Add sample defaults for config.toml

- Allow including defaults in `src/bootstrap/defaults` using `profile = "..."`
- Add default config files
- Combine config files using the merge dependency.
- Add comments to default config files
- Add a README asking to open an issue if the defaults are bad
- Give a loud error if trying to merge `.target`, since it's not
  currently supported
- Use an exhaustive match
- Use `<none>` in config.toml.example to avoid confusion
- Fix bugs in `Merge` derives

Previously, it would completely ignore the profile defaults if there
were any settings in `config.toml`. I sent an email to the `merge` maintainer
asking them to make the behavior in this commit the default.

This introduces a new dependency on `merge` that hasn't yet been vetted.

I want to improve the output when `include = "x"` isn't found:

```
thread 'main' panicked at 'fs::read_to_string(&file) failed with No such file or directory (os error 2) ("configuration file did not exist")', src/bootstrap/config.rs:522:28
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
failed to run: /home/joshua/rustc/build/bootstrap/debug/bootstrap test tidy
Build completed unsuccessfully in 0:00:00
```

However that seems like it could be fixed in a follow-up.

7 files changed, 103 insertions, 25 deletions

diff --git a/src/bootstrap/Cargo.toml b/src/bootstrap/Cargo.toml
index faec2c53742..0177a9dab97 100644
--- a/src/bootstrap/Cargo.toml
+++ b/src/bootstrap/Cargo.toml
@@ -48,6 +48,7 @@ lazy_static = "1.3.0"
 time = "0.1"
 ignore = "0.4.10"
 opener = "0.4"
+merge = "0.1.0"
 
 [target.'cfg(windows)'.dependencies.winapi]
 version = "0.3"
diff --git a/src/bootstrap/config.rs b/src/bootstrap/config.rs
index 7e2cb772186..d925af19a84 100644
--- a/src/bootstrap/config.rs
+++ b/src/bootstrap/config.rs
@@ -16,6 +16,7 @@ use crate::flags::Flags;
 pub use crate::flags::Subcommand;
 use crate::util::exe;
 use build_helper::t;
+use merge::Merge;
 use serde::Deserialize;
 
 macro_rules! check_ci_llvm {
@@ -278,10 +279,31 @@ struct TomlConfig {
     rust: Option<Rust>,
     target: Option<HashMap<String, TomlTarget>>,
     dist: Option<Dist>,
+    profile: Option<String>,
+}
+
+impl Merge for TomlConfig {
+    fn merge(&mut self, TomlConfig { build, install, llvm, rust, dist, target, profile: _ }: Self) {
+        fn do_merge<T: Merge>(x: &mut Option<T>, y: Option<T>) {
+            if let Some(new) = y {
+                if let Some(original) = x {
+                    original.merge(new);
+                } else {
+                    *x = Some(new);
+                }
+            }
+        };
+        do_merge(&mut self.build, build);
+        do_merge(&mut self.install, install);
+        do_merge(&mut self.llvm, llvm);
+        do_merge(&mut self.rust, rust);
+        do_merge(&mut self.dist, dist);
+        assert!(target.is_none(), "merging target-specific config is not currently supported");
+    }
 }
 
 /// TOML representation of various global build decisions.
-#[derive(Deserialize, Default, Clone)]
+#[derive(Deserialize, Default, Clone, Merge)]
 #[serde(deny_unknown_fields, rename_all = "kebab-case")]
 struct Build {
     build: Option<String>,
@@ -321,7 +343,7 @@ struct Build {
 }
 
 /// TOML representation of various global install decisions.
-#[derive(Deserialize, Default, Clone)]
+#[derive(Deserialize, Default, Clone, Merge)]
 #[serde(deny_unknown_fields, rename_all = "kebab-case")]
 struct Install {
     prefix: Option<String>,
@@ -338,7 +360,7 @@ struct Install {
 }
 
 /// TOML representation of how the LLVM build is configured.
-#[derive(Deserialize, Default)]
+#[derive(Deserialize, Default, Merge)]
 #[serde(deny_unknown_fields, rename_all = "kebab-case")]
 struct Llvm {
     skip_rebuild: Option<bool>,
@@ -365,7 +387,7 @@ struct Llvm {
     download_ci_llvm: Option<bool>,
 }
 
-#[derive(Deserialize, Default, Clone)]
+#[derive(Deserialize, Default, Clone, Merge)]
 #[serde(deny_unknown_fields, rename_all = "kebab-case")]
 struct Dist {
     sign_folder: Option<String>,
@@ -389,7 +411,7 @@ impl Default for StringOrBool {
 }
 
 /// TOML representation of how the Rust build is configured.
-#[derive(Deserialize, Default)]
+#[derive(Deserialize, Default, Merge)]
 #[serde(deny_unknown_fields, rename_all = "kebab-case")]
 struct Rust {
     optimize: Option<bool>,
@@ -434,7 +456,7 @@ struct Rust {
 }
 
 /// TOML representation of how each build target is configured.
-#[derive(Deserialize, Default)]
+#[derive(Deserialize, Default, Merge)]
 #[serde(deny_unknown_fields, rename_all = "kebab-case")]
 struct TomlTarget {
     cc: Option<String>,
@@ -523,27 +545,31 @@ impl Config {
         }
 
