Warn if an already loaded package is attempted to be loaded from a different path (#44329)

Author: KristofferC

base/loading.jl

@@ -1502,6 +1502,7 @@ function _tryrequire_from_serialized(modkey::PkgId, build_id::UInt128)
     assert_havelock(require_lock)
     loaded = nothing
     if root_module_exists(modkey)
+        warn_if_already_loaded_different(modkey)
         loaded = root_module(modkey)
     else
         loaded = start_loading(modkey)
@@ -1532,6 +1533,7 @@ function _tryrequire_from_serialized(modkey::PkgId, path::String, ocachepath::Un
     assert_havelock(require_lock)
     loaded = nothing
     if root_module_exists(modkey)
+        warn_if_already_loaded_different(modkey)
         loaded = root_module(modkey)
     else
         loaded = start_loading(modkey)
@@ -1975,11 +1977,50 @@ function __require_prelocked(uuidkey::PkgId, env=nothing)
         # After successfully loading, notify downstream consumers
         run_package_callbacks(uuidkey)
     else
+        warn_if_already_loaded_different(uuidkey)
         newm = root_module(uuidkey)
     end
     return newm
 end
 
+const already_warned_path_change_pkgs = Set{UUID}()
+# warns if the loaded version of a module is different to the one that locate_package wants to load
+function warn_if_already_loaded_different(uuidkey::PkgId)
+    uuidkey.uuid ∈ already_warned_path_change_pkgs && return
+    pkgorig = get(pkgorigins, uuidkey, nothing)
+    if pkgorig !== nothing && pkgorig.path !== nothing
+        new_path = locate_package(uuidkey)
+        if !samefile(fixup_stdlib_path(pkgorig.path), new_path)
+            if isnothing(pkgorig.version)
+                v = get_pkgversion_from_path(dirname(dirname(pkgorig.path)))
+                cur_vstr = isnothing(v) ? "" : "v$v "
+            else
+                cur_vstr = "v$v "
+            end
+            new_v = get_pkgversion_from_path(dirname(dirname(new_path)))
+            new_vstr = isnothing(new_v) ? "" : "v$new_v "
+            warnstr = """
+            $uuidkey is already loaded from a different path:
+              loaded:    $cur_vstr$(repr(pkgorig.path))
+              requested: $new_vstr$(repr(new_path))
+            """
+            if isempty(already_warned_path_change_pkgs)
+                warnstr *= """
+                This might indicate a change of environment has happened between package loads and can mean that
+                incompatible packages have been loaded"""
+                if JLOptions().startupfile < 2 #(0 = auto, 1 = yes)
+                    warnstr *= """. If this happened due to a startup.jl consider starting julia
+                    directly in the project via the `--project` arg."""
+                else
+                    warnstr *= "."
+                end
+            end
+            @warn warnstr
+            push!(already_warned_path_change_pkgs, uuidkey.uuid)
+        end
+    end
+end
+
 mutable struct PkgOrigin
     path::Union{String,Nothing}
     cachepath::Union{String,Nothing}