --crate-name bar bar/src/lib.rs [..]--crate-type lib --emit=[..]link[..]-C codegen-units=5 [..]
 [COMPILING] bdep [..]
-[RUNNING] `[..] rustc --crate-name bdep bdep/src/lib.rs [..]--crate-type lib --emit=[..]link[..]-C codegen-units=5 -C debuginfo=2 [..]
+[RUNNING] `[..] rustc --crate-name bdep bdep/src/lib.rs [..]--crate-type lib --emit=[..]link[..]-C codegen-units=5 [..]
 [COMPILING] foo [..]
-[RUNNING] `[..] rustc --crate-name build_script_build build.rs [..]--crate-type bin --emit=[..]link[..]-C codegen-units=5 -C debuginfo=2 [..]
+[RUNNING] `[..] rustc --crate-name build_script_build build.rs [..]--crate-type bin --emit=[..]link[..]-C codegen-units=5 [..]
 [RUNNING] `[..]/target/debug/build/foo-[..]/build-script-build`
 [foo 0.0.1] foo custom build PROFILE=debug DEBUG=true OPT_LEVEL=0
 [RUNNING] `[..] rustc --crate-name foo src/lib.rs [..]--crate-type lib --emit=[..]link -C panic=abort[..]-C codegen-units=1 -C debuginfo=2 [..]
@@ -173,11 +173,11 @@ fn profile_selection_build_all_targets() {
 [COMPILING] bar [..]
 [RUNNING] `[..] rustc --crate-name bar bar/src/lib.rs [..]--crate-type lib --emit=[..]link[..]-C codegen-units=1 -C debuginfo=2 [..]
 [RUNNING] `[..] rustc --crate-name bar bar/src/lib.rs [..]--crate-type lib --emit=[..]link -C panic=abort[..]-C codegen-units=1 -C debuginfo=2 [..]
-[RUNNING] `[..] rustc --crate-name bar bar/src/lib.rs [..]--crate-type lib --emit=[..]link[..]-C codegen-units=5 -C debuginfo=2 [..]
+[RUNNING] `[..] rustc --crate-name bar bar/src/lib.rs [..]--crate-type lib --emit=[..]link[..]-C codegen-units=5 [..]
 [COMPILING] bdep [..]
-[RUNNING] `[..] rustc --crate-name bdep bdep/src/lib.rs [..]--crate-type lib --emit=[..]link[..]-C codegen-units=5 -C debuginfo=2 [..]
+[RUNNING] `[..] rustc --crate-name bdep bdep/src/lib.rs [..]--crate-type lib --emit=[..]link[..]-C codegen-units=5 [..]
 [COMPILING] foo [..]
-[RUNNING] `[..] rustc --crate-name build_script_build build.rs [..]--crate-type bin --emit=[..]link[..]-C codegen-units=5 -C debuginfo=2 [..]
+[RUNNING] `[..] rustc --crate-name build_script_build build.rs [..]--crate-type bin --emit=[..]link[..]-C codegen-units=5 [..]
 [RUNNING] `[..]/target/debug/build/foo-[..]/build-script-build`
 [foo 0.0.1] foo custom build PROFILE=debug DEBUG=true OPT_LEVEL=0
 [RUNNING] `[..] rustc --crate-name foo src/lib.rs [..]--crate-type lib --emit=[..]link -C panic=abort[..]-C codegen-units=1 -C debuginfo=2 [..]`
@@ -295,12 +295,12 @@ fn profile_selection_test() {
     p.cargo("test -vv").with_stderr_unordered("\
 [COMPILING] bar [..]
 [RUNNING] `[..] rustc --crate-name bar bar/src/lib.rs [..]--crate-type lib --emit=[..]link[..]-C codegen-units=3 -C debuginfo=2 [..]
-[RUNNING] `[..] rustc --crate-name bar bar/src/lib.rs [..]--crate-type lib --emit=[..]link[..]-C codegen-units=5 -C debuginfo=2 [..]
+[RUNNING] `[..] rustc --crate-name bar bar/src/lib.rs [..]--crate-type lib --emit=[..]link[..]-C codegen-units=5 [..]
 [RUNNING] `[..] rustc --crate-name bar bar/src/lib.rs [..]--crate-type lib --emit=[..]link -C panic=abort[..]-C codegen-units=3 -C debuginfo=2 [..]
 [COMPILING] bdep [..]
-[RUNNING] `[..] rustc --crate-name bdep bdep/src/lib.rs [..]--crate-type lib --emit=[..]link[..]-C codegen-units=5 -C debuginfo=2 [..]
+[RUNNING] `[..] rustc --crate-name bdep bdep/src/lib.rs [..]--crate-type lib --emit=[..]link[..]-C codegen-units=5 [..]
 [COMPILING] foo [..]
-[RUNNING] `[..] rustc --crate-name build_script_build build.rs [..]--crate-type bin --emit=[..]link[..]-C codegen-units=5 -C debuginfo=2 [..]
+[RUNNING] `[..] rustc --crate-name build_script_build build.rs [..]--crate-type bin --emit=[..]link[..]-C codegen-units=5 [..]
 [RUNNING] `[..]/target/debug/build/foo-[..]/build-script-build`
 [foo 0.0.1] foo custom build PROFILE=debug DEBUG=true OPT_LEVEL=0
 [RUNNING] `[..] rustc --crate-name foo src/lib.rs [..]--crate-type lib --emit=[..]link -C panic=abort[..]-C codegen-units=3 -C debuginfo=2 [..]
@@ -492,13 +492,13 @@ fn profile_selection_check_all_targets() {
     //
     p.cargo("check --all-targets -vv").with_stderr_unordered("\
 [COMPILING] bar [..]
-[RUNNING] `[..] rustc --crate-name bar bar/src/lib.rs [..]--crate-type lib --emit=[..]link[..]-C codegen-units=5 -C debuginfo=2 [..]
+[RUNNING] `[..] rustc --crate-name bar bar/src/lib.rs [..]--crate-type lib --emit=[..]link[..]-C codegen-units=5 [..]
 [RUNNING] `[..] rustc --crate-name bar bar/src/lib.rs [..]--crate-type lib --emit=[..]metadata[..]-C codegen-units=1 -C debuginfo=2 [..]
 [RUNNING] `[..] rustc --crate-name bar bar/src/lib.rs [..]--crate-type lib --emit=[..]metadata -C panic=abort[..]-C codegen-units=1 -C debuginfo=2 [..]
 [COMPILING] bdep[..]
-[RUNNING] `[..] rustc --crate-name bdep bdep/src/lib.rs [..]--crate-type lib --emit=[..]link[..]-C codegen-units=5 -C debuginfo=2 [..]
+[RUNNING] `[..] rustc --crate-name bdep bdep/src/lib.rs [..]--crate-type lib --emit=[..]link[..]-C codegen-units=5 [..]
 [COMPILING] foo [..]
-[RUNNING] `[..] rustc --crate-name build_script_build build.rs [..]--crate-type bin --emit=[..]link[..]-C codegen-units=5 -C debuginfo=2 [..]
+[RUNNING] `[..] rustc --crate-name build_script_build build.rs [..]--crate-type bin --emit=[..]link[..]-C codegen-units=5 [..]
 [RUNNING] `[..]target/debug/build/foo-[..]/build-script-build`
 [foo 0.0.1] foo custom build PROFILE=debug DEBUG=true OPT_LEVEL=0
 [RUNNING] `[..] rustc --crate-name foo src/lib.rs [..]--crate-type lib --emit=[..]metadata -C panic=abort[..]-C codegen-units=1 -C debuginfo=2 [..]
@@ -596,12 +596,12 @@ fn profile_selection_check_all_targets_test() {
     //
     p.cargo("check --all-targets --profile=test -vv").with_stderr_unordered("\
 [COMPILING] bar [..]
-[RUNNING] `[..] rustc --crate-name bar bar/src/lib.rs [..]--crate-type lib --emit=[..]link[..]-C codegen-units=5 -C debuginfo=2 [..]
+[RUNNING] `[..] rustc --crate-name bar bar/src/lib.rs [..]--crate-type lib --emit=[..]link[..]-C codegen-units=5 [..]
 [RUNNING] `[..] rustc --crate-name bar bar/src/lib.rs [..]--crate-type lib --emit=[..]metadata[..]-C codegen-units=3 -C debuginfo=2 [..]
 [COMPILING] bdep[..]
-[RUNNING] `[..] rustc --crate-name bdep bdep/src/lib.rs [..]--crate-type lib --emit=[..]link[..]-C codegen-units=5 -C debuginfo=2 [..]
+[RUNNING] `[..] rustc --crate-name bdep bdep/src/lib.rs [..]--crate-type lib --emit=[..]link[..]-C codegen-units=5 [..]
 [COMPILING] foo [..]
-[RUNNING] `[..] rustc --crate-name build_script_build build.rs [..]--crate-type bin --emit=[..]link[..]-C codegen-units=5 -C debuginfo=2 [..]
+[RUNNING] `[..] rustc --crate-name build_script_build build.rs [..]--crate-type bin --emit=[..]link[..]-C codegen-units=5 [..]
 [RUNNING] `[..]target/debug/build/foo-[..]/build-script-build`
 [foo 0.0.1] foo custom build PROFILE=debug DEBUG=true OPT_LEVEL=0
 [RUNNING] `[..] rustc --crate-name foo src/lib.rs [..]--crate-type lib --emit=[..]metadata[..]-C codegen-units=3 -C debuginfo=2 [..]
@@ -642,13 +642,13 @@ fn profile_selection_doc() {
     p.cargo("doc -vv").with_stderr_unordered("\
 [COMPILING] bar [..]
 [DOCUMENTING] bar [..]
-[RUNNING] `[..] rustc --crate-name bar bar/src/lib.rs [..]--crate-type lib --emit=[..]link[..]-C codegen-units=5 -C debuginfo=2 [..]
+[RUNNING] `[..] rustc --crate-name bar bar/src/lib.rs [..]--crate-type lib --emit=[..]link[..]-C codegen-units=5 [..]
 [RUNNING] `rustdoc [..]--crate-name bar bar/src/lib.rs [..]
 [RUNNING] `[..] rustc --crate-name bar bar/src/lib.rs [..]--crate-type lib --emit=[..]metadata -C panic=abort[..]-C codegen-units=1 -C debuginfo=2 [..]
 [COMPILING] bdep [..]
-[RUNNING] `[..] rustc --crate-name bdep bdep/src/lib.rs [..]--crate-type lib --emit=[..]link[..]-C codegen-units=5 -C debuginfo=2 [..]
+[RUNNING] `[..] rustc --crate-name bdep bdep/src/lib.rs [..]--crate-type lib --emit=[..]link[..]-C codegen-units=5 [..]
 [COMPILING] foo [..]
-[RUNNING] `[..] rustc --crate-name build_script_build build.rs [..]--crate-type bin --emit=[..]link[..]-C codegen-units=5 -C debuginfo=2 [..]
+[RUNNING] `[..] rustc --crate-name build_script_build build.rs [..]--crate-type bin --emit=[..]link[..]-C codegen-units=5 [..]
 [RUNNING] `[..]target/debug/build/foo-[..]/build-script-build`
 [foo 0.0.1] foo custom build PROFILE=debug DEBUG=true OPT_LEVEL=0
 [DOCUMENTING] foo [..]