         #[cfg(test)]
-        let toml = TomlConfig::default();
+        let get_toml = |_| TomlConfig::default();
         #[cfg(not(test))]
-        let toml = flags
-            .config
-            .map(|file| {
-                use std::process;
-
-                let contents = t!(fs::read_to_string(&file));
-                match toml::from_str(&contents) {
-                    Ok(table) => table,
-                    Err(err) => {
-                        println!(
-                            "failed to parse TOML configuration '{}': {}",
-                            file.display(),
-                            err
-                        );
-                        process::exit(2);
-                    }
+        let get_toml = |file: PathBuf| {
+            use std::process;
+
+            let contents = t!(fs::read_to_string(&file), "configuration file did not exist");
+            match toml::from_str(&contents) {
+                Ok(table) => table,
+                Err(err) => {
+                    println!("failed to parse TOML configuration '{}': {}", file.display(), err);
+                    process::exit(2);
                 }
-            })
-            .unwrap_or_else(TomlConfig::default);
+            }
+        };
+
+        let mut toml = flags.config.map(get_toml).unwrap_or_else(TomlConfig::default);
+        if let Some(include) = &toml.profile {
+            let mut include_path = config.src.clone();
+            include_path.push("src");
+            include_path.push("bootstrap");
+            include_path.push("defaults");
+            include_path.push(format!("config.toml.{}", include));
+            let included_toml = get_toml(include_path);
+            toml.merge(included_toml);
+        }
 
         let build = toml.build.unwrap_or_default();
 
diff --git a/src/bootstrap/defaults/README.md b/src/bootstrap/defaults/README.md
new file mode 100644
index 00000000000..a91fc3538eb
--- /dev/null
+++ b/src/bootstrap/defaults/README.md
@@ -0,0 +1,11 @@
+# About bootstrap defaults
+
+These defaults are intended to be a good starting point for working with x.py,
+with the understanding that no one set of defaults make sense for everyone.
+
+They are still experimental, and we'd appreciate your help improving them!
+If you use a setting that's not in these defaults that you think others would benefit from, please [file an issue] or make a PR with the changes.
+Similarly, if one of these defaults doesn't match what you use personally,
+please open an issue to get it changed.
+
+[file an issue]: https://github.com/rust-lang/rust/issues/new/choose
diff --git a/src/bootstrap/defaults/config.toml.codegen b/src/bootstrap/defaults/config.toml.codegen
new file mode 100644
index 00000000000..a9505922ca7
--- /dev/null
+++ b/src/bootstrap/defaults/config.toml.codegen
@@ -0,0 +1,13 @@
+# These defaults are meant for contributors to the compiler who modify codegen or LLVM
+[llvm]
+# This enables debug-assertions in LLVM,
+# catching logic errors in codegen much earlier in the process.
+assertions = true
+
+[rust]
+# This enables `RUSTC_LOG=debug`, avoiding confusing situations
+# where adding `debug!()` appears to do nothing.
+# However, it makes running the compiler slightly slower.
+debug-logging = true
+# This greatly increases the speed of rebuilds, especially when there are only minor changes. However, it makes the initial build slightly slower.
+incremental = true
diff --git a/src/bootstrap/defaults/config.toml.compiler b/src/bootstrap/defaults/config.toml.compiler
new file mode 100644
index 00000000000..4772de8a2cb
--- /dev/null
+++ b/src/bootstrap/defaults/config.toml.compiler
@@ -0,0 +1,8 @@
+# These defaults are meant for contributors to the compiler who do not modify codegen or LLVM
+[rust]
+# This enables `RUSTC_LOG=debug`, avoiding confusing situations
+# where adding `debug!()` appears to do nothing.
+# However, it makes running the compiler slightly slower.
+debug-logging = true
+# This greatly increases the speed of rebuilds, especially when there are only minor changes. However, it makes the initial build slightly slower.
+incremental = true
diff --git a/src/bootstrap/defaults/config.toml.library b/src/bootstrap/defaults/config.toml.library
new file mode 100644
index 00000000000..e4316f4d864
--- /dev/null
+++ b/src/bootstrap/defaults/config.toml.library
@@ -0,0 +1,10 @@
+# These defaults are meant for contributors to the standard library and documentation.
+[build]
+# When building the standard library, you almost never want to build the compiler itself.
+build-stage = 0
+test-stage = 0
+bench-stage = 0
+
+[rust]
+# This greatly increases the speed of rebuilds, especially when there are only minor changes. However, it makes the initial build slightly slower.
+incremental = true
diff --git a/src/bootstrap/defaults/config.toml.user b/src/bootstrap/defaults/config.toml.user
new file mode 100644
index 00000000000..6647061d88f
--- /dev/null
+++ b/src/bootstrap/defaults/config.toml.user
@@ -0,0 +1,9 @@
+# These defaults are meant for users and distro maintainers building from source, without intending to make multiple changes.
+[build]
+# When compiling from source, you almost always want a full stage 2 build,
+# which has all the latest optimizations from nightly.
+build-stage = 2
+test-stage = 2
+doc-stage = 2
+# When compiling from source, you usually want all tools.
+extended = true