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diff --git a/src/bootstrap/defaults/README.md b/src/bootstrap/defaults/README.md
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+# 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
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+++ b/src/bootstrap/defaults/config.toml.codegen
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+# 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
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+++ b/src/bootstrap/defaults/config.toml.compiler
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+# 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
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+++ b/src/bootstrap/defaults/config.toml.library
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+# 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
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+++ b/src/bootstrap/defaults/config.toml.user
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+# 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