bootstrap: revert rust.download-rustc global default to false

And only default library and tools profile to `rust.download-rustc =
"if-unchanged"`.

Author: jieyouxu

config.example.toml

@@ -496,15 +496,18 @@
 #
 #debug = false
 
-# Whether to download the stage 1 and 2 compilers from CI.
-# This is useful if you are working on tools, doc-comments, or library (you will be able to build
-# the standard library without needing to build the compiler).
+# Whether to download the stage 1 and 2 compilers from CI. This is useful if you
+# are working on tools, doc-comments, or library (you will be able to build the
+# standard library without needing to build the compiler).
 #
-# Set this to "if-unchanged" if you are working on `src/tools`, `tests` or `library` (on CI, `library`
-# changes triggers in-tree compiler build) to speed up the build process.
+# Set this to "if-unchanged" if you are working on `src/tools`, `tests` or
+# `library` (on CI, `library` changes triggers in-tree compiler build) to speed
+# up the build process if you don't need to build a compiler from the latest
+# commit from `master`.
 #
-# Set this to `true` to always download or `false` to always use the in-tree compiler.
-#download-rustc = "if-unchanged"
+# Set this to `true` to always download or `false` to always use the in-tree
+# compiler.
+#download-rustc = false
 
 # Number of codegen units to use for each compiler invocation. A value of 0
 # means "the number of cores on this machine", and 1+ is passed through to the

src/bootstrap/defaults/config.compiler.toml

@@ -19,6 +19,9 @@ lto = "off"
 # Forces frame pointers to be used with `-Cforce-frame-pointers`.
 # This can be helpful for profiling at a small performance cost.
 frame-pointers = true
+# Compiler contributors often want to build rustc even without any changes to
+# e.g. check that it builds locally and check the baseline behavior of a
+# compiler built from latest `master` commit.
 download-rustc = false
 
 [llvm]

src/bootstrap/defaults/config.dist.toml

@@ -16,6 +16,7 @@ download-ci-llvm = false
 # We have several defaults in bootstrap that depend on whether the channel is `dev` (e.g. `omit-git-hash` and `download-ci-llvm`).
 # Make sure they don't get set when installing from source.
 channel = "nightly"
+# Never download a rustc, distributions must build a fresh compiler.
 download-rustc = false
 lld = true
 # Build the llvm-bitcode-linker

src/bootstrap/defaults/config.library.toml

@@ -10,7 +10,9 @@ bench-stage = 0
 incremental = true
 # Make the compiler and standard library faster to build, at the expense of a ~20% runtime slowdown.
 lto = "off"
-download-rustc = false
+# Download rustc by default for library profile if compiler-affecting
+# directories are not modified. For CI this is disabled.
+download-rustc = "if-unchanged"
 
 [llvm]
 # Will download LLVM from CI if available on your platform.

src/bootstrap/defaults/config.tools.toml

@@ -3,6 +3,9 @@
 [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
+# Most commonly, tools contributors do not need to modify the compiler, so
+# downloading a CI rustc is a good default for tools profile.
+download-rustc = "if-unchanged"
 
 [build]
 # Document with the in-tree rustdoc by default, since `download-rustc` makes it quick to compile.

src/bootstrap/src/core/config/config.rs

@@ -2786,8 +2786,12 @@ impl Config {
 
         // If `download-rustc` is not set, default to rebuilding.
         let if_unchanged = match download_rustc {
-            None => self.rust_info.is_managed_git_subrepository(),
-            Some(StringOrBool::Bool(false)) => return None,
+            // Globally default for `download-rustc` to `false`, because some contributors don't use
+            // profiles for reasons such as:
+            // - They need to seemlessly switch between compiler/library work.
+            // - They don't want to use compiler profile because they need to override too many
+            //   things and it's easier to not use a profile.
+            None | Some(StringOrBool::Bool(false)) => return None,
             Some(StringOrBool::Bool(true)) => false,
             Some(StringOrBool::String(s)) if s == "if-unchanged" => {
                 if !self.rust_info.is_managed_git_subrepository() {