Implement --remap-path-prefix

Remove experimental -Zremap-path-prefix-from/to, and replace it with
the stabilized --remap-path-prefix=from=to variant.

This is an implementation for issue of #41555.

src/doc/man/rustc.1

@@ -125,6 +125,16 @@ Print version info and exit.
 \fB\-v\fR, \fB\-\-verbose\fR
 Use verbose output.
 .TP
+\fB\-\-remap\-path\-prefix\fR \fIfrom\fR=\fIto\fR
+Remap source path prefixes in all output, including compiler diagnostics, debug information,
+macro expansions, etc. The \fIfrom\fR=\fIto\fR parameter is scanned from right to left, so \fIfrom\fR
+may contain '=', but \fIto\fR may not.
+
+This is useful for normalizing build products, for example by removing the current directory out of
+pathnames emitted into the object files. The replacement is purely textual, with no consideration of
+the current system's pathname syntax. For example \fI\-\-remap\-path\-prefix foo=bar\fR will
+match \fBfoo/lib.rs\fR but not \fB./foo/lib.rs\fR.
+.TP
 \fB\-\-extern\fR \fINAME\fR=\fIPATH\fR
 Specify where an external rust library is located. These should match
 \fIextern\fR declarations in the crate's source code.

src/doc/unstable-book/src/compiler-flags/remap-path-prefix.md

@@ -1,37 +0,0 @@
-# `remap-path-prefix`
-
-The tracking issue for this feature is: [#41555](https://github.com/rust-lang/rust/issues/41555)
-
-------------------------
-
-The `-Z remap-path-prefix-from`, `-Z remap-path-prefix-to` commandline option
-pair allows to replace prefixes of any file paths the compiler emits in various
-places. This is useful for bringing debuginfo paths into a well-known form and
-for achieving reproducible builds independent of the directory the compiler was
-executed in. All paths emitted by the compiler are affected, including those in
-error messages.
-
-In order to map all paths starting with `/home/foo/my-project/src` to
-`/sources/my-project`, one would invoke the compiler as follows:
-
-```text
-rustc -Zremap-path-prefix-from="/home/foo/my-project/src" -Zremap-path-prefix-to="/sources/my-project"
-```
-
-Debuginfo for code from the file `/home/foo/my-project/src/foo/mod.rs`,
-for example, would then point debuggers to `/sources/my-project/foo/mod.rs`
-instead of the original file.
-
-The options can be specified multiple times when multiple prefixes should be
-mapped:
-
-```text
-rustc -Zremap-path-prefix-from="/home/foo/my-project/src" \
-      -Zremap-path-prefix-to="/sources/my-project" \
-      -Zremap-path-prefix-from="/home/foo/my-project/build-dir" \
-      -Zremap-path-prefix-to="/stable-build-dir"
-```
-
-When the options are given multiple times, the nth `-from` will be matched up
-with the nth `-to` and they can appear anywhere on the commandline. Mappings
-specified later on the line will take precedence over earlier ones.