Fix doc warnings and link failures (#1948)

Also add configuration to CI to fail doc generation if any links are
broken. Unfortunately we can't blanket deny all warnings in rustdoc
since some are unconditional warnings, but for now this is hopefully
good enough.

Closes #1947

cranelift/codegen/src/ir/extfunc.rs

@@ -383,7 +383,7 @@ pub struct ExtFuncData {
     /// flag is best used when the target is known to be in the same unit of code generation, such
     /// as a Wasm module.
     ///
-    /// See the documentation for [`RelocDistance`](machinst::RelocDistance) for more details. A
+    /// See the documentation for [`RelocDistance`](crate::machinst::RelocDistance) for more details. A
     /// `colocated` flag value of `true` implies `RelocDistance::Near`.
     pub colocated: bool,
 }

cranelift/codegen/src/ir/globalvalue.rs

@@ -66,7 +66,7 @@ pub enum GlobalValueData {
         ///
         /// If `true`, some backends may use relocation forms that have limited range: for example,
         /// a +/- 2^27-byte range on AArch64. See the documentation for
-        /// [`RelocDistance`](machinst::RelocDistance) for more details.
+        /// [`RelocDistance`](crate::machinst::RelocDistance) for more details.
         colocated: bool,
 
         /// Does this symbol refer to a thread local storage value?

cranelift/codegen/src/isa/aarch64/abi.rs

@@ -86,9 +86,9 @@
 //!   formal arguments, would:
 //!   - Accept a pointer P to the struct return area in x0 on entry.
 //!   - Return v3 in x0.
-//!   - Return v2 in memory at [P].
-//!   - Return v1 in memory at [P+8].
-//!   - Return v0 in memory at [P+16].
+//!   - Return v2 in memory at `[P]`.
+//!   - Return v1 in memory at `[P+8]`.
+//!   - Return v0 in memory at `[P+16]`.
 
 use crate::ir;
 use crate::ir::types;

cranelift/codegen/src/machinst/buffer.rs

@@ -12,13 +12,13 @@
 //!   from the branch itself.
 //!
 //! - The lowering of control flow from the CFG-with-edges produced by
-//!   [BlockLoweringOrder], combined with many empty edge blocks when the register
-//!   allocator does not need to insert any spills/reloads/moves in edge blocks,
-//!   results in many suboptimal branch patterns. The lowering also pays no
-//!   attention to block order, and so two-target conditional forms (cond-br
-//!   followed by uncond-br) can often by avoided because one of the targets is
-//!   the fallthrough. There are several cases here where we can simplify to use
-//!   fewer branches.
+//!   [BlockLoweringOrder](super::BlockLoweringOrder), combined with many empty
+//!   edge blocks when the register allocator does not need to insert any
+//!   spills/reloads/moves in edge blocks, results in many suboptimal branch
+//!   patterns. The lowering also pays no attention to block order, and so
+//!   two-target conditional forms (cond-br followed by uncond-br) can often by
+//!   avoided because one of the targets is the fallthrough. There are several
+//!   cases here where we can simplify to use fewer branches.
 //!
 //! This "buffer" implements a single-pass code emission strategy (with a later
 //! "fixup" pass, but only through recorded fixups, not all instructions). The
@@ -41,7 +41,7 @@
 //!   by the emitter (e.g., vcode iterating over instruction structs). The emitter
 //!   has some awareness of this: it either asks for an island between blocks, so
 //!   it is not accidentally executed, or else it emits a branch around the island
-//!   when all other options fail (see [Inst::EmitIsland] meta-instruction).
+//!   when all other options fail (see `Inst::EmitIsland` meta-instruction).
 //!
 //! - A "veneer" is an instruction (or sequence of instructions) in an "island"
 //!   that implements a longer-range reference to a label. The idea is that, for