make download-rustc="if-unchanged" default for library profile

Signed-off-by: onur-ozkan <work@onurozkan.dev>

src/bootstrap/defaults/config.library.toml

@@ -8,6 +8,9 @@ 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
+# Download rustc from CI instead of building it from source.
+# For stage > 1 builds, this cuts compile times significantly when there are no changes on "compiler" tree.
+download-rustc = "if-unchanged"
 # Make the compiler and standard library faster to build, at the expense of a ~20% runtime slowdown.
 lto = "off"
 

src/bootstrap/defaults/config.tools.toml

@@ -4,7 +4,7 @@
 # This greatly increases the speed of rebuilds, especially when there are only minor changes. However, it makes the initial build slightly slower.
 incremental = true
 # Download rustc from CI instead of building it from source.
-# This cuts compile times by almost 60x, but means you can't modify the compiler.
+# For stage > 1 builds, this cuts compile times significantly when there are no changes on "compiler" tree.
 # Using these defaults will download the stage2 compiler (see `download-rustc`
 # setting) and the stage2 toolchain should therefore be used for these defaults.
 download-rustc = "if-unchanged"