diff --git a/config.example.toml b/config.example.toml
index d3233ad17b511..6dd0088b067c2 100644
--- a/config.example.toml
+++ b/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
diff --git a/src/bootstrap/defaults/config.compiler.toml b/src/bootstrap/defaults/config.compiler.toml
index 57a0ca5a7183b..a737de3bd0856 100644
--- a/src/bootstrap/defaults/config.compiler.toml
+++ b/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]
diff --git a/src/bootstrap/defaults/config.dist.toml b/src/bootstrap/defaults/config.dist.toml
index 4346a9c2dd11a..7b381b416ca8a 100644
--- a/src/bootstrap/defaults/config.dist.toml
+++ b/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
diff --git a/src/bootstrap/defaults/config.library.toml b/src/bootstrap/defaults/config.library.toml
index 67ceb22052409..b43796d6f206b 100644
--- a/src/bootstrap/defaults/config.library.toml
+++ b/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.
diff --git a/src/bootstrap/defaults/config.tools.toml b/src/bootstrap/defaults/config.tools.toml
index 76b47a841b3f2..64097320caba7 100644
--- a/src/bootstrap/defaults/config.tools.toml
+++ b/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.
diff --git a/src/bootstrap/src/core/config/config.rs b/src/bootstrap/src/core/config/config.rs
index e706aba977b6e..9b2d49e762023 100644
--- a/src/bootstrap/src/core/config/config.rs
+++ b/src/bootstrap/src/core/config/config.rs
@@ -1771,8 +1771,37 @@ impl Config {
                 std_features: std_features_toml,
             } = rust;
 
-            config.download_rustc_commit =
-                config.download_ci_rustc_commit(download_rustc, config.llvm_assertions);
+            // FIXME(#133381): alt rustc builds currently do *not* have rustc debug assertions
+            // enabled. We should not download a CI alt rustc if we need rustc to have debug
+            // assertions (e.g. for crashes test suite). This can be changed once something like
+            // [Enable debug assertions on alt
+            // builds](https://github.com/rust-lang/rust/pull/131077) lands.
+            //
+            // Note that `rust.debug = true` currently implies `rust.debug-assertions = true`!
+            //
+            // This relies also on the fact that the global default for `download-rustc` will be
+            // `false` if it's not explicitly set.
+            let debug_assertions_requested = matches!(rustc_debug_assertions_toml, Some(true))
+                || (matches!(debug_toml, Some(true))
+                    && !matches!(rustc_debug_assertions_toml, Some(false)));
+
+            if debug_assertions_requested {
+                if let Some(ref opt) = download_rustc {
+                    if opt.is_string_or_true() {
+                        eprintln!(
+                            "WARN: currently no CI rustc builds have rustc debug assertions \
+                            enabled. Please either set `rust.debug-assertions` to `false` if you \
+                            want to use download CI rustc or set `rust.download-rustc` to `false`."
+                        );
+                    }
+                }
+            }
+
+            config.download_rustc_commit = config.download_ci_rustc_commit(
+                download_rustc,
+                debug_assertions_requested,
+                config.llvm_assertions,
+            );
 
             debug = debug_toml;
             rustc_debug_assertions = rustc_debug_assertions_toml;
@@ -2778,6 +2807,7 @@ impl Config {
     fn download_ci_rustc_commit(
         &self,
         download_rustc: Option<StringOrBool>,
+        debug_assertions_requested: bool,
         llvm_assertions: bool,
     ) -> Option<String> {
         if !is_download_ci_available(&self.build.triple, llvm_assertions) {
@@ -2786,8 +2816,12 @@ 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 `download-rustc` to `false`, because some contributors don't use
+            // profiles for reasons such as:
+            // - They need to seamlessly 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() {
@@ -2845,6 +2879,14 @@ impl Config {
             return None;
         }
 
+        if debug_assertions_requested {
+            eprintln!(
+                "WARN: `rust.debug-assertions = true` will prevent downloading CI rustc as alt CI \
+                rustc is not currently built with debug assertions."
+            );
+            return None;
+        }
+
         Some(commit)
     }
 
diff --git a/src/bootstrap/src/utils/change_tracker.rs b/src/bootstrap/src/utils/change_tracker.rs
index 7f62ffb20db80..41a541d726940 100644
--- a/src/bootstrap/src/utils/change_tracker.rs
+++ b/src/bootstrap/src/utils/change_tracker.rs
@@ -305,4 +305,9 @@ pub const CONFIG_CHANGE_HISTORY: &[ChangeInfo] = &[
         severity: ChangeSeverity::Info,
         summary: "`rust.llvm-tools` is now enabled by default when no `config.toml` is provided.",
     },
+    ChangeInfo {
+        change_id: 133068,
+        severity: ChangeSeverity::Warning,
+        summary: "Revert `rust.download-rustc` global default to `false` and only use `rust.download-rustc = \"if-unchanged\"` default for library and tools profile. As alt CI rustc is built without debug assertions, `rust.debug-assertions = true` will now inhibit downloading CI rustc.",
+    },
 ];
diff --git a/src/ci/docker/host-x86_64/x86_64-gnu-tools/Dockerfile b/src/ci/docker/host-x86_64/x86_64-gnu-tools/Dockerfile
index 17fc1a5749299..2a09cd54b139a 100644
--- a/src/ci/docker/host-x86_64/x86_64-gnu-tools/Dockerfile
+++ b/src/ci/docker/host-x86_64/x86_64-gnu-tools/Dockerfile
@@ -84,7 +84,10 @@ ENV RUST_CONFIGURE_ARGS \
   --enable-new-symbol-mangling
 
 ENV HOST_TARGET x86_64-unknown-linux-gnu
-ENV FORCE_CI_RUSTC 1
+
+# FIXME(#133381): currently rustc alt builds do *not* have rustc debug
+# assertions enabled! Therefore, we cannot force download CI rustc.
+#ENV FORCE_CI_RUSTC 1
 
 COPY host-x86_64/dist-x86_64-linux/shared.sh /scripts/
 COPY host-x86_64/dist-x86_64-linux/build-gccjit.sh /scripts